Animal Disease Ecology & Transmission

Clarissa Yvonne J. DomingoProfessor, CVSM-CLSU

Terms to Reckon:

infectionpresence of a pathogen

in the body but does not

necessarily mean the

presence of clinical signs

disease

occurrence of cases and outbreaks (with clinical signs) caused by the pathogen in a population

ecology branch of biology that deals with the relations of organisms to one another and to their physical surroundings

transmission passing of a communicable disease from an infected host individual or group to a particular susceptible individual or group

Diseases affecting livestock can have a

devastating impact on:

1. animal productivity and production,

2. trade in live animals, meat and other animal products,

3. human health and, consequently,

4. the overall process of economic development.

Concepts for interplay of ecology to diseases

transmission

Macro Level Consequence1. growing populations Livestock, fastest growing

subsector of agriculture(true in developed countries)2. income gains and

urbanization

3. result of globalization Unprecedented increase of emerging and re-emerging animal diseases and zoonoses4. climate change the world

Biological factors1. Environmental, climate, weather changes (temperature & humidity).

Climate change also has the following effects on disease occurrences:

Increased parasitic zoonoses

(e.g. helminthoses, tick-borne diseases)

Increased spread of vectors and vector-borne diseases

(e.g. Rift Valley fever, West Nile virus)

- Increased humidity increases vector survival; increased rain may increase larval habitat and vector population size by creating new habitat

Increased incidence of tropical diseases in areas previously considered as having temperate climate conditions

(e.g. Old and New World screwworm)

•Alteration of disease eco-epidemiology, new disease patterns, and change in vector capacity.

- Decreased rain can increase container-breeding mosquitoes by forcing increased water storage

- Epic rainfall events can synchronize vector host-seeking and virus transmission

•Change in geographic range, host range and pathogen virulence.

- at higher temperature changes there is decreased extrinsic incubation period of pathogen inside the vector during transmission season.

2. Ecological and ecosystem changes. Changes in the current conditions of the ecosystem affect many animals and cause higher risk for emergence of diseases and the occurrence of pathogens and parasites.

3. Animal density and biomass.

The optimal density of animals in a specific location is vital to animal health.

Overcrowding or high densities pose high risks and allow faster transfer of parasites and pests.

4. Changes in host-pathogen interactions.

The emergence and re-emergence of diseases can be attributed to host-pathogen interactions.

When these interactions are interrupted or altered, the resistance of animals can be weakened and places them in vulnerable states.

5. Microbial adaptation to new environmental conditions.

• Intrinsic properties of the pathogens that are associated with this are:

a. genetic shift as a result of mutation & reassortment, as it occurs in avian influenza virus;

b. conjugation, transformation, and transduction in bacteria.

c. adaptation to new vectors and hosts

d. development of increased virulence or drug resistance

6. Changes in land use and use of natural resources.

• Human economic activities cause major changes in land use.

Encroachment into natural animal habitat due to

a. forest clearings and

b. land conversions for various uses such as industry, pasture, residential space and agriculture

changes in animal and human density and distribution

alters local biodiversity through the introduction of exotic and invasive species that could cause extinction of one or more species.

Changes in Environment

• Malaysia, 1999, intensive pig farming intrudes into the natural habitat of fruit bats carrying the Nipah virus

• Entire pig herd of the country slaughtered ($120M); 105 human deaths

7. Increased contacts between pathogens and host population (urbanization, rural periurban-urban continuum)

• As a result of urbanization and industrial conversion animals are kept in densely populated areas

• This increases the contacts between pathogens and hosts that lead to increased exposure and risk.

8. Social and cultural practices

Social and cultural practices of man add to the risk of zoonotic transmission. These are:• food habits• religious beliefs• Halal meat• free range eggs and meat• exotic meat• raw milk

Abiotic factors 1. Globalization, international trade, and level of economic

development. Due to globalization, changes in the movements of human and animal populations enhance transmission. Likewise, movements are attributed to:

a. extent of ownership and movement of pets

b. extent of air travel

c. extent of ecotourism, hunting, camping, etc.

d. Introduction of infected host in to new ecosystem. eg : Marburg Disease

e. natural movement of wild animals including migratory birds, and by anthropogenic movement of animals.

Changes in Movements of human and animal populations

• Distance and speed of travel increased 1000 fold since 1800

• 1.4 billion air travelers/year

2. Lack or inadequate policies for control and confinement measures.

Policies must focused on managing and implementing movement restrictions of animals and products from one place to another, especially across borders.

Policies and strategies should be improved to safeguard animal health, public health, livelihoods and food systems.

3. Political and civil strife and instability.

There is disruption of health services in war-torn or impoverished countries.

Example: War-torn Kosovo led to epizootic rodent tularemia.

4. Breakdown of public services in charge of disease control.

In case of natural disasters, health infrastructures are damaged and control of diseases that emerged from the calamity cannot be mitigated efficiently and effectively.

5. Management of animal waste.

Popularity of organic farming andintensive tilapia inland farming,poultry manure and other animalmanure became a source of addedincome to livestock farmers.

Transporting manure to otherplaces because of demand is a riskfor transmitting the pathogens.

Wet manure is seeded with viableviral, bacterial, fungal and protozoalpathogens that can be harmful toboth man and susceptible animals.

Changes in Management of animal wastes

Manure-related fish kills , typical in many countries

6. Poor or change in public health management.

Poor or change in management results in lack of integration with animal health surveillance,fundingin the public health sector and sustained funding on scientific studies to answer public health questions and build expertise.

7. Animal welfare philosophies affecting animal production system.

Animal welfare societies have been loud in espousing practices thatshould not hinder the natural behavior of animals. Due to increase practiceof range farming however, soil-borne parasites that have been controlledbefore occurred such as Trichinella , Toxoplasma, cysticercosis, etc. have re-emerged.

8. Changes in production systems.Production systems are currently under

pressure to adjust to rapidly evolvingsocio-economic conditions and largeintensive livestock production units.Increasing demand for animal protein hasled to changes in:

a. farming practices (e.g. large “open” poultry production units in Asia)

b. animal marketsc. bush meat consumptiond. global tradee. pig and poultry production have

emerged in many developing regions in response to the rapidly growing demand for livestock products.

As the intensity of production increases, there is greater complexity and severity of diseases.

The macro-environment can influence the pattern of disease events.Extreme conditions of the physical environment such as

a. climate,b. topography,c. vegetation andd. soil

can directly cause illness or disease to animals.Factors affecting pathogen vectors, endemicity, and pathogen vectordiffusion include

a. temperature,b. relative humidity,c. vegetation cover andd. plant species ande. chemical composition of soils.

Macro-environment and diseases

Predicting the presence ofdisease using a set ofparameters could accuratelymodel the observed diseasesituation in a certain area.Predictive mapping could beuseful for the managementcontrol planning in a largearea

Micro-environment and diseases - interaction between biological supports (host animal, vector, and pathogen)which determine the establishment, multiplication, spread or reduction, andeven elimination of infections.

Host’s capacity to respond to infectious attack:

Susceptibility. The ability to acquire a pathogen and to show a pathological status.

Tolerance. The relative capacity to control the development of a pathogen and to limit its pathological effects.

Resistance. The ability to reduce the growth rate, fecundity, and persistence of a population of pathogens.

Resilience. The ability to grow and be productive despite the presence of a normal pathogen charge.

Refractoriness. The impossibility to acquire an infection because the biological support inhibits the multiplication of the pathogen.

Micro-environmental interactions and competitions:

Coccidia and helminths.

There is equilibrium at the level of the gastrointestinal tract. Treatments against coccidia positively affect the multiplication of gastrointestinal helminths and vice versa.

Tsetse flies, intestinal symbionts and trypanosome infections:

There are symbiotic micro-organisms (Wigglesworthia glossinidia) inside the intestine of tsetse flies give a degree of immunity to acquire and transmit trypanosome infections

Transmission

•The habitat in which an infectious agent normally lives, grows, and multiplies.

•The reservoir may or may not be the source from which an agent is transferred to a host.

•For example, the reservoir of clostridium botulinumis soil, but the source of most botulism infections is improperly canned food containing C. Botulinum spores.

Reservoir

Reservoir

1. Human reservoirs- cases and carriers

2. Animal reservoirs- cases and carriers

3. Environment reservoirs-plants, soil and water

•Carriers commonly transmit disease because they do not recognize they are infected and consequently take no special precautions to prevent transmission.

•Symptomatic persons are usually less likely to transmit infection

Human Carriers

Animal reservoirsExamples• brucellosis (cows, dogs, pigs)• trichinosis (swine)• rabies (dogs and cats).

•Many fungal agents, such as those causing histoplasmosis, live and multiply in the soil.

•The primary reservoir of Cryptosporidiosis is water

Environmental reservoirs

Portals of Exit

•Gastrointestinal tract

•Respiratory tract

•Genitourinary tract

•Skin and mucous membranes

•Blood

•Transplacental

Mode of Transmission

•The route of infection refers to the path which the disease agent used to gain access to the body of the susceptible host

•Mode of transmission to the method of transference.

Example: Route vs Mode •The route from Point A to Point B is through the main highway (route of infection).

•However, the journey from point A to point B using the main highway can be completed using any of the transportation options: by public bus, railway transit or private vehicle (mode of transmission).

Mode and route

Interventions to improve livestock and agriculture production systems require policies that address interventions in • socio-economic dimensions,• institutional support, • training and information, • land use, land tenure, and

natural resource management, and

• increasing agricultural production.

END OF LECTURE