The animal trypanosomiases and their chemotherapy: a revieweprints.gla.ac.uk/120436/1/120436.pdf ·...

The animal trypanosomiases and their chemotherapy:a review

FEDERICA GIORDANI1, LIAM J. MORRISON2, TIM G. ROWAN3,HARRY P. DE KONING1 and MICHAEL P. BARRETT1*1Wellcome Trust Centre for Molecular Parasitology, Institute of Infection, Immunity and Inflammation, College ofMedical, Veterinary and Life Sciences, University of Glasgow, Glasgow G12 8TA, UK2Roslin Institute, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush, Midlothian EH259RG, UK3Global Alliance for Livestock Veterinary Medicines (GALVmed), Doherty Building, Pentlands Science Park, Bush Loan,Edinburgh EH26 0PZ, UK

(Received 30 March 2016; revised 6 June 2016; accepted 23 June 2016; first published online 10 October 2016)

SUMMARY

Pathogenic animal trypanosomes affecting livestock have represented a major constraint to agricultural development inAfrica for centuries, and their negative economic impact is increasing in South America and Asia. Chemotherapy andchemoprophylaxis represent the main means of control. However, research into new trypanocides has remained inad-equate for decades, leading to a situation where the few compounds available are losing efficacy due to the emergenceof drug-resistant parasites. In this review, we provide a comprehensive overview of the current options available for thetreatment and prophylaxis of the animal trypanosomiases, with a special focus on the problem of resistance. The keyissues surrounding the main economically important animal trypanosome species and the diseases they cause are also pre-sented. As new investment becomes available to develop improved tools to control the animal trypanosomiases, we stressthat efforts should be directed towards a better understanding of the biology of the relevant parasite species and strains, toidentify new drug targets and interrogate resistance mechanisms.

Key words: animal trypanosomiases, veterinary trypanocide, drug resistance, Trypanosoma congolense, Trypanosomavivax, Trypanosoma brucei.

INTRODUCTION

The animal trypanosomiases (or trypanosomoses)include a variety of wasting diseases caused byunicellular protozoan parasites of the genusTrypanosoma (order Kinetoplastida). All relevantanimal pathogenic trypanosomes (T. vivax – sub-genus Duttonella, T. congolense – subgenus Nanno-monas and T. brucei spp. – subgenus Trypanozoon)(Fig. 1) belong to the Salivaria group (Haag et al.1998), so-called because their transmission to thevertebrate host occurs principally via the infectedsaliva of blood-sucking insects. Most valuable do-mestic livestock (bovines, ovines, caprines, equids,camelids and suids) are susceptible to infectionwith one or more of these Trypanosoma species.This can lead to acute and/or chronic forms ofwasting disease, causing high morbidity, mortalityand infertility in the absence of treatment (Leachand Roberts, 1981; Connor, 1992). By affecting agri-cultural production and animal husbandry, theanimal trypanosomiases have a high economic and

social impact in vast areas of the tropics and subtrop-ics where transmission occurs. Africa has historicallysuffered the greatest burden (Steverding, 2008), butthe negative effects are also increasing in SouthAmerica and South-East Asia, where unrestrictedanimal movements favour the spread of some tryp-anosome species.Chemotherapy and chemoprophylaxis represent

the mainstay of animal trypanosomiases control, en-suring animal health and production in enzooticcountries. However, the available veterinary trypa-nocides (Table 1) are inadequate and outmoded.Only six compounds are currently licensed, andtheir narrow therapeutic indices restrict their use, es-pecially when even low-level resistance arises. Byfar, the most usage is of two compounds, diminazeneaceturate and isometamidium chloride, largelyapplied against animal trypanosomiases in Africa(Holmes et al. 2004), with suramin also being rela-tively widely used to treat T. b. evansi infections.Worryingly, an increasing number of reports of re-sistance to this handful of existing chemicals, par-ticularly diminazene and isometamidium, indicatetheir future utility to be in jeopardy (Geerts et al.2001; Delespaux and de Koning, 2007).It has been estimated that as many as 35 million

doses of trypanocides are used annually in sub-Saharan Africa alone (Holmes, 2013), which

* Corresponding author: Wellcome Trust Centre forMolecular Parasitology, Institute of Infection, Immunityand Inflammation, College of Medical, Veterinary andLife Sciences, Sir Graeme Davies Building, Universityof Glasgow, 120 University Place, Glasgow G12 8TA,UK. E-mail: [email protected]

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Parasitology (2016), 143, 1862–1889. ©Cambridge University Press 2016. This is an Open Access article, distributed under the terms ofthe Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribu-tion, and reproduction in any medium, provided the original work is properly cited.doi:10.1017/S0031182016001268

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represents a figure suitable to treat only around one-third of the cattle at risk (Swallow, 2000). Inclusionof trypanocides sold informally in the Africanmarket may substantially increase the total numberof doses sold annually, which may be as high as 70million doses (Frans van Gool, personal communica-tion, 2015). Despite this demand, the high costsof drug development and the low anticipatedprofit from the sale of chemotherapeutics in develop-ing countries have disincentivized commercialpharmaceutical investments in this field (Connor,1992). In recent years, a public–private partnership,GALVmed (Global Alliance for LivestockVeterinary Medicines), supported by funding fromthe Bill & Melinda Gates Foundation and the UKDepartment for International Development, hasemerged to fill the gap, and has committed to the de-velopment of new therapeutic and prophylactic try-panocidal drugs (http://www.galvmed.org/en/).However, even in the best case scenario, a novellicensed compound is unlikely to be available forseveral years yet; hence the rational, correct use ofthe trypanocides already available is of paramountimportance.

THE ANIMAL TRYPANOSOMIASES :

DISTRIBUTION, TRANSMISSION, HOSTS ,

PATHOLOGY AND ECONOMIC IMPACT

Animal African trypanosomiasis (AAT, nagana)

AAT[also callednagana, fromtheZuluword ‘N’gana’which means ‘powerless/useless’ (Steverding, 2008)],is caused by trypanosome species T. congolense,T. vivax and, to a lesser extent, T. brucei spp.

(Fig. 1). The disease is widespread in sub-SaharanAfrica (Fig. 2), where it is cyclically transmitted bythe tsetse fly (Glossina spp.), the same vector respon-sible for the transmission of human-infective trypano-somes (T. brucei gambiense and T. b. rhodesiense, theaetiological agents of humanAfrican trypanosomiasis,HAT, or sleeping sickness) (Barrett et al. 2003). Inanimals, tsetse flies can also transmit trypanosomesmechanically when they begin a blood meal on aninfected host and end it on another one, providedthat the time between the two meals is short enoughtoensure survival ofparasites in the insectmouthparts,as shown in experimental infections in goats (Molooet al. 2000). Unlike other trypanosomes, T. vivaxdoes not multiply in the tsetse midgut, but remainsconfined to the insect proboscis, where it completesits short life cycle (Gardiner, 1989). This is thereason why this species can also be transmitted mech-anically by other haematophagous flies, in particularhorseflies (Tabanus spp.) and stable flies (Stomoxysspp.). Mechanical transmission has allowed T. vivaxto spread far beyond the limits of the African tsetsebelt: this parasite is now established in Mauritius andin 13 South American countries (Fig. 2), where itprobably arrived in the 18th or 19th century viainfected Zebu cattle exported from West Africa(Jones and Davila, 2001; Osorio et al. 2008), anorigin corroborated by phylogenetic studies (Cortezet al. 2006). Although T. vivax remains enzootic inSouth America primarily due tomechanical transmis-sion, other potential modes of transmission includeperinatal and iatrogenic routes or via alternative, asyet unidentified vectors (Osorio et al. 2008). Thislack of definitive knowledge greatly hampers the

Fig. 1. Morphological characteristics of the bloodstream form trypomastigote of the three most important livestocktrypanosomes. T. brucei group trypanosomes (T. b. brucei, T. b. evansi, T. b. equiperdum) are morphologicallyindistinguishable (with the exception of the non-proliferative stumpy-form in T. b. brucei). The trypomastigote is thedisease-relevant form and the target of therapy.

1863Chemotherapy of the animal trypanosomiases


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Table 1. Currently available veterinary trypanocides.

Name Trade namesa StructureAdministrationroute Action

Dosage(mg Kg−1)b Indication/animal

Adverseeffects/otherinformation

Treatment ofrelapses

Diminazeneaceturate

Berenil, Veriben,Pirocide,Ganaseg,Azidin, Trypan

IM, SCT 3·5 (up to 8

for resistanttrypanosomes,5–10 forT. b. evansi)

T. congolense,T. vivax (less activeon T. b. brucei,T. b. evansi)/Cattle,sheep, goats, dogs

Toxic to horses,donkeys, dogsand camels. Alsobabesicidal

Isometamidiumchloride

Homidiumbromide

Homidiumchloride

Ethidium

Novidium

IM (deep,cattle), IV(sheep, goats,pigs)

T, (P) 1 T. vivax,T. congolense (lessactive onT. b. brucei)/Cattle,sheep, goats, pigs

IM toxic tohorses.Potentiallycarcinogenic

Diminazeneaceturate,Isometamidiumchloride

Isometamidiumchloride

Trypamidium,Samorin,Veridium,Securidium

IM (deep) P, T 0·25–1 (T), 0·5–1(P)

T. congolense,T. vivax (less activeon T. b. brucei,T. b. evansi)/Cattle,sheep, goats,horses, camels

Toxic above 2mgKg–1. Avoidsubcutaneousadministration.Highly irritant.Possible localreactions incattle.

Diminazeneaceturate

Quinapyraminesulphate

Quinapyraminesulphate:chlor-ide (3:2 w/w)

Antrycide,Trypacide,Noroquin,Quintrycide,Tribexin,Triquin-S,M7555,Trypacideprosalt

SC T

P

3–5 (T) (20–40for T. simiae)(Camels,horses, pigs,dogs: dosedivided andgiven at 6 hintervals), 7·4(P)

T. b. evansi,T. vivax,T. congolense,T. brucei,T. b. equiperdum,T. simiae/Camels,horses, pigs, dogs,cattle(discouraged)

Toxic at highdoses. Fastresistanceacquisition

Isometamidiumchloride,Suraminsodium

Suramin sodium Naganol, Bayer205, Germanin

IV T (P) 10 (horses: 3doses/1 week)

T. b. evansi,T. b. brucei,T. b. equiperdum/Camels, horses

IM can causesevere necrosisat injection site.May be toxic tohorses

Quinapyraminesulphate

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implementation of surveillance and control strategies(Jones and Davila, 2001). Non-tsetse transmitted T.vivax infection in cattle is also recognized in parts ofAfrica, for example in regions of Ethiopia, Chad andSudan (Ahmed et al. 2016). Mechanical transmissionof T. congolense has been shown under experimentalconditions (Desquesnes andDia, 2003) and can there-fore not be excluded from contributing to its spread inAfrica (Desquesnes et al. 2009).The host range is wide (Uilenberg, 1998).

Trypanosoma congolense is considered the mostpathogenic trypanosome in cattle (followed by T.vivax), but it also causes infections in horses,sheep, goats, pigs and dogs. Apart from bovines,T. vivax can affect sheep, goats, horses and camels(Osorio et al. 2008). Trypanosoma b. brucei is foundin various domestic ungulates but it is particularlyvirulent in dogs, camels and horses, the latter oftensuccumbing to infection within a few months inthe absence of treatment. In areas where more thanone trypanosome species is present, mixed infectionsin domestic animals are often encountered (Kihuraniet al. 1994; Auty et al. 2008; Biryomumaisho et al.2013; Takeet et al. 2013; Moti et al. 2015) andmodern molecular techniques (Desquesnes andDavila, 2002) facilitate speciation. Many wildanimal species in Africa also host one or more tryp-anosome species and can serve as reservoirs for bothhuman and domestic animal infective trypanosomes(Mulla and Rickman, 1988; Auty et al. 2012).Similarly, wild South American fauna can harbourT. vivax and act as reservoir of infection (Osorioet al. 2008).Belonging to the same Nannomonas subgenus as

T. congolense, T. simiae is the only trypanosomespecies to be extremely pathogenic to pigs, which re-present the main host, although other domesticspecies can harbour the parasite (Joshua and Kayit,1984; Salim et al. 2014). In pigs, T. simiae causes ahyperacute, often fatal infection, with death oftenoccurring within 48 h of the appearance of symp-toms (Leach and Roberts, 1981). For this reason,chemoprophylaxis is preferred to curative treatment.The pathogenicity of trypanosomal infections

varies considerably depending on several factors, in-cluding parasite-related aspects (species and viru-lence), host (species, breed, age, immunologicalstatus, nutritional status, presence of co-infectionand physical condition), vector (species, density, in-fection rate and host preference), epidemiologicalsituation (endemic or epidemic) and the environ-ment (e.g. the availability of food and water andthe season) (Leach and Roberts, 1981; Van denBossche and Delespaux, 2011). Anaemia is themost prominent pathological feature of AAT(Taylor and Authié, 2004) and, in conjunctionwith other systemic lesions, can contribute to deaththrough eventual congestive heart failure. Othersymptoms include pyrexia, lymph node and spleenM

elarsomine

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T.b.

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T.b.

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T.b.

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/Cam

els,buffalo,

goats,pigs,horses,

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IM,intram

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enlargement, ataxia, lethargy, weight loss, oedema,immunosuppression, abortion and decrease in milkproduction. The immunosuppression caused by try-panosomes can affect animal health by interferingwith vaccination against other diseases (Singlaet al. 2010), or by increasing susceptibility of thehost to other infections. Inflammatory, degenerativelesions are also observed, and can damage variousorgans such as heart, central nervous system(CNS), eyes, testes, ovary and pituitary gland.Death may occur within weeks from onset of theacute disease. Otherwise the animal enters achronic phase (spontaneous recovery is rare but notunknown), characterized by intermittent or sub-patent parasitaemia, general malaise and infertility,and may last months or years prior to death(Taylor and Authié, 2004).While mortality due to the disease is clearly im-

portant, the impact upon overall cultivation andcrop production due to reduced draught power isthe most significant contributor to the economicimpact of AAT (Swallow, 2000). This is consideredthe livestock disease with the highest impact on agri-cultural production and animal husbandry in Africa,causing annual losses which run to billions of US$(Shaw et al. 2014). Across the tsetse belt as manyas 55 million cattle are at risk of infection (Cecchiand Mattioli, 2009), plus 30 million sheep and 40million goats. Of these cattle, 3 million die everyyear from AAT. The disease has devastating effectson the livelihoods of local farmers, for whom cattlerepresent not only a source of food (meat andmilk), manure, and draught power, but have alsofundamental social roles as ‘living banks’ and areused for social obligations (e.g. dowry and ritualuse) (Swallow, 2000; Grace et al. 2009; Mungubeet al. 2012).

Infection with T. vivax is considered an emergingdisease in South America where it has a significantimpact on cattle farming, but where it also affectshorses and other ruminants (Batista et al. 2007,2009, 2012; Da Silva et al. 2011). In a region includingthe Brazilian Pantanal and the Bolivian lowlands,where cattle ranching is the singlemost important eco-nomic activity (11 million head of cattle are reared inthe region), the losses caused to the industry by asingle outbreak of T. vivax in 1995 were calculatedat more than US$ 160 million (Seidl et al. 1999).The gross financial burden of T. vivax in SouthAmerica, however, is not known with any degree ofcertainty.

Surra

Surra (from the Hindi word for ‘rotten’) is the mostwidely used of a plethora of names given toT. b. evansi infection in animals (Desquesnes et al.2013b). As seen for T. vivax, T. b. evansi (a T.brucei subspecies) has also evolved a mechanicalmechanism of transmission that has allowed thisspecies to spread beyond Africa by export of infectedanimals (Lun et al. 2010). Trypanosoma b. evansi istoday the pathogenic animal trypanosome with thebroadest geographical distribution (Fig. 2), whichstretches from North-East Africa to much of Asiain the east (Luckins, 1988; Payne et al. 1991; Lunet al. 1993) and to Latin America in the west(Desquesnes et al. 2013b), and it is spreading stead-ily. In Europe, recent imported cases of surra havebeen documented and vigilance remains necessaryafter outbreaks in the Canary Islands, mainlandSpain, France and Germany (Desquesnes et al.2008; Gutierrez et al. 2010; Tamarit et al. 2011;Defontis et al. 2012).

Fig. 2. Countries where the most important livestock trypanosomes are present. Modified from (Auty et al. 2015), basedon PubMed search and including countries where data were not available and parasite presence is inferred. To note that thereal geographical distribution in some countries is limited (as, for example, for T. congolense in South Africa, Namibia andBotswana and for T. b. evansi in Russia). Cases of eradicated outbreaks of T. b. evansi in Europe (i.e. in France) are notindicated.

1866Federica Giordani and others



Several probable or suggested methods of surratransmission exist: by biting insects includinghorseflies and stable flies (the major creditedroute), by vampire bats, by iatrogenic (e.g. as aresult of a vaccination intervention), sexual, horizon-tal or vertical transmission, or by per-oral contamin-ation in the case of carnivores eating infected meat(Desquesnes et al. 2013a).Trypanosoma b. evansi can parasitize a wide range

of wild and domestic animal hosts, but the infectionis particularly pathogenic in horses, camels andAsian water buffaloes (Desquesnes et al. 2013b).There is increasing evidence that common rodentsare an important reservoir host for T. b. evansi andother trypanosomes (Jittapalapong et al. 2008;Maia da Silva et al. 2010; Kocher et al. 2015;Pumhom et al. 2015), such as T. lewisi, a parasiteof rats also found in atypical human infections(Howie et al. 2006; Sarataphan et al. 2007). Thesefindings revive the important question of rodentsas reservoirs of other T. brucei species. Rare casesof human infection with T. b. evansi (Joshi et al.2005; Haridy et al. 2011; Van Vinh et al. 2016),where individuals were infected through trypano-some-carrying animal blood, have been reportedand, in at least one case, infection was associatedwith a null mutation in the trypanosome lyticfactor blood component Apolipoprotein L1(APOL1), which normally protects humans fromanimal trypanosome infections (Vanhollebeke et al.2006; Truc et al. 2013). In a more recent case, nomutations in APOL1 were found to explain theunusual infection (Van Vinh et al. 2016).Symptoms of surra overlap those previously

described for AAT and their intensity can varygreatly between and within host species anddepend on the geographical area and epidemiologicalsituation (Desquesnes et al. 2013b).In the Philippines, outbreaks of surra cause high

morbidity and mortality in water buffaloes andother large ruminants, greatly affecting the liveli-hood of local small-scale farmers (Dargantes et al.2009; Desquesnes et al. 2013a). In the BrazilianPantanal T. b. evansi affects over 6000 horses peryear (of the 50 000 present), with serious conse-quences to the local economy, horses being essentialfor herding livestock. The total impact ofT. b. evansiinfection in horses in this region was estimated at US$ 2·4 million per year (Seidl et al. 1998). Surra is alsoone of the most frequent diseases affecting camels inNorth Africa, causing severe economic damage.

Dourine

Dourine is a disease caused by the subspeciesT. bruceiequiperdum, the only Salivarian trypanosome whosetransmission cycle avoids invertebrate vectors com-pletely. Instead, this parasite is transmitted amonghorses and other equids during mating (Claes et al.

2005). Of note, vertical or perinatal transmission oftrypanosomes other than T. b. equiperdum in the re-productive tissues has been reported (Griffin, 1983;Melendez et al. 1993; Lindner and Priotto, 2010;Biteau et al. 2016), although the role and relative im-portance of this mode of transmission in the field isnot clear.Trypanosoma b. equiperdum is an important veter-

inary trypanosome endemic in Africa and Asia, andis also found in the Middle-East, South-EastEurope and South America. Strict control policieshave eradicated T. b. equiperdum from WesternEurope in the past century (Claes et al. 2005), butthe risk of reintroduction remains, as shown by arecent outbreak in Italy (Pascucci et al. 2013).The infection presents with typical oedema of the

genital organs as well as weakness, emaciation, ur-ethral discharge, characteristic plaques in the skinand neurological symptoms such as lack of coordin-ation of the hind legs (Hagos et al. 2010). Dourine inhorses is generally fatal without treatment but it isusually subclinical in donkeys and mules (Brunet al. 1998).Considering the transmission mechanism and the

absence of a reservoir in other species, the controlstrategies for the disease follow a different approachas compared with other insect-borne forms of tryp-anosomiasis (Claes et al. 2005). The World HealthOrganization for Animal Health (OIE) recommendsbreeding and movement restrictions, compulsorynotification and slaughter of infected animals toblock new infection outbreaks or achieve eradication.Additionally, pharmacological therapy is not advisedas this may result in clinical improvement but not incomplete cure, leaving the animal as a potentialcarrier of the parasite. However, the feasibility oreffectiveness of this strict policy in developing coun-tries, where horses have a significant role in transportand agriculture, is questionable. Here, chemotherapymay help to sustain animal health and productivity.Although no official cure for dourine is available,studies have indicated the efficacy of melarsomine inthe treatment of acute and chronic T. b. equiperduminfection in horses (Hagos et al. 2010).

ANIMAL TRYPANOSOME SPECIES : VIRULENCE,

TISSUE DISTIBUTION, BIOLOGY AND

LABORATORY TOOLS

Trypanosoma congolense and T. simiae

Trypanosoma congolense is the smallest of the patho-genic trypanosomes (see Fig. 1 for its morphology).The species is divided into three main subgroups(i.e. Savannah, Forest and Kilifi) based on molecularmarkers (Hide and Tait, 2004; Auty et al. 2015), theSavannah subgroup being the most virulent (Bengalyet al. 2002a, b) and the most clinically important incattle. However, even within the same Savannah




subgroup substantial differences in virulence exist,with some strains causing only mild infections(Masumu et al. 2006), highlighting the complexityand subtlety of the balance between the level of para-site persistence and the host immune system.In the vertebrate host, T. congolense parasites

remain confined to the vascular system, where theybind to circulating erythrocytes (Banks, 1979) andto endothelial cells (Hemphill et al. 1994) throughtheir flagellum, causing damage at the adhesion site(Banks, 1980). Attachment of the bloodstreamform is also observed in in vitro culture, where para-sites adhere to the bottom of the flask, a phenotypeunique to T. congolense among trypanosome species(Coustou et al. 2010).Today, long-term culture of the pathogenic

bloodstream form is possible only for a limitednumber of strains (e.g. IL3000 and STIB910)(Coustou et al. 2010). Genetic tools have been devel-oped for this species, including a gene overexpres-sion system (Coustou et al. 2010) and RNAinterference (although, in this case, only for the pro-cyclic insect form) (Inoue et al. 2002; Coustou et al.2010). A draft genome sequence of strain IL3000 hasalso been published (Jackson et al. 2012) and offersthe potential to accelerate discovery of biomarkersfor diagnosis and targets for new drugs. However,despite the veterinary importance of T. congolense,the data available to understand its biology andpathogenicity and, therefore, to improve treatment,are scanty. It appears that this parasite has a carbo-hydrate metabolism that differs significantly fromthat of the far more widely studied T. brucei(Agosin and von Brand, 1954), with indications ofa more pronounced mitochondrial activity in itsbloodstream form. These dissimilarities may haverelevance in the very different responses of thesespecies to trypanocides (Leach and Roberts, 1981)and in the identification of potential drug targets.Of note, T. congolense lacks an orthologue of the T.brucei TbAT1 gene that encodes the P2 nucleosidetransporter (see subsection Diminazene aceturatebelow), which is central to the uptake of the trypano-cidal drug diminazene (Munday et al. 2013).Trypanosoma congolense has a correspondinglyreduced sensitivity to diminazene, which is not accu-mulated to the same degree in these parasites.Similarly, the closely related T. simiae does not

easily infect common laboratory rodents and, there-fore, little data on this organism is available.However, a method for the axenic in vitro culture ofthe bloodstream form of this parasite has been pub-lished (Zweygarth et al. 1992), offering the means toaccelerate our ability to dissect the parasite’s biology.

Trypanosoma vivax

Among African trypanosomes, T. vivax (Fig. 1) isthe most phylogenetically distinct species (Fig. 3).

Specific isolates present with different pathogenicityin cattle, in some cases causing chronic, sub-clinicalinfections and in others acute, haemorrhagic infec-tions (Wellde et al. 1983; Magona et al. 2008).Although T. vivax (as T. congolense) has been con-

sidered typically to remain confined to the vascularsystem of the host, some strains may, especially inlate infections, also reach extravascular locations(e.g. lymph nodes, eyes and cerebrospinal fluid)where they may directly damage tissues and wherethey are less accessible to drug treatment(Whitelaw et al. 1988; Osorio et al. 2008;D’Archivio et al. 2013).Trypanosoma vivax is generally difficult to culti-

vate in the laboratory and this has restricted bio-logical studies into this parasite. Short-term, axenicculture systems for the bloodstream form havebeen reported (Brun and Moloo, 1982; Zweygarthet al. 1991; D’Archivio et al. 2011) but they havebeen difficult to reproduce in other laboratoriesand have not entered routine use. Most studies onthis trypanosome species are, therefore, conductedin in vivo laboratory models; however, very few T.vivax strains have been isolated that readily infectrodents and most published in vivo work on thisspecies comprises the very few mouse-infectivestrains, the main one being Y486 and its derivatives(Gibson, 2012). A simplified system for in vitro cul-tivation of the insect form of T. vivax was recentlydescribed and genetic manipulation methodologyimplemented (D’Archivio et al. 2011). As with T.congolense, studies into the biochemical physiologyof T. vivax have lagged behind those in T. bruceibut significant differences with the metabolism ofbloodstream form T. brucei were clear from earlystudies (Desowitz, 1956), which probably explainsincongruence in potency of different chemicalclasses against these species.

Trypanosoma brucei spp.

Trypanosoma brucei spp. (Fig. 1) include bothanimal (T. b. brucei, T. b. evansi, T. b. equiperdum)and human (T. b. rhodesiense, T. b. gambiense) infect-ive subspecies. UnlikeT. vivax (most strains at least)or T. congolense, T. brucei group trypanosomes arefound in both the vascular system and in othertissues, and can parasitize the brain in experimentalinfections (Moulton, 1986; Grab and Kennedy,2008; Coles et al. 2015); descriptions of this clinicalcondition in field settings are limited, other than forequids, which are particularly susceptible toT. brucei (Tuntasuvan et al. 1997; Ranjithkumaret al. 2014). As the most widely used drugs totreat animal trypanosomes (diminazene and isometa-midium) do not cross the blood–brain barrier, thepresence of parasites in sites other than the blood-stream represents a potentially important issue fortreatment of T. brucei. Parasites from inaccessible




body sites including the CNS may eventually re-es-tablish infection in the bloodstream and causerelapse following treatment with these drugs(Myburgh et al. 2013). Trypanosoma b. equiperdumis quite unique, it being mainly a tissue parasite,found in the capillaries of the urogenital tract andrarely in peripheral blood (Brun et al. 1998). Thismakes diagnosis, parasite isolation and treatmentparticularly difficult.Trypanosoma b. brucei is the most extensively

studied trypanosome. Some lineages (e.g. Lister427) are well adapted to laboratory in vitro cultureand have been used as model organism to studymany eukaryotic cell processes. The genome of thisspecies was published in 2005 (Berriman et al.2005) and its metabolism has been widely studied(Shameer et al. 2015). It has long been known that,in its bloodstream form,T. brucei species depend en-tirely on glycolysis for energy production, whileKrebs cycle and oxidative phosphorylation areactive only in the insect stages. New, comprehensivemetabolomics approaches (Creek et al. 2015) aremodifying this paradigm and, in conjunction withtranscriptomic approaches, a clearer understandingof trypanosome metabolism is emerging.Trypanosoma b. evansi and T. b. equiperdum can

be considered petite mutants of T. brucei, so

named after petite mutants of yeast that have lostmitochondrial respiratory function. These parasiteshave lost part (dyskinetoplastic parasites) or all (aki-netoplastic parasites) of their kinetoplast DNA(kDNA), which constitutes the mitochondrialgenome and comprises a network of circular conca-tenated mini- and maxi-circles (Schnaufer et al.2002; Lai et al. 2008). Although long consideredas two separate species, it has been proposed thatT. b. evansi and T. b. equiperdum be reclassifiedas subspecies of T. brucei, based on phylogeneticanalysis of sequenced genomes (Carnes et al.2015), and we have adopted this convention here.As the kinetoplast genome encodes for an essentialsubunit (F0-A6) of the mitochondrial F1F0 ATPsynthase, T. b. evansi and T. b. equiperdum cannotcomplete their life cycle in the fly and are lockedin the trypomastigote stage, which relies on glycoly-sis for ATP production. A compensating mutationin the nuclear genome-encoded γ-subunit of theATP synthase allows these parasites to maintaintheir mitochondrial membrane potential irrespect-ive of the F0-A6 subunit and, therefore, to survivein the absence of the kinetoplast genome (Deanet al. 2013). It is for this reason that these parasiteslost their dependency on the tsetse fly fortransmission.

Fig. 3. Phylogenetic tree based on SSU rRNA sequences from trypanosome species. Modified from (Cortez et al. 2006).




CONTROL STRATEGIES AND

TRYPANOTOLERANCE

All of the important livestock trypanosomesdescribed above are extracellular parasites inmammals and evade the host immune defences bycontinuously changing their surface coat (Horn,2014), one of the immune-evading mechanismsthat essentially preclude the development of conven-tional vaccines (La Greca and Magez, 2011; Cnopset al. 2015). Hence, control of animal trypanoso-miases relies primarily on the use of insecticides ortraps to control the vector (especially in the case oftsetse-transmitted trypanosomiases), and on the useof trypanocides to control the parasite (Holmes,2013). (The control strategy for dourine followsa completely different approach and has beendescribed separately; see subsection Dourineabove). Since vector control can be expensive whenused on a large scale and is not always sustainableor effective, administration of trypanocidal drugsrepresents the main intervention tool in most poorrural endemic areas, ensuring maximum effects atrelatively little cost (Grace et al. 2009; Van denBossche and Delespaux, 2011). The cost-effective-ness of this practice was shown both in Africa (atleast under certain circumstances) (Shaw et al.2015) and elsewhere (Seidl et al. 1998, 1999;Dobson et al. 2009). Control of parasites with che-motherapeutic and chemoprophylactic agents hasthe double effect of limiting the losses caused bythe infection and of eliminating the transmissibletrypanosome reservoir (Welburn et al. 2015).Effective treatment of the acute phase of infectionusually leads to prompt recovery of the animal; theuse of trypanocides in the chronic phase, however,usually clears parasitaemia, but clinical recovery inthese instances may require a significantly longertime, depending on the severity of symptoms suchas weight loss and organ damage.Some indigenous African livestock breeds (e.g.

N’Dama, Muturu and Dahomey) are more resistantto trypanosome infection than imported breeds(classically temperate ‘European’ taurine breedsbut also including Asian-derived Bos indicusbreeds, relatively new to trypanosome endemicareas, such as Boran). This phenomenon is called‘trypanotolerance’ and is defined as the ‘capacity tosurvive and remain productive after trypanosome in-fection’ (Murray et al. 1982). A major factor enab-ling these animals to cope with trypanosomeinfections is a better capacity to limit both anaemiaand parasitaemia (Naessens, 2006). The use of trypa-notolerant breeds has helped livestock productivityin various endemic regions in Africa and elsewhere,and it is often advocated as an important controlstrategy. Wild animals, which have co-evolvedwith trypanosomes, are also usually trypanotolerantand rarely suffer from clinical disease when infected.

TREATMENT STRATEGIES AND CHALLENGES

Treatment and prophylaxis of pathogenic trypano-some infections in animals relies on only six com-pounds (Table 1), most dating back to the first halfof the 20th century (Leach and Roberts, 1981).Moreover, several factors limit their use. Thecurrent drugs all have small therapeutic indicesand can also cause local irritancy at the injectionsite. Most importantly, extensive utilization in thepast has led to the appearance of resistant parasitesin the field, and the fact that many of these trypano-cides are chemically related has exacerbated the situ-ation with cross-resistance onset (Peregrine, 1994).A number of currently used compounds appear totarget the kinetoplast, causing its loss (Shapiro andEnglund, 1990; Chitambo and Arakawa, 1992b),but the actual mode of action of these trypanocidesand the biochemical mechanisms underpinning re-sistance are largely unclear. As noted above, differ-ences in biochemical physiology and host organdistribution discriminate each of the veterinary try-panosomes and, therefore, the different trypanocideshave divergent ability to kill based on specificpotency against each species and pharmacokineticparameters affecting distribution.Most trypanocides have therapeutic rather than

prophylactic activity, but the phenanthridine isome-tamidium is mostly used for its prophylactic effects(Stevenson et al. 1995). Unfortunately, these drugsare less active against T. b. evansi (Toro et al.1983) and are less used outside of sub-SaharanAfrica (Reid, 2002). The decision as to whether touse therapeutic or prophylactic drugs depends onseveral factors, including the risk of infection, drugavailability and distribution logistics (Gu et al.1999). Ideally, in areas of low prevalence, onlythose animals that present with clinical disease at-tributable to trypanosomes and/or have confirmedinfection should be treated with therapeutic drugs;instead, in areas of high challenge, prophylacticdrugs applied to the whole herd are more cost-effective, providing much greater reduction of mor-tality and morbidity and avoiding the adverse effectsof infection on productivity (Gu et al. 1999). Single-dose therapeutic and prophylactic products for cattleare preferred, as multiple-dose administration regi-mens are often not practical in developing countries,where animal handling facilities are typically verylimited.As new compounds are not likely to become avail-

able in the near future (i.e. the most optimisticoutlook is at least 3–5 years before a new compoundcould realistically be expected to be registeredthrough current initiatives), prudent use of thosealready on the market is paramount. However, infield settings drug usage is often difficult tomonitor and regulate. In hyperendemic Africancountries, trypanocides are usually administered




directly by farmers, who can easily obtain them atlocal markets for a relatively affordable price (forless than US$ 1 per treatment). Unfortunately,most livestock keepers in the affected regions havelimited access to tools which (a) enable accuratediagnosis, and frequently farmers are reliant solelyon clinical signs, which are often not pathognomon-ic; and (b) provide information or training regardingoptimal drug usage and dosage, and this combin-ation of factors can lead to drug misuse (Van denBossche et al. 2000; Grace et al. 2009). Moreover,in an unregulated market, poor quality or counterfeittrypanocides are widespread in some areas, especial-ly in Africa, where documented product specifica-tions are scarce (Sutcliffe et al. 2014; Tchamdjaet al. 2016). To improve veterinary drug standardsand tackle the issue of counterfeit drugs two labora-tories for trypanocide quality control checks were re-cently set up in Africa (one in Dakar and one in DarEs Salaam) thanks to a GALVmed-FAO (Food andAgriculture Organization of the United Nations) ini-tiative with other collaborating partners (Sutcliffeet al. 2014).Besides correct dosage administration, various

other options to extend the life of current trypano-cides exist. Different approaches (such as deliverysystems including complexing to polymeric sub-stances promoting slow release or alternative formu-lations) have been considered in order to improvetherapeutic efficacy (Peregrine, 1994; Geerts et al.1999; Kroubi et al. 2011; Unciti-Broceta et al.2015). These could allow the use of lower quantitiesof trypanocide in a more effective way and, conse-quently, pose a decreased risk of toxicity and pos-sibly decreased resistance development.Unlike the situation with HAT, where the nifurti-

mox–eflornithine combination therapy (NECT) isnow the preferred first line treatment for second-stage disease (Priotto et al. 2009; Alirol et al.2013), no drug combinations are currently used forthe animal trypanosomiases. Instead, alternatinguse of compounds, particularly diminazene andisometamidium (called a ‘sanative pair’), with lowrisk of cross-resistance, is recommended where pos-sible. In particular, in the case of relapse the animalshould be treated with a different drug class from theone previously administered, in order not to re-inforce drug resistance selection (Leach andRoberts, 1981). Due to the chemical relatedness ofseveral veterinary trypanocides, however, this ap-proach is not always practicable. Thus, in order tomaintain the efficacy of the currently used com-pounds, it is important that chemotherapeutic andchemoprophylactic dosage regimens are rationalizedon the basis of the drug-susceptibility phenotypeof trypanosome populations in a given locality.However, such rationalization is not possible,because the systems that are currently available tocharacterize the drug resistance phenotype of

trypanosome populations are not field applicable(Peregrine, 1994). Limited numbers of field isolatescan be characterized and all of the systems take manymonths to provide definitive data (see section Testsfor resistance detection below). There is therefore arequirement for new assays that will rapidly quantifythe drug resistance phenotype of large numbers oftrypanosome isolates.

VETERINARY TRYPANOCIDES : DOSAGE,

PHARMACOKINETICS , MODE OF ACTION AND

RESISTANCE

Diminazene aceturate

Diminazene aceturate (Table 1) was introduced forthe treatment of babesiosis and African trypanosom-iasis in livestock in 1955. It belongs to the diamidineclass of compounds, a member of which (pentami-dine) has also been used for HAT since the 1930s(Steverding, 2010). Ironically, it was pursuing astructure-activity iterative synthesis from a com-pound belonging to a different class, Surfen C [atthe time of its introduction in the 1930s, the bestavailable agent against T. congolense infections(Bennett, 1936)], that led to diminazene develop-ment (Hawking, 1963). Although it was anti-T. con-golense activity in experimental rodents that initiallydrove development, today’s in vitro systems, whereanti-parasite potency can be tested without con-founding issues related to pharmacokinetic behav-iour in hosts, show that diminazene is substantiallyless potent against T. congolense than it is againstT. brucei group trypanosomes. This feature is attrib-utable to the fact that its uptake into the latter para-sites via the P2/TbAT1 transporter (see later) allowsconcentrative and rapid uptake (De Koning et al.2004). In T. congolense, which lacks an orthologueof TbAT1 (Munday et al. 2013), uptake is lessrobust, explaining its lower activity.Diminazene is today the most commonly used try-

panocide in cattle, sheep and goats, due to its activityagainst both T. congolense and T. vivax and its rela-tively low toxic side effects. The compound alsoeffectively cures surra and is, for example, the main-stay of treatment of T. b. evansi in the Philippines(Reid, 2002). The recommended therapeutic doseis 3·5 mg kg−1 body weight for AAT due to T. con-golense and T. vivax (7 mg kg−1 may be recom-mended against resistant isolates) and 7 mg kg−1 isindicated for AAT due to T. brucei and for surra,administered by intramuscular or subcutaneous in-jection (Connor, 1992). The common practice ofadministering 3·5 mg kg−1 of the drug to treatT. b. evansi infections is considered an underdosing,and this misuse may have contributed to the emer-gence of resistant strains in South-East Asia(Desquesnes et al. 2013a). The fact that higherdoses appear to be needed to treat T. brucei group




trypanosomes, in spite of these parasites being moresensitive to the drug, probably relates to their widertissue dispersal compared with T. congolense and T.vivax, underlining the key role of hostpharmacokinetics.Diminazene is only applied as a curative agent and

is not used for prophylaxis, as it is rapidly metabo-lized and excreted (Peregrine and Mamman, 1993).After rapid absorption (the peak blood level isreached within 1 h of dosing), elimination follows abiphasic or triphasic behaviour depending on theanimal species and formulation; elimination half-life values following intramuscular administrationvaried from 11–19 h in sheep and goats, to 74to >200 h in cattle (Mamman et al. 1993; Peregrineand Mamman, 1993; Mdachi et al. 1995; ElBanna et al. 1999). Cattle excrete diminazenemainly in the urine, together with two main metabo-lites: p-aminobenzamidine and p-amino-benzamide(Kellner et al. 1985). Diminazene residues maypersist for several weeks in the edible tissues ofcattle and other food-producing animals, especiallyin the liver and kidney, whereas the drug levels inmilk peak at 6 h and fall to below detection limitsafter 48 h (FAO, 1990). For this reason it isadvised that cattle and sheep destined for humanconsumption are subject to a 21–35 days pre-slaugh-ter withdrawal (discard) from drug, while a 3-daymilk discard period is recommended (FAO, 1990;Peregrine and Mamman, 1993); however, product-specific withdrawal periods as given on productlabels should be adhered to.The trypanocidal mode of action of diminazene

has not been completely elucidated. The compoundbinds the minor groove of the DNA at AT-rich sites(Wilson et al. 2008). In trypanosomes, the kDNA isa known target of the drug, and kDNA binding cancause inhibition of replication and kDNA loss(Shapiro and Englund, 1990), possibly exacerbatedby an inhibitory effect on mitochondrial type IItopoisomerase (Portugal, 1994). It had long beenbelieved that loss of the kinetoplast might not besufficient to kill trypanosomes, as viable dyskineto-plastic strains do occur naturally and also can be pro-duced artificially in the laboratory (Schnaufer et al.2002). However, the discovery in laboratory gener-ated-dyskinetoplastic T. b. brucei of a compensatingmutation in the nuclear genome-encoded γ-subunitof the mitochondrial ATP synthase (Dean et al.2013) meant that the kinetoplast has been resur-rected as the potential drug target of diminazene.These dyskinetoplastic lines do indeed show signifi-cant in vitro resistance to diamidines (includingdiminazene aceturate) and phenanthridines (Gouldand Schnaufer, 2014). Furamidine (DB75), aclosely related diamidine, whose fluorescent proper-ties enabled tracking of its cellular distribution, wasshown to bind to T. b. brucei kDNA and nuclearDNA in situ, and also to accumulate in other

organelles identified as acidocalcisomes (Mathiset al. 2006). The compound was also shown to inter-fere with the mitochondrial membrane potential(Lanteri et al. 2008). Interestingly, it has been sug-gested that diminazene can also modulate the hostimmune response by dampening pro-inflammatorycytokines and excessive immune activation, whichmight also influence the in vivo effects of the drug(Kuriakose et al. 2012).Chemically, diminazene is an aromatic diamidine

made of two benzamidine moieties linked by a tria-zene bridge. Due to its charged nature, diminazenecan only cross membranes via specific carriers andthis has three important consequences: (a) the drugis not active on CNS infections as it cannot crossthe blood–brain barrier; (b) the compound is select-ively toxic to trypanosomes, as they express trans-porters that specifically accumulate diminazene;and (c) trypanosomes may become resistant to thedrug by losing these transporters or their activity.As mentioned above, diminazene uptake in T.brucei mainly occurs via an aminopurine transportercalled P2 or TbAT1, which is also implicated in theuptake of the related diamidine pentamidine and themelaminophenyl arsenical melarsoprol, two drugslicensed for HAT (Carter et al. 1995; Barrett andFairlamb, 1999; De Koning, 2008). Diminazeneuptake into T. brucei is fast, with a Km of 0·45 µMand a Vmax of 0·049 pM 10

7 cells−1 s−1 (De Koninget al. 2004) and is inhibited by pentamidine and ad-enosine, the main physiological substrate of thiscarrier. Loss of P2/TbAT1 activity was shown tocause diminazene resistance in T. b. brucei (Matovuet al. 2003), T. b. equiperdum (Barrett et al. 1995;Stewart et al. 2010) and T. b. evansi (Witola et al.2004). Another gene, named TeDR40, has alsobeen implicated in resistance in T. b. evansi(Witola et al. 2005). However, using that gene tosearch for orthologues in other trypanosomatidsat the TriTrypDB database (www.tritrypdb.org),indicates that it is actually a variant surface glycopro-tein (VSG) gene, part of the parasite’s system ofantigenic variation whereby it avoids host immunity.It is possible that, in the process of selection of resist-ance, the parasites switched expression of a VSGgene independently of the resistance selection,which explains the massive increase in expressionof that gene.The application to T. brucei of a genome-wide

RNA interference target sequencing (RIT-seq)screen, where any gene whose loss of function isidentified by reduced drug sensitivity, was able toidentify additional plasma membrane proteins(P-type H+-ATPases), as well as a putativeprotein phosphatase, that were linked to the actionof the related diamidine pentamidine (Alsfordet al. 2012). The HAPT1/TbAQP2 carrier (DeKoning, 2001b), encoded by the TbAQP2 gene(Baker et al. 2012), has a key role in uptake of



http://www.tritrypdb.orghttp://dx.doi.org/10.1017/S0031182016001268http:/www.cambridge.org/corehttp:/www.cambridge.org/core/terms

pentamidine and the melaminophenyl arsenicals inT. brucei, although its role in diminazene uptake isless pronounced (Teka et al. 2011; Munday et al.2014) and loss of P2/TbAT1 alone is sufficient togive high level of resistance to this latter drug(Matovu et al. 2003). It has recently been proposedthat TbAQP2 acts as a receptor for pentamidine,with high affinity, and its uptake then occurs via re-ceptor-mediated endocytosis (Song et al. 2016);further work is needed to confirm or refute this hy-pothesis, although other evidence points to pent-amidine actually entering through the channel,enabled by a unique selectivity filter and the highdegree of flexibility of the pentamidine chain(Munday et al. 2014, 2015a).Trypanosoma congolense appears to lack a func-

tional equivalent of TbAQP2. A putative P2/TbAT1-type transporter, TcoAT1, was identifiedin T. congolense and a particular allele proposed tobe associated with diminazene resistance (Delespauxet al. 2006). This conclusion was curious, given thatthe so-called resistance allele was not always asso-ciated with resistant form parasites isolated in oneregion (Delespaux et al. 2006) and was also abundantin areas where diminazene had not been used(Chitanga et al. 2011). Furthermore, TcoAT1 isnot the orthologue of TbAT1, instead correspond-ing to a related, but distinct, member of the nu-cleoside transporter family (Munday et al. 2013).Its heterologous expression has proven that theencoded protein does not enable diminazeneuptake, instead facilitating the uptake of adenosineand inosine (Munday et al. 2013). Hence, it can bedefinitively ruled out that the gene misnamedTcoAT1 has any role in diminazene uptake, action,or resistance.Diminazene resistance is generally believed to be

difficult to produce experimentally in T. congolense(in contrast to T. brucei). High levels of resistanceto the drug were obtained in mice infected withT. b. evansi, but only when using immunocomprom-ised animals, a result which stresses the importanceof the link between immunity and chemotherapy,as the efficacy of trypanocides appears to bereduced by immunosuppression, hence favouringdevelopment of resistance (Osman et al. 1992). Invitro experiments with T. b. brucei and T. b. evansidemonstrated that a shared mechanism of internal-ization accounts for the cross-resistance betweendiminazene and other diamidines as well as melami-nophenyl arsenicals (melarsoprol and melarsomine)(Matovu et al. 2003). By contrast, no cross-resistancewas observed with other chemically unrelated com-pounds including suramin or quinapyramine. Adegree of cross-resistance has been observedbetween isometamidium and diminazene in T.brucei group trypanosomes, although the functionalbasis of this is not clear (Zhang et al. 1991; Witolaet al. 2004).

Homidium salts

Homidium bromide or ethidium bromide, alsoavailable as a chloride salt (Novidium®, Table 1),was introduced for field use in 1952, as an improve-ment to previous phenanthridine-based trypanoci-dal agents (Wainwright, 2010). It is widely used inAfrica to treat T. congolense and T. vivax infectionsin cattle, sheep and goats, in spite of its proven mu-tagenic and possible carcinogenic properties as aDNA intercalator (Sutcliffe et al. 2014). Due to its po-tential toxicity, the use of homidium is today highlydiscouraged (Sutcliffe et al. 2014). Widespread resist-ance to the drug in the 1960s and 1970s reduced itsusage. Today, the number of doses of homidiumused annually is reported to be down to around 10%of the total African trypanocide market, but thisvalue may be a significant underestimate of its realuse (Frans van Gool, personal communication, 2015).Although used as a curative drug, homidium also

possesses chemoprophylactic properties, but theseare less pronounced than those of isometamidium(see subsection Isometamidium chloride below). Forboth purposes, homidium is administered at thedose of 1 mg kg−1 by a single, deep intramuscular in-jection (Peregrine, 1994). Homidium excretion isfaster than isometamidium, its serum concentrationdeclining rapidly over the first 24 h following bothintravenous and intramuscular injection at a stand-ard dosage (Murilla et al. 1999). Elimination half-life ranged from 178 h in Boran cattle to 488 h inFriesian cattle following intramuscular injection(Murilla et al. 1999). However, low levels of thedrug (0·1–0·3 ng mL−1) do persist in circulation forseveral weeks when given intramuscularly, provid-ing an 8–17-week prophylaxis period (Dolan et al.1990; Murilla et al. 1999). Homidium has an exten-sive extravascular distribution and accumulates pre-dominantly in the liver and the kidneys (Murillaet al. 1996), a factor which presents some risk in pro-ducts from treated animals destined for human con-sumption. Homidium can be used as sanative pairwith diminazene, but not with isometamidium,where the shared phenanthridine core underliescross-resistance (Peregrine et al. 1997).Intracellular localization of homidium can be

monitored by microscopy, exploiting the intrinsicfluorescence of the compound. Work on T. bruceishowed that homidium localizes in the nucleus andthe kinetoplast of treated trypanosomes (Cox et al.1984; Boibessot et al. 2002). Treatment with thedrug induces dyskinetoplasty in a similar way toother phenanthridines and diamidines (Riou et al.1980; Shapiro and Englund, 1990) and disruptionof genome function has long been believed to under-lie its trypanocidal effects. Indeed, it was found thathomidium blocks both kinetoplast and nuclear DNAreplication in T. brucei by distorting and changingthe double helix topology (Roy Chowdhury et al.




2010). The inhibition of minicircle replication and,consequently, loss of the kinetoplast network, wasfound to be the primary killing mechanism at lowdoses (0·02 µg mL−1), but at higher doses homidiumwas also shown to affect nuclear DNA, which couldaccount for its ability to kill dyskinetoplastic trypa-nosomes (Roy Chowdhury et al. 2010). The reasonfor the initial targeting of the kinetoplast over thenucleus is believed to be the result of the preferentialaccumulation of lipophilic cations (such as homi-dium) in the mitochondrion, as shown with other ex-perimental trypanocides (Lanteri et al. 2008;Ibrahim et al. 2011; Alkhaldi et al. 2016). The mech-anism of resistance to homidium is not known, but itis likely to be similar to that of the related compoundisometamidium.

Isometamidium chloride

Isometamidium chloride hydrochloride is a hybridphenanthridine with amphiphilic and cationic prop-erties, synthesized by coupling homidium with thediazotized p-aminobenzamide moiety of dimina-zene, modified with the amidine group in the metaposition (see Table 1 for structures). It has bothcurative and prophylactic properties and, since itslaunch in the 1960s, it has remained the only drugavailable for chemoprophylaxis of AAT, after quina-pyramine was discontinued due to problems linkedto toxicity and, particularly, the induction ofmulti-drug resistance (Peregrine, 1994; Geerts andHolmes, 1998). The veterinary formulations are typ-ically a mixture of four phenanthridine compounds:isometamidium chloride hydrochloride [8-(3-m-amidinophenyl-2-triazeno)-3-amino-5-ethyl-6-phe-nylphenanthridinium chloride hydrochloride], thepositional red isomer [3-(3-m-amidinophenyl-2-triazeno)-8-amino-5-ethyl-6-phenylphenanthridiniumchloride hydrochloride], the blue isomer [7-(m-amidinophenyldiazo)-3,8-diamino-5-ethyl-6-phenyl-phenanthridinium chloride hydrochloride], and thedisubstituted compound [3,8-di(3-m-amidinophenyl-triazeno)-5-ethyl-6-phenylphenanthridinium chloridedihydrochloride]. A protocol for their individualpurification from the mixture and a detailed struc-tural analysis of each compound were described ina recent publication (Igoli et al. 2015). In commer-cial products isometamidium is the principal compo-nent (guidelines establish it must be at least 55% ofthe total material), with the other componentsaccounting for less than 40% (Sutcliffe et al. 2014).As the in vitro and in vivo trypanocidal activity onT. congolense is lower for the red and blue isomer itis paramount that the product composition followsstrict quality standards (Sahin et al. 2014). The di-substituted compound has poor trypanocidal activ-ity but it has a good prophylactic effect, possiblybecause it can act as a pro-drug that is cleaved to iso-metamidium in vivo (Sahin et al. 2014).

Isometamidium is used primarily to treat andprevent T. congolense and T. vivax infections in live-stock in Africa. Its activity against T. brucei spp. isless marked, but this drug can also be utilizedagainst some T. b. evansi strains, although notwhen these have reached the CNS, as the compounddoes not cross the blood–brain barrier. The drug isadministered to cattle at single doses of 0·25–1·0mg kg−1 for cure, and at doses of 0·5–1 mg kg−1 forprophylaxis (Leach and Roberts, 1981). The dosagefor T. b. evansi infections is generally 1–2 mg kg−1,but in horses it is recommended not to exceed 0·5mg kg−1 due to toxicity issues (Uilenberg, 1998;Desquesnes et al. 2013a). Multiple intramuscularadministrations of isometamidium can cause severefibrous lesions, hence damaging the carcass andmeat quality from livestock. Intravenous administra-tion has been successfully used to abrogate musculardamage, but it has been suggested that this couldresult in compromised prophylactic activity, due tothe lack of a drug depot at the injection site (Dowleret al. 1989; Munstermann et al. 1992). The durationof prophylactic activity following intramuscular ad-ministration in cattle is typically 2–3 months andmay be up to 6 months, but can vary greatly, depend-ing on the formulation and dosage used and on theparasite strain, as well as on other factors, includingsusceptibility of the particular breed and its generalhealth status (Toro et al. 1983; Kinabo and Bogan,1988).Isometamidium plasma concentrations reach their

peak within 1 h after administration and then fallrelatively quickly during the first week post-treat-ment and thereafter more gradually (Kinabo, 1993;Eisler et al. 1994). Three months after cattle hadbeen injected, the circulating drug concentrationwas measured at 0·75 ng mL−1 (Eisler et al. 1994).This study showed that the serum concentrationfits a bi-exponential model, with half-life of approxi-mately 25 days for the second phase in cattle (Eisleret al. 1994), while another study (Eisler, 1996) indi-cated an elimination half-life of 9–19 days. In sheepand goats isometamidium appears to be eliminatedmore rapidly than in cattle (Wesongah et al. 2004).The drug accumulates in the liver, kidneys andspleen as well as at the injection site, and from hereit is slowly released to the plasma exerting itsprophylactic activity (Kinabo and Bogan, 1988).Persistence of isometamidium residues is muchlonger than for diminazene. For this reason, a with-drawal period of 30 days was established for con-sumption of produce from cattle treated with thedrug (FAO, 1990), although in practice the with-drawal (discard) period is always product-specific.Excretion occurs mainly via bile and levels in cattlemilk are generally very low (Kinabo, 1993).Isometamidium may be used as part of a sanative

pair with diminazene, the two drugs being used se-quentially to minimize the risk of resistance




development (Leach and Roberts, 1981; Peregrine,1994). Despite this recommendation, there are mul-tiple reports of field isolates, from many Africancountries, indicating isometamidium resistance, par-ticularly in T. congolense but also in T. brucei speciesand T. vivax, sometimes detailing cross-resistancewith diminazene (Ainanshe et al. 1992; Clausenet al. 1992; Codjia et al. 1993; Afewerk et al. 2000;Sinyangwe et al. 2004; Mamoudou et al. 2008).However, other reports found no cross-resistance(e.g. Gray et al. 1993; Joshua et al. 1995) and we con-clude that cross-resistance does not necessarilyoccur, but may be a consequence of the level of re-sistance that has been established, whereas in othercases resistance to both drugs may have beeninduced separately. In addition, the chance ofcross-resistance developing may be different for thevarious animal trypanosome species, given theirknown differences in biochemical physiology anddrug transport.By taking advantage of isometamidium’s intrinsic

fluorescence, accumulation in the kinetoplast wasobserved (Wilkes et al. 1995; Boibessot et al.2002). Although closely related to the intercalatingphenanthridine homidium, isometamidium is notknown to be carcinogenic, and was reported tobind kDNA with an unconventional ‘sideways’geometry (Dougherty and Waring, 1982). Its highaffinity for the kDNA might underlie its trypanoci-dal activity. Linearization of kDNA minicircles inT. b. equiperdum following interaction of the drugwith the kinetoplast was observed (Shapiro andEnglund, 1990). Moreover, naturally occurring dys-kinetoplastic T. b. evansi (Brun and Lun, 1994) andin vitro-generated T. b. brucei lacking a functionalkinetoplast (Gould and Schnaufer, 2014) are highlyresistant to the drug. Efficacy against someT. b. evansi strains might relate to these parasitesretaining kDNA (albeit dispersed in dyskinetoplas-tidy) while others are akinetoplastic (i.e. retain nokDNA at all) and may be less susceptible to thedrug. However, the drug would still accumulatepreferentially in the mitochondrion, as the mito-chondrial membrane potential is unaffected by theloss of the kinetoplast in cells carrying a compensa-tory mutation in the γ-subunit of the F1F0-ATPsynthase (Dean et al. 2013), providing a drivingforce for cations. A mutation in this ATP synthasesubunit is sufficient to cause a substantial level of iso-metamidium and homidium resistance, althoughfurther drug pressure was shown to increase thiseven further. Interestingly, this very high level of re-sistance is indeed associated with a loss of mitochon-drial membrane potential, preventing furtherisometamidium accumulation in this organelle (Ezeet al. 2016).Despite possessing the recognition motif for the

P2/TbAT1 transporter and despite being a high-affinity inhibitor of this carrier (De Koning,

2001a), the internalization of isometamidiumdepends at most partially on this route (Delespauxand de Koning, 2007). Passive diffusion across themembrane may be feasible but is not likely, giventhe two positive charges on the molecule andpartial characterization of isometamidium transport,linking drug resistance, at least in part, to reduceduptake (Sutherland et al. 1992; Wilkes et al. 1995,1997). High-throughput RIT-seq (Baker et al.2015) failed to identify involvement of any of the re-ceptor-mediated endocytosis pathways as previouslyidentified for suramin (see subsection Suraminsodium below) using this approach (Alsford et al.2012), although alternative endocytic routes couldnot be ruled out.Resistance to isometamidium is encountered in

the field. InT. congolense a mechanism behind resist-ance was proposed to relate to diminished mitochon-drial membrane potential (Wilkes et al. 1997). This,in turn, would diminish the accumulation of drug inthe mitochondrion, having a net effect of reduceduptake at the plasma membrane, presumably dueto rapid equilibration of intracellular and extracellu-lar concentrations when the mitochondrial sink islost. Active extrusion by plasma membrane trans-porters has also been proposed (Sutherland andHolmes, 1993). A recent application of the RIT-seq approach, conducted on T. brucei, identifiedmutations to many subunits of the vacuolarATPase (found in the lysosomes and acidocalci-somes), in the trafficking protein AP-3 (an adaptinthat mediates delivery of proteins to lysosome-related organelles) and in EMC (an ER membranecomplex) that reduced drug activity, potentially con-tributing to dug resistance (Baker et al. 2015).Secondary loss of kDNA was found to be possibleonce vATPase and AP-3 subunits are lost from thecells, pointing to an intriguing, but as yet ill-defined, interaction between the vacuolar systemand mitochondrion. The fact that kDNA is lost incells selected for resistance to isometamidium wasclassically interpreted to point to its role as target.However, the discovery that kDNA loss can occuras a consequence of changes to the vacuolar systemcomplicates this interpretation.

Quinapyramine sulphate

Quinapyramine sulphate was developed from theearly trypanocide Surfen C (Curd and Davey,1950) and came into use around 1950. The com-pound was applied to treat cattle infected with trypa-nosomes until 1976, when it was withdrawn frommany areas due to emergence of widespread resist-ance (Connor, 1992). The drug was subsequentlyreintroduced in 1984 to treat T. b. evansi in camelsand horses (Peregrine, 1994), and is still used today(Ranjithkumar et al. 2014). In horses with acuteinfections of T. brucei spp. quinapyramine is




considered the most effective treatment (5 mg kg−1

via subcutaneous injection), although the druginduces severe but transient side effects in theseanimals (Auty et al. 2008). The prosalt form of qui-napyramine (a mixture of the soluble sulphate andthe insoluble chloride salts) was the first prophylac-tic drug available for animal infections. A 7·4 mgkg−1 dose of this prosalt suspension has both a cura-tive and a prophylactic (up to 4 months) effect onT. b. evansi infections in horses and camels(Williamson, 1970).Quinapyramine is a quinoline pyrimidine

(Table 1) and, as isometamidium and diminazene,a dication at physiological pH (homidium ismonocationic). As seen for the other charged trypa-nocides, quinapyramine is unable to cross the blood-brain barrier, which explains its failure to cureT. b. evansi infections in equids when the CNS isaffected (Ranjithkumar et al. 2014). However, it isimportant to note that some cationic trypanocidesdo penetrate the blood–brain barrier, the clearestexample being compound DB829 (Wenzler et al.2013). Pentamidine has actually been used to treat‘early-late stage’ HAT (Doua et al. 1996) but itsmovement across the blood–brain barrier is counter-acted by active efflux mechanisms, including P-glycoprotein and multi-drug resistance transporters(Sanderson et al. 2009).Plasma levels of quinapyramine decline rapidly

after dosing and, in the case of the prosalt, its persist-ence is probably due to slow release from the sub-cutaneous depot formed at the injection site(Spinks, 1950). Quinapyramine accumulates in theliver and kidneys, where its concentration remainshigh for weeks and can cause organ-specific toxicity.Excretion occurs mainly via urine (Spinks, 1950).Quinapyramine’s mode of action remains

unknown. Hypotheses include the interferencewith nucleic acid synthesis and inhibition of cyto-plasmic ribosomes (and, therefore, protein synthe-sis) (Newton, 1962, 1966). However, its dicationic/aromatic nature would strongly suggest a mitochon-drial accumulation, as with the phenanthridines andbis-benzamidines.Trypanosoma congolense and T. b. evansi lines re-

sistant to the drug can easily be obtained by in vivoselection in mice (Ndoutamia et al. 1993; Liao andShen, 2010). As quinapyramine resistant T. congo-lense trypanosomes show cross-resistance to isometa-midium, homidium and diminazene, the use of thiscompound to treat infections in cattle is not recom-mended (Peregrine et al. 1997). Given the lack ofcross-resistance between diminazene and homidium,the fact that quinapyramine is cross-resistant to bothis intriguing. Although the mechanism underpin-ning quinapyramine resistance remains unknown,it is likely that all these trypanocides have a mito-chondrial target and that any single change that dra-matically reduces the mitochondrial membrane

potential, or the loss of organic cation carriers inthe inner mitochondrial membrane, could result inresistance to all of them.

Suramin sodium

Suramin sodium is a symmetrical polyanionic sulfo-nated naphthylamine (Table 1). It is the oldest try-panocide still in use, having been introduced in1921 for the treatment of surra in camels and re-placing the then-standard treatment of intravenoustartar emetic (potassium antimonyl tartrate)(Uilenberg, 1998). A single dose of 6–10 g ofsuramin sodium per camel was described as 100%effective (Bennett, 1930). Suramin is also the stand-ard treatment for equine trypanosomiasis (T. bruceispp.), being more effective than diminazene andless toxic than quinapyramine (Williamson, 1970).The current treatment for camels and horses is 10mg kg−1, administered intravenously. Intramuscularadministration is avoided as it causes intense local ir-ritation. Suramin has further been used for cure andprophylaxis of onchocerciasis and other microfilarialinfections including Brugia pahangi (Delespaux andde Koning, 2007), as well as for the treatment ofearly stage HAT since 1922 (Apted, 1970). Althoughsuramin is effective against T. b. gambiense (Kno-bloch et al. 1984; Pepin and Khonde, 1996), it ismostly used against HAT due to T. b. rhodesiense,for which it is still available today (Voogd et al.1993), whereas it was replaced with pentamidinefor the form due to T. b. gambiense. Although thedrug has good efficacy against T. simiae in pigs(Stephen, 1966; Williamson, 1970), it is relativelyineffective against T. congolense and T. vivax(Leach and Roberts, 1981), presumably due to theaforementioned differences in biochemical physi-ology that distinguish T. brucei group organismsfrom these other species.Old work showed that suramin can be used as a

prophylactic agent when administered subcutane-ously as an insoluble complex with one of the cation-ic trypanocides (e.g. with quinapyramine, in a 1:3molecular proportion, reflecting the six negativecharges of suramin vs the two cationic charges ofquinapyramine), resulting in 3–6 months protectionat 40 mg kg−1 of quinapyramine in pigs(Williamson, 1970) and >160 days protection incattle (Williamson and Desowitz, 1956). This ap-proach could be effective for the eradication ofT. b. gambiense in pigs, which are reportedly actingas reservoir hosts of this species (Mehlitz et al.1982). Complexes of suramin with homidium, qui-napyramine and prothidium also gave protection inexperimental infections in cattle (Desowitz, 1957).The pharmacokinetic parameters of suramin in

animals (Kinabo, 1993) have not been subject tothe same extensive characterization as occurred inhumans, where the compound has also been trialled




for the treatment of AIDS and cancer (Barrett et al.2007). Most of the drug (>99%) binds to plasma pro-teins yielding a slow clearance. The terminal half-lifein humans ranges between 40 and 50 days or more,depending on the infusion protocol applied (Jodrellet al. 1994). This slow clearance underpins limited(i.e. several weeks) prophylactic action in animalstoo when the drug is used on its own. Suramindoes suppress infection, but is dependent on thehost’s immune response to be fully effective (Leachand Roberts, 1981). Because of its large molecularsize and highly anionic nature, suramin does notcross the blood–brain barrier.Suramin strongly binds to human serum proteins

and various trypanosome enzymes by electrostaticinteraction (Voogd et al. 1993). The drug was pro-posed to enter trypanosomes via receptor-mediateduptake bound to LDL and to accumulate in the lyso-some (Vansterkenburg et al. 1993). This hypothesis,however, looked doubtful after it was demonstratedthat inT. brucei (procyclic form at least) suramin andLDL uptake are not coupled (Pal et al. 2002). Adefinitive mode of action for the compound has notbeen determined. Fairlamb and Bowman proposedthat suramin curbs glycolytic ATP production inT. brucei by inhibiting glycerol-3-phospate oxidaseand NAD+-dependent glycerol-3-phosphate de-hydrogenase (Fairlamb and Bowman, 1980).However, being highly charged, suramin bindsmany enzymes when assayed and a multitude of pu-tative targets have been proposed (Gutteridge,1985), including 6-phosphogluconate dehydrogen-ase, of the pentose phosphate pathway, of which itis a competitive inhibitor (Hanau et al. 1996).More recently, a RIT-seq screen in bloodstream T.brucei identified 28 genes that contribute tosuramin action, including: a surface glycoproteinfamily (ISG75), which appears to be the ligand towhich the drug binds; cathepsin L, believed torelease the drug from ligand within the lysosomalsystem; a number of deubiquitinating enzymes andvarious proteins involved in the endocytic pathway(Alsford et al. 2012). It appears that inhibitinguptake of suramin, or its normal passage throughthe endocytic pathway following binding to aspecific receptor, is sufficient to render parasites re-sistant to the drug, although it remains unknownhow suramin kills once accumulated intracellularly.Extensive use of the compound in the first half of

the 20th century resulted in emergence of wide-spread resistance in T. b. evansi in Africa (Boidet al. 1989; El Rayah et al. 1999) and South-EastAsia (Gill, 1971; Zhou et al. 2004), in some casesleading to withdrawal of suramin as a treatment (ElRayah et al. 1999). However, even in the absenceof drug pressure, the resistance phenotype has per-sisted in the field, as found for some SudaneseT. b. evansi strains (El Rayah et al. 1999). Stabilityof the suramin resistance phenotype was also

observed in T. brucei lines generated in vitro (Scottet al. 1996) and in T. b. evansi parasites selected inmice (Mutugi et al. 1994). However, the drug waseffective against T. b. evansi isolates in Brazil,where it had not been used (Faccio et al. 2013).

Melarsomine dihydrochloride

An early reported case of an attempt to cure ananimal afflicted with trypanosomiasis was that ofDr David Livingstone, the Scottish missionarywhose travels in Southern Africa in the mid-19thcentury were exceptionally well recorded. In aletter to the British Medical Journal in 1858 hedescribed the use of arsenic oxide (Fowler’s solu-tion) to treat a case of ‘fly disease’ in a horse(Livingstone, 1858). Although the treated horsewas not cured, there was a temporary relief in symp-toms. Over 50 years later, once the trypanosome hadbeen implicated, H. W. Thomas and A. Breinl, andthen P. Ehrlich, revisited arsenic chemistry to seektrypanocides in the early days of chemotherapy(Williamson, 1970). By the 1950s melarsoprol hadbeen introduced for the treatment of late-stageHAT; the drug was created by coupling of melarsenoxide to 2,3-dimercaptopropanol (Steverding,2010). The formulation displayed diminished tox-icity while retaining potent trypanocidal activity.Melarsomine dihydrochloride (Table 1) is a mela-

mino-phenylarsine, synthesized by linking melarsenoxide (Barrett et al. 2007) to two equivalent ofcysteamine (Berger and Fairlamb, 1994). The com-pound has improved aqueous solubility over melar-soprol. It was introduced to the market in 1992 andis the latest addition to the veterinary trypanocidallist. The drug (Immiticide®) is also used in the treat-ment of heartworms in dogs, where it kills adultworms (McCall et al. 1994), albeit with a lowmargin of safety. It is registered for use againstT. b. evansi in camels at a dose of 0·25 mg kg−1,but it has also been evaluated and proven efficaciousagainst T. b. evansi infections in cattle (Desquesneset al. 2011), goats (Gutierrez et al. 2008) andhorses (Tamarit et al. 2010), although at higherdosages than that applied to camels (i.e. 0·5 mgkg−1 or above). Melarsomine also proved curativein cattle infected with T. b. evansi strains resistantto suramin (Payne et al. 1994). Moreover, treatmentregimens with both 0·25 and 0·5 mg kg−1 of the drugwere proven effective in curing acute and chronicT. b. equiperdum infections in horses, resulting in areduction of neurological symptoms (Hagos et al.2010) and offering a possible treatment for theseinfections. Side effects to the drug are usually mild(salivation, lacrimation, muscle tremors, increasedgut motility and frequent urination), but a severeadverse reaction has also been documented (Berlinet al. 2010). Reports of neurological sequelae indogs (Hettlich et al. 2003), albeit perhaps not




analogous to the reactive encephalopathy associatedwith melarsoprol treatment of humans (Blum et al.2001), are notable. Should the reduced neurotoxicityof melarsomine be replicated in man it might be con-sidered as a replacement for melarsoprol, althoughit is doubtful that comparative clinical trials of thetwo arsenicals would receive ethical clearance, espe-cially since melarsoprol is being phased out in favourof nifurtimox–eflornithine combination therapy(Simarro et al. 2012). The paucity of compoundsthat kill adult filarial worms is of note too, and,should the safety profile of melarsomine be accept-able, it could be considered for use against thehuman filariases.The mode of action of melarsomine is unknown.

As for other trypanocidal arsenicals, the disruptionof the thiol-redox balance is a possible mechanism(Fairlamb, 2003). The drug (or, rather, its metabol-ite melarsen oxide) enters T. brucei via the same P2/TbAT1 adenosine nucleoside transporter (Carterand Fairlamb, 1993; De Koning and Jarvis, 1999)and TbAQP2 (Munday et al. 2014) that carry othermelaminophenyl arsenicals and the diamidine trypa-nocides. Selective uptake probably accounts for mostof the selective toxicity of the arsenicals (Baker et al.2013). Reduction of P2/TbAT1 activity is a knownreason behind onset of cross-resistance between thecompounds that enter via this route: trypanosomesof the T. brucei group resistant to melarsomine areoften also less sensitive to diamidines and other ar-senical drugs as melarsoprol, but not to suramin(Zhang et al. 1991; Pospichal et al. 1994). In vitroand in vivo selected melarsomine-resistantT. b. evansi (Suswam et al. 2001) revealed that thedecrease in P2/TbAT1 transporter activity waslinked both to reduced transporter expression andchanges in binding properties (Suswam et al.2003). In a T. b. brucei strain selected for melarso-mine resistance in mice the TbAT1 gene was stillpresent but its transcript was lost (Stewart et al.2010). The lack of authentic orthologues of TbAT1and TbAQP2 in T. congolense and T. vivax (see sub-section Diminazene aceturate above) may explainwhy the drug is less potent against these parasites.

DRUG RESISTANCE IN THE FIELD: DEFINITION

AND EXTENT OF THE PROBLEM

Drug resistance is suspected when treatment failureoccurs using standard drug dosages. However, in thefield, this interpretation can be erroneous, as treat-ment failure can result from many factors otherthan the parasite’s increased tolerance to drugs.For example, the presence of parasites in treatedanimals could correspond to a new infection ratherthan to recrudescence, particularly in areas of highchallenge (Rowlands et al. 2001). Using microsatel-lite DNA markers to strain type T. congolense fromcattle in Ethiopia following treatment with

diminazene, essentially equal occurrences of new in-fection (40%) and actual relapse (37·5%) wereproposed (Moti et al. 2015). Other causes of treat-ment failure not linked to true drug resistancecould be related to the poor health state of theanimal (e.g. malnutrition, immunosuppression, con-current infections), or to incorrect drug use (e.g. ir-regular treatment or prolonged intervals betweentreatments), or to under-dosage. The latter canresult from poor drug quality (either due to inappro-priate storage or to the use of counterfeit products)(Sutcliffe et al. 2014), or from incorrect drug usage(wrong dilution, use of unsterilized water or errone-ous dosage due to inaccurate estimation of the animalweight) (Van den Bossche et al. 2000; Grace et al.2009). For phenanthridines, in particular isometa-midium, the adverse reaction which often appearsat the injection site might possibly alter drug absorp-tion and diminish the levels of drug in circulation(Kinabo, 1993), thus determining under-dosage. Itis widely believed that under-dosing could representa major determinant in drug resistance developmentin the field through parasite exposure to sub-curativedrug concentrations (Leach and Roberts, 1981). Asimilar phenomenon could derive from failure tocomply with strict dose timing, which could leadto periods where sub-prophylactic drug levels arepresent (Leach and Roberts, 1981). Moreover, as amutagen, homidium might also directly contributeto resistance appearance through induction of muta-tions in parasites that are then selected under drugpressure. Constant parasitological monitoring is ne-cessary to distinguish treatment failure from appear-ance of true resistance.In the previous section, we have outlined that

there are issues associated with selected resistanceto each of the drugs used against the animal trypano-somiases. Cases of resistance to veterinary trypano-cides started to be reported in the field soon aftertheir introduction, and their numbers have been in-creasing ever since (Delespaux et al. 2008b). Areview of available literature in 2008 reported lossof efficacy of the available AAT trypanocides in atleast 17 African countries (Delespaux et al. 2008b).Available data, in 2001, indicated that resistance toisometamidium was more widespread than resist-ance to diminazene (Geerts et al. 2001), however,this may no longer be so, as prevalence of resistancemay change substantially over a few years(Delespaux et al. 2008a). Treatment failure againstT. congolense and T. vivax infections with either ofthese drugs has been observed in both West(Kupper and Wolters, 1983; Pinder and Authie,1984; Knoppe et al. 2006; Mungube et al. 2012;Vitouley et al. 2012) and East A

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