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Dr. Nik Ahmad Irwan Izzauddin Nik Him

Room 4o3, nikirwan@usm.my

BVT 211 Biology of Vectors & Parasites

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Introduction and important aspects of vectors in disease transmission – important concepts, terms and definitions

Arthropod vectors – Mosquitoes as disease vectors-its role as important human disease vector for dengue, microfilaria and malaria

Course outline

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Introduction and important aspects of vectors in disease transmission – important concepts, terms and

definitions

Objective

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IntroductionThe most successful animal

groupArthropods have huge impact

on health of humans & domestic animals Irritation & diseases

Relatively few species involved but serious social & economic consequences Transmit diseases (vectors) Inject venoms & transmit

allergens Cause wounds Create nuisance & phobias

Other arthropod groups also very important

Leishmaniasis

Myiasis

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IntroductionMajor insect orders

Diptera (flies)Hemiptera (true bugs)Phthiraptera (lice)Siphonaptera (fleas)

Diseases & causative pathogens

Other arthropod groupsAcari (ticks & mites)Araneae (spiders)Scorpiones

(scorpions)

Protozoan causing sleeping sickness

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Nuisance & phobiasNuisance mostly related to

high densities & not real hazards Justified in case of biting,

venomous & filth-frequenting species

Major causes of nuisance & irritation Blood-feeding species Lachrymal-feeders Immunological reactions Phobic responses (delusory

parasitosis)

Large industries are focused on pest control

Mosquito feeding frenzy

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Vector - Definition: Organism that are capable of transmitting other organisms that cause disease in vertebrate host

Carrier, bearer

In parasitology An organism or vehicle that transmits the causative agent or disease-causing organism from reservoir to the host

Introduction

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Vectors generally don’t become “ill” from carrying their various viral, protozoan and nematode infections.

They might accrue some damage to their tissues, but in some cases this “damage” actually makes them more likely to transmit and infect.

A mosquito with problems in its feeding apparatus will need to take additional bites to complete a blood meal.

A flea with a gut clogged with plague bacteria will regurgitate more.

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Some example of vectors

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Primary vector- If they are proved to be transmitting a pathogen to man

or other animal

Secondary vector- If they play a supplementary role in transmission, but would be unable to maintain a disease in the absence of primary vector

Vector

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Case sample: In the transmission of dengue fever in Malaysia

Aedes egypti is the main vector…

and Aedes albopictus is the secondary vector

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In instances where a disease of unknown cause is occurring, certain general characteristics help to identify its vector

Establishment of the relationship of arthropod populations to transmission of a particular disease agent is called vector incrimination

The process of knowing which species of arthropod is serving as a vector of a particular disease

Vector Incrimination

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• There are 4 major criteria for incriminating arthropods as vectors of human disease

Barnett, H. C. 1956. The transmission of western equine encephalitis virus by the mosquito Culex tarsalis Coq. Am. J. Trop. Med. Hyg. 5: 86-98.

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Identifying contact between arthropod and host

- suck blood for meal YES

Having a biological association in time

- the rate of cases higher in correlation to the higher number of Aedes YES

Repeated demonstration of disease between arthropod and host

- The infective stage have been found consistently in Aedes and they transferred it during feeding YES

Replicable under experimental conditions

- Can be rearing in the lab and the and can be replicated YES

Study case: Aedes as a vector for dengue fever

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The time that elapses between the invasion of a susceptible host by an infectious agent and the onset of symptoms of the disease caused by that agent is called the incubation period

The length of the incubation period varies greatly

Extrinsic Incubation period – a period in a vector during which the disease-producing organism/parasite increases or transforms to a point where it can be transmitted.

Intrinsic incubation period – a period in the vertebrate host before disease is expressed clinically

Incubation Period

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Case sample: Malaria

Intrinsic incubation period

Extrinsic incubation period

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Vectorial capacity describes the potential of agroup of arthropods to transmit a given

pathogenVector effectivenessThere are 6 main determinants:

– Abundance

– Host preference and host-feeding patterns

– Reproductive capacity

– Longevity

– Dispersal

– Vector competence

Vectorial Capacity

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The more vectors there are, the higher the probability of disease transmission

because it has a direct bearing on the probability of vector-host contact

Vectorial Capacity: Abundance

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Knowing what the vectors feed upon allows for identification of disease transmission

Patterns of host feeding that are determined by identification of blood meals in vectors are usually the end result of many factors, including host preference and host availability.

Vectorial Capacity: Host preference and host-feeding patterns

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A measure of the rate at which a population of vectors increases

Fecundity is a related term which relates to the number of generations, broods, or litters produced per unit of time.

The net reproductive rate of a population of vectors is a combination of fecundity and survival.

Survival is influenced by various mortality factors, including predation and diseases, as well as accidents and natural aging

Vectorial Capacity: Reproductive capacity

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Longevity is knowing how long the stages of the vector’s life cycle last

A vector must feed more than once to transmit pathogens

It is essential that a vector live a sufficient period of time

Maximal longevity will permits vectors to serve as essential parts of the reservoir of infection.

Vectorial Capacity: Longevity

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Dispersal is knowing how far a vector can fly or move about freely– The greater the movement, the greater chance for spread of disease

Vectors that can move freely and for long distances will have greater chances for contact with humans, and will be more likely to move between infected and noninfected hosts.

Superior mobility aids in the dissemination of pathogens over a wide area, so that their associated diseases are not limited or focal in nature.

Flying vectors (e.g. mosquitoes, flies) generally make a good vector; however crawling vectors (e.g. fleas, lice, mites and ticks) are distributed by the relative mobility of their hosts.

Vectorial Capacity: Dispersal

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The susceptibility of a group of arthropods to a given strain of pathogen and the ability of those arthropods to transmit the pathogen

These traits are under genetic control, and although infection and transmission in vectors will vary with temperature, vector competence is considered to be an innate characteristic for a particular vector for a given microorganism

Vectorial Capacity: Vector competence

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Mechanical transmissionBiological transmission

- Cyclodevelopment transmission

- Propagative transmission- Cyclopropagative transmission

Methods of Transmission

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This is when the pathogen adheres to body hairs, spines, sticky pads, or other structures of insects – In the case of certain insects, transmission may be by regurgitation or defecation

- Biting flies may transmit pathogens by biting with contaminated mouthparts

Nearly all mechanically transmitted diseases can also be transmitted in other ways (e.g., contaminated food and water)

e.g. houseflies, cockroaches

Mechanical transmission

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The parasites undergo several molts in the body of the vector

No multiplication takes place within the body of the vector

The only pathogens that are transmitted this way are filarial nematodes

- Start out as a microfilariae and develops into an infectious larvae

Also known as Cyclical transmission

Biological transmission: Cyclodevelopment transmission

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In this type of transmission, the pathogen multiplies within the body of the vector, but does not undergo any changes in form

Most viral diseases fall into this category

Plague (a bacterial disease) is also an example

Any stage of these pathogens can infect a vertebrate host

Biological transmission: Propagative transmission

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Both multiplication and changes in the life form of the pathogen occur within the vector

Examples are malaria, leishmaniasis, both caused by protozoan parasites

Also known as Propagative and Cyclical transmission

Biological transmission: Cyclopropagative transmission

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Horizontal and vertical transmission describe the pathway a pathogen takes among vectors and hosts.

Horizontal transmission- Horizontal transmission involves the pathogen being transmitted by a vector to a host in a cyclical pathway

Ventricle transmission- Vertical transmission is more direct and does not involve a host, but occurs directly from infected mother/female to offspring

– This is also called transovarial transmission

Modes of Transmission

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Horizontal and vertical transmission mechanisms

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Dead end host – a vertebrate that harbors the pathogen and is severely affected by it, yet the level of pathogen in its body is too low for blood sucking vector to become infective after feeding on the host.

Amplifying host – a vertebrate that has high level of pathogen that a feeding vector will likely become infectious.

Silent host – one that harbors the pathogen, but shows no obvious signs of disease.

Types of host

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Resistant host – one that is not naturally affected by a pathogen.

Partially resistant host – one that harbors the pathogens for a long period before being overcome by it.

Susceptible host – a victim of the pathogen

Types of host

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Reservoir refer to any vector or host, capable of maintaining a pathogen for considerable periods of time.

Usually, they show no evidence of serious disease, or a reservoir of infection may be maintained by continuous transmission among a group of severely affected animals.

Most vectors are short-lived and would not serve as important components of reservoirs, except possibly through aestivation or hibernation.

Reservoirs

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At the end of the nineteenth century, it was discovered that certain species of insects, other arthropods and freshwater snails were responsible for the transmission of some important diseases.

The discovery of the insecticide dichlorodiphenyltrichloroethane (DDT) in the 1940s was a major breakthrough in the control of vector-borne diseases.

In 1950s and early 1960s -->

-To eradicate the diseases or to reduce transmission to such low level that control could be maintained through the general health care facilities without the need of control measures.

Problem resistance expensive

Vector Control

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Vector control methods suitable for community involvement should:

— be effective;— be affordable;— use equipment and materials that can be

obtained locally;— be simple to understand and apply;— be acceptable and compatible with local

customs, attitudes and beliefs;— be safe to the user and the environment.

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Before starting any vector control activity, it is important to ask two questions:

What result do you want to achieve: merely to protect yourself or your family from biting pests and the diseases they carry, or to reduce disease in the community?

Are the health authorities already carrying out control measures and do you want to provide the community or your family with additional protection from disease?

Control measures

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Important factors to take into consideration

- Biological factors- Availability of technical tools- Socio-economic considerations

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Species affectedDensity of susceptible speciesWildlife reservoirVector transmissionTransmissibilityCurrent extent of diseaseSurvival in the environmentCarrier stateEase of clinical recognition

Vector control: Biological factors

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Diagnostic testsVaccinesTreatmentEffectiveness of isolation/ quarantineDisinfection

Vector control: Availability of technical tools

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Cost and benefits of interventionEase of implementationStake holder engagementPolitical will

Vector control: Socio-economic considerations

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2 control methods1. Self-protection2. Community control

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Self-protection measures are used to protect yourself, your family or a small group of people living or working together from insect pests or vectors of disease.

These measures include personal protection, 1. the prevention of contact between the human body

and the disease vector, 2. and environmental measures to prevent pests and

vectors from entering, finding shelter in, or breeding in or around your house.

These measures are usually simple and inexpensive, and can often be adopted without help from specialized health workers.

Self-protection

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Example: Repellents Protective clothing Insecticide vaporizers Mosquito nets Insecticides - Insecticide spraying of walls - Space-spraying with insecticides - Treating fabrics with an insecticide Anti-mosquito screening Prevention of breeding - Source reduction - Biological control - Larvicides - Insect growth regulator

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The type of control may be the same as for the protection of an individual or a family, but is, of course, larger.

Before investing resources in community-wide control efforts, advice should be obtained from health workers on the type of measures most likely to be successful under local conditions.

Before investing resources in community-wide control efforts, advice should be obtained from health workers on the type of measures most likely to be successful under local conditions.

Community control

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Many factors need to be taken into account:1. the vector species and its behaviour, 2. the compatibility of control methods with

the local culture, 3. affordability in the long term, the need for

expert advice

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THANK YOU