Disease ecology and strategies for disease control

Question 1

Methods of transmission (2)(6)

Answer 1

1. Vertical
2. Horizontal - Direct: blood-borne, muco-cutaneous - Indirect: vector-borne, water-borne, food-borne
3. Ingestion
4. Airborne
5. Contact (muco-cutaneous)
6. Inoculation (vector-borne)
7. Iatrogenic
8. Sexual

Question 2

Routes of infection (3)

Answer 2

1. Oral
2. Respiratory
3. Muco-cutaneous

Question 3

Determinants of disease - host, pathogen, environment

Answer 3

Host: genotype, age, sex (hormonal, behavioural), species and breed, etc

Agent: virulence/pathogenicity, gradient of infection (sub-clinical/clinical), outcome of infection (carrier state - incubatory/convalescent, latent infection), colonization

Environmental: Location (influences occupation and husbandry, vector distribution), Climate (drought, flood), husbandry, stress?

Question 4

Hosts - definitive, intermediate, paratenic, aberrant, amplifier, reservoir

Answer 4

Definitive/primary/final: host where parasite undergoes sexual reproduction (parasitology); maintenance host of agent (other pathogens)

Intermediate host: host where parasite undergoes asexual reproduction and is still essential to transmission

Paratenic host: can support asexual development of parasite and can pass it on but is not essential for maintenance and transmission of the agent (basically a substitute for normal intermediate host)

Aberrant host: dead-end host; can support infection but can’t pass it on

Amplifier host: host where pathogen can multiply to high levels

Reservoir: host where pathogen normally lives and multiplies

Question 5

Vector - definition, types (2)

Answer 5

Invertebrate host involved in transmission, frequently definitive/intermediate hosts.

1. Mechanical: physically carry agent but do not support multiplication/development of agent inside
2. Biological - support multiplication/development of agent inside

Question 6

Infectiousness - latent period, incubation period, serial interval

Answer 6

Infectiousness refers to the duration of infection and relative amount of infectious agent excreted from the host.

• Latent period/prepatent period (parasites)/eclipse phase(virus): time from infection to when animal becomes infectious
• Incubation period: time from infection to when animal develops clinical signs
• Extrinsic incubation period: time between infection of vector and availability of agent for transmission to next host
• Generation time/serial interval: period between onset of symptoms and infection of next host

Question 7

Maintenance strategies (5)

Answer 7

1. Avoidance of external environment (vertical/venerial/vector transmission and ingestion)
2. Development of resistant forms (spores - protect against external environment e.g. Bacillus/Clostridium, cysts - internal environment e.g. Toxoplasma)
3. Rapidly in/out (no time for host response e.g. upper respiratory viruses, downside: requires continuous supply of hosts)
4. Persistance in host (induce immunosuppression e.g. TB in humans; or tolerance e.g. prenatal infection with LCM; antigenic variation; multiplication in sites inaccessible to the immune response)
5. Extension of host range (e.g. zoonoses)

Question 8

Basic epidemic theory - key concepts (3)

Answer 8

1. Kendall’s threshold theorem: minimum density of susceptible animals is required to allow a contact-transmitted epidemic to commence.
2. As epidemic proceeds the number of susceptible animals decreases (immunity or death) at which time the epidemic will die out.
3. Replacement of susceptible population through birth and migration allows subsequent epidemics to occur.

Question 9

Basic reproduction number (R0)

Answer 9

Number of new individuals infected (on average) by one individual during its infectious period. R0 = c x p x d where c = contact rate (# contacts per unit time) p = probability of transmission per contact d = duration of infectiousness cxp often called beta (# effective contacts per unit time)

If R0 > 1 epidemic starts, if R0 < 1 dies out

Question 10

Effective contact

Answer 10

Conditions under which transmission is likely to occur. Depends on probability of transmission per contact and contact rates.

Question 11

Herd immunity - definition (3), implications

Answer 11

1. Proportion immune among individuals in a population.
2. Particular threshold proportion of immune individuals that should lead to a decline in incidence of infection.
3. Pattern of immunity that should protect a population from invasion of a new infection.

A common implication of the term is that the risk of infection among susceptible individuals in a population is reduced by the presence and proximity of immune individuals

Question 12

Epidemic curve - shapes (4)

Answer 12

• Common source: all cases infected from same source
• Point source: small exposure window
• Continuous source: longer exposure window
• Propagating: contagious diseases spread from one animal to another

Question 13

Epidemic curve - factors influencing shape

Answer 13

1. Incubation period
2. Infectivity of agent
3. Proportion susceptible
4. Animal density

Question 14

Control/eradication - definitions

Answer 14

Control (ongoing): reduction of morbidity and mortality from disease

Eradication (time limited):

1. Extinction in nature (smallpox, rinderpest)
2. Reduction in prevalence to extent that transmission doesn’t occur
3. Reduction in prevalence to extent that disease is not a health problem (through transmission still occurs)
4. Regional extinction (most common in vet med)

Question 15

Major strategies for infectious disease control (13)

Answer 15

1. Do nothing (natural burn out, seasonal changes)
2. Quarantine of infected/suspected/at-risk (period depends on incubation period, time to diagnosis, time to become non-infectious)
3. Slaughter a. Test and removal (infected animals only) b. Pre-emptive (exposed animals) e.g. blanket culling where animals on properties surrounding IP are slaughtered
4. Strategic deployment of vaccine (emergency vaccination - higher potency) in areas which do not routinely vaccinate a. Ring vaccination - fully surrounding affected area b. Barrier vaccination - partially surrounding affected area c. Dampening down vaccination - within and around outbreak area NB vaccination may also be used in countries which do vaccinate - to boost immunity, or protect against novel subtypes
5. Chemo-prophylaxis - e.g. abx in feed to promote growth, warblecides
6. Host movement - e.g. seasonal movement of herds to avoid tsetse
7. Restriction of movement e.g. within zones, trade restrictions
8. Grazing strategies a. Mixed grazing - susceptible and resistant animals at same time (e.g. calves and adults) to reduce pasture contamination b. Alternate grazing - different species c. Sequential - susceptible and resistant animals at different time
9. Control of biologic/mechanical vectors
10. Improved biosecurity practices (cleanliness, disinfection, maintenance of perimeter fencing, testing/isolation of animals before introduction into herd, waste management, limiting visitors, training staff, ensuring animals can be traced)
11. Niche filling e.g. competitive exclusion
12. Genetic improvement
13. Minimal disease methods - combines disinfection of premises + treatment/removal of infected stock + uninfected replacement animals (intensive production only)

Question 16

Control and eradication - considerations, decision to vaccinate or slaughter

Answer 16

Decision based on technical and economic considerations and influence by agricultural policy (including appropriate legislation). Decision to vaccinate or slaughter:

• knowledge of natural history of disease
• diagnostic feasibility (including availability & characteristics of diagnostic test)
• availability of vaccines (& how impacts diagnosis)
• adequate surveillance
• economic constraints
• veterinary infrastructure
• availability of replacement stock producers/societies
• opinions & cooporation
• public health significance
• legislation that provides for compensation

Question 17

Stamping out

Answer 17

Carrying out under the authority of the Veterinary Authority, on confirmation of a disease, the killing of the animals which are affected and those suspected of being affected in the herd and, where appropriate, those in other herds which have been exposed to infection by direct animal to animal contact, or by indirect contact of a kind likely to cause the transmission of the causal pathogen. All susceptible animals, vaccinated or unvaccinated, on an infected premises should be killed and their carcasses destroyed by burning or burial, or by any other method which will eliminate the spread of infection through the carcasses or products of the animals killed. This policy should be accompanied by the cleansing and disinfection procedures defined in the Terrestrial Code.

Question 18

Biosecurity plan

Answer 18

Plan that identifies potential pathways for the introduction and spread of disease in a zone or compartment, and describes the measures which are being or will be applied to mitigate the disease risks. The biosecurity plan should describe in detail:

1. potential pathways for introduction and spread, including animal movements, rodents, fauna, aerosols, arthropods, vehicles, people, biological products, equipment, fomites, feed, waterways, drainage or other means. Consideration should also be given to the survivability of the agent in the environment;
2. the critical control points for each pathway;
3. measures to mitigate exposure for each critical control point;
4. standard operating procedures including: implementation, maintenance, monitoring of the measures, application of corrective actions, verification of the process, record keeping;
5. contingency plan addressing any potential future changes in the risk factors;
6. reporting procedures to the Veterinary Authority;
7. the programme for educating and training workers to ensure that all persons involved are knowledgeable and informed on biosecurity principles and practices;
8. the surveillance programme in place

Question 19

Zoning and compartmentalization - containment zone - definition, tools (4)

Answer 19

Established in the event of limited outbreaks occurring in a country/zone previously free of disease for purposes of trade.

Tools:

1. standstill of movement and commodities upon notification of suspicion and demonstration of containment to zone on traceback/traceforward studies;
2. stamping out or other method to eradicate infection within zone;
3. appropriate measures to prevent spread to outside zone;
4. surveillance inside containment zone

Deemed effective when no new cases in containment zone within a minimum of 2 incubation periods

Question 20

Zoning and compartmentalization - protection zone - definition, tools (5)

Answer 20

Established to preserve the health status of animals from adjacent countries or zones of different animal health status.

Tools:

1. animal identification/tracability,
2. vaccination of at risk populations,
3. testing and/or vaccination of animals before being moved,
4. enhanced biosecurity measures (disinfection of transport, compulsory routes),
5. surveillance of wildlife/vectors

Question 21

Disposal of dead animals - considerations (6)

Answer 21

1. Timeliness
2. OH&S
3. Pathogen inactivation
4. Environmental concerns
5. Financial/human resources & infrastructure
6. Scavengers & vectors

Question 22

Disposal of dead animals - methods (5)

Answer 22

1. Rendering: mechanical and thermal treatment of animal tissues in designated facility leading to stable, sterilized products (inactivates all pathogens except prions)
2. Incineration: whole dead animals or parts of animals are completely burned and reduced to ash in designated facility
3. Pyre burning: open system of burning dead animals
4. Composting: natural biological decomposition (B. anthracis, MTB can survive)
5. Burial: whole dead animals are buried and covered by soil