14 - Disease Control in Populations Flashcards
1
Q
What do we need for an infection to occur?
A
- Susceptible host
- Effective contact with infectious host
- *probability of contact and likelihood of transmission
2
Q
What does probability of contact with an infectious host depend on?
A
- Number of contacts with others in population
- Prevalence of infection in that population
3
Q
What does the likelihood of transmission given contact depend on?
A
- Number of organisms to which animal is exposed
- Characteristics of infectious agent
- Route of transmission: presence of innate resistance or natural barriers
4
Q
‘endemic stability’ concept
A
- Common mistake is to interpret a sudden appearance of clinical cases as an indicator that a NEW infectious agent has been introduced
- *actually, unrecognized CHANGES IN DISEASE ECOLOGY have result in clinical cases caused by an UBIQUITOUS INFECTIOUS AGENT (that has been present all along)
5
Q
What are the opportunities for control of infection?
A
- Remove agent
a. Effective case finding or depopulation - Stop transmission
a. Education, hygiene, quarantine, vector control
b. Contact with infected horse, indirect with contaminated environment and vectors - Enhance host resistance
a. Inherent
b. Acquired: passive and active
6
Q
What makes control programs motivating? (iceberg)
A
- Bovine influenza
o Lots under the iceberg!
7
Q
What are the methods to control disease in populations?
A
- *Selective slaughter
- Depopulation
- Quarantine
- *Mass treatment
- *Mass immunization
- *Environmental control
- Education
- Applied ecology
- Genetic improvement
8
Q
Why less genetic improvement in beef cattle compared to Holsteins?
A
- not much genetic variation in Holstein’s (poultry, or swine)
- *beef cattle=all over the map!
o Lots of variability=good thing!
9
Q
Selective slaughter
A
- “test and slaughter”
- Deliberate killing of minority of infected animals to project health MAJORITY
- **Need a method of ‘case finding’ (live animal test, NOT rabies)
- Works well EARLY in disease outbreaks and slowly spreading disease
o Ex. did for Brucellosis, but not now
o Ex. Johne’s disease (testing, culling and slaughtering) or Neospora outbreak
10
Q
Mass treatment
A
- Treating all (sick and well)
- Combats disease occurring at very HIGH prevalence where depopulation and slaughter are not economical or viable
- Need SAFE, CHEAP and EFFECTIVE therapeutic agents
o Ex. parasite control, dry cow therapy, heartworm medications - Potential problem of disease RESISTANCE
11
Q
Mass immunization
A
- Creating immunity in population which limits spread and impact of disease
- Has been successful in past
o Canine distemper, parvo virus, rabies - *safer compared to mass treatment
- *herd immunity
12
Q
Herd immunity: rabies vaccination example in dogs
A
- Outbreaks will not propagate if between 39-57% of dogs are vaccinated
- WHO recommends immunization of 70%
13
Q
Basic reproductive ration (R0)
A
- Average number of susceptible individuals that are infected by each INFECTED individual when all others are susceptible
- *ease of transmission of an infectious agent
- Ex. FMD=70 vs. IBR=7, human flu=2
14
Q
What determines R?
A
- P=Probability of infection on contact
- C=Rate of contact
- D=Duration of infectiousness
- R0=pc*D
15
Q
What is need for communicable infections to establish in a population?
A
- On average each infected individual must infect 1 or more susceptible individuals
- If less than 1=outbreak will die out
16
Q
What is effective reproductive ratio (R*)?
A
- Average number of susceptible individuals that are infected by each infected individual in CURRENT EPIDEMIOLGOICAL CONTEXT
- *want to reduce it to be less than 1 (doesn’t need to be 0)
17
Q
What does effective reproductive ratio depend on?
A
- Probability of contact
- Probability of transmission given contact
- Duration of infectiousness
- ***% of population that is susceptible
18
Q
What is critical fraction?
A
- *don’t need to know formula
- The proportion of the population that immunity is needed to achieve herd immunity OR prevent an outbreak from progressing
19
Q
Environmental control
A
- Utilization of host, agent and environment
- Management, environmental control, feeding, husbandry
- *many health management programs revolve around environmental hygiene
o Ex. ventilation, and laminitis control - Disinfection of fomites; surgical and sterilization
20
Q
What are some environmental factors potentially affecting disease control programs?
A
- Population density
- Housing
- Environmental conditions
21
Q
Population density: to control disease example
A
- Deer feeding stations and TB in Michigan
22
Q
Housing: examples to control disease
A
- Ventilation
- Sanitation
- Bedding quality
- Floor quality
- Overcrowding
23
Q
Environmental conditions: example to control disease
A
- Temperature
- Humidity
- Wind velocity
- Precipitation
- Climatic changes
24
Q
Examples: rabies control program in Ontario
A
- Focused on vaccination
o Vaccine baits (airplane, helicopter, by hand)
o Trap-vaccinate-release program
o *Red fox and skunk rabies - Raccoon rabies
o Trap-vaccinate-release
o Point infection control
Euthanize those in 5km area
Trap-vaccinate-release in 5-10km area
o Aerial drops of baits containing vaccine
o *boat and trailers to/from US and garbabe/pet food - *PUBLIC EDUCATION
25
Q
What methods of disease control were used for the rabies control program in Ontario?
A
- Depopulation
o Since NO test (NOT selective slaughter)
o All animals in specific area - Mass immunization
- Education
26
Q
TB in Manitoba: Riding Mountain National Park elk and white-tailed deer example
A
- Started with the cattle and then went to the wildlife
- 60% of elk going outside of the park to eat
o NEED to put up barrier fences (hard to open gates when lots of snow)
o Hay needed to be removed from field to get crop insurance - Increased hunting opportunities
- Improved environment of park to make it more attractive to elk
- Stop baiting of elk
- Studies to enhance knowledge of elk movements and behaviour patterns
- Got hunters to submit samples for TB testing
- ZONING: ‘split statis’
27
Q
What methods of disease control were used for TB control in MB?
A
- *environmental control
- Depopulation: cow herd
- Quarantine
- Education
- (selective slaughter: did something with the positive wildlife)
28
Q
Chronic wasting disease (CWD) transmission (prion disease)
A
- Animal to animal OR animal to environment to animal
- Pastures, feed or water equipment remains contaminated for years
- *route of transmission unknown: likely feces and/or saliva
- *bison not susceptible=put on ‘old’ elk farms
- *wolves=mechanical vector, contribute to SPREAD!
- CARIBOU: very susceptible as they stay in large herds
29
Q
CWD in SK
A
- Big problem!
- *universally fatal
o Those that die slower, just spread it more - Vaccines: live longer, but shed more and then die
o NOT HELPFUL since it doesn’t’ reduce the shedding
30
Q
CWD and infecting people?
A
- Could happen, NOT strong evidence
- *test before you eat!
- Hunting and environmental perspective (poop in your yard)
31
Q
What methods of disease control could be done for CWD?
A
- Increased provincial surveillance
- Environmental control
o Manage contaminated premises
o Reduced feeding and baiting stations (and grain bags or spillage) - Quarantine the premises (keep wildlife out)
- *depopulation: didn’t work well as they just came back
- Selective slaughter: more SUCCESSFUL
o Increase hunting license of MALE deer (from data)
