Infectious disease control and prevention in populations Flashcards
What is needed for infection to occur?
-a susceptible host
-effective contact with an infectious host
Factors affecting probability of contact with infectious host
-number of contacts
-prevalence of infection in the population
What does likelihood of transmission depend on?
-number of organisms to which the animal is exposed
-characteristics of the infectious agent
-route of transmission
Endemic stability
**not typically due to new agent coming in, typically due to changes in disease ecology allowing for ubiquitous infectious agent to emerge that was already present
Opportunities for control
- remove agent- remove or treat of infected hosts. Includes find and cull
2.stop transmission
*direct contact, indirect contact through environ, contact with vectors
- Enhance host resistance
*inherent
*acquired (active or passive)
Methods to control disease in populations
-selective slaughter
-depopulation
-quarantine
-mass treatment
-mass immunization
-environmental control
-education
-applied ecology
-genetic improvement
Selective slaughter
Test and slaughter= kill minority of infected animals to protect the health majority (eg. Johne’s disease, Neospora)
-need case finding (diagnostic screening test)
-works well early in disease outbreaks and in slowly spreading diseases
eg. Brucellosis eradication early, but not used now
Mass treatment
Treat all animals whether sick or well
-eg. mastitis, shipping fever, parasites
-combats disease when depopulation and slaughter are not economical or viable.
-need safe, cheap, effective therapeutic agents. But be aware of treatment/disease resistance
Mass immunization
Create immunity in population which limits spread and impact on disease
eg. Canine distemper, parvo virus, rabies
Basic Reproductive Ratio (R0)
The avg number of susceptible individuals that are infected by each infected individual when all others are susceptible
**the ease of transmission of an infectious agent
eg. IBR: R0=7 ,FMD: R0=70
What determines R?
-probability of infection on contact
-rate of contact
-duration of infectiousness
R0<1
outbreak will die out
R0>1
outbreak will take off
Effective Reproductive Ratio
The avg number of susceptible individuals that are infected by each infected individual in the current epidemiological context
What does effective R depend on?
-probability of contact
-probability of transmission given contact
-duration of infectiousness
-% of population that is susceptible
R* (effective reproductive number) >1
Needed for an infectious agent to survive in a group
GOAL of control strategies is to get R* <1
Critical fraction
TO achieve herd immunity or prevent an outbreak from progressing we need to create immunity in this proportion of the population
Environmental control
-Use Triad (host, agent, environment) to control disease
*management, environment control, feeding, husbandry
*hygiene (ventilation, comfort, disinfect fomites, surgical sterilization)
Environmental factors affecting disease control programs
- pop density (eg. deer feeding stations)
- Housing (eg. ventilation, sanitation, bedding, flooring, overcrowding)
- Environmental conditions (eg. temp, humidity, wind, precipitation, climate change)
Ontario rabies control program
**focus on skunk and fox strains
-vaccine bait distribution
-trap-vaccinate-release program
*raccoons
-trap, vaccinate, release
-point control= positive, would kill others within 2km… depopulation within a positive case perimeter
-aerial drops of baits
Educations role
-education population about how exposure might occur
eg. Rabies in raccoons… garbage and pet food, boats and trailers
TB in Manitoba
Positive cases in elk and white tailed deer in Riding Mountain National Park
-linked to historic pasture cattle in the park
-saw 7 infected cattle herds since 1997
Management of TB in Manitoba
1.More than 60% of elk come outside the park to feed… need to have barrier fences to protect stored winter feed supplies
- Prescribed burns to improve habitats
- Increase hunting opportunities of elk
- require hay be removed from fields to be eligible for crop insurance
- Manitoba conservation regulations on baiting elk for hunting
6.education on elk movements/behaviours
TB testing management
Put zones in place around Riding Mountain National Park and Duck mtn provincial park
-hunters were to submit samples for testing from these zones
Manitoba TB split zone
Had split status for TB
-Rocky mtn= TB accredited advanced (low prevalence)
-Rest of MB= free
**Now considered TB free
TB in MB today
-Mostly surveillance through hunter submissions (hunter killed surveillance)
Chronic wasting disease transmission
-animal to animal or animal to environment to animal
*likely through feces and/or saliva
-pastures or feed and water equipment remain contaminated for years
Who is most susceptible to chronic wasting disease?
-caribou most susceptible because they move in large herds
-rare in moose because they are normally on their own
Chronic waste disease and TB surveillance
Province expanded CWD surveillance program in wildlife management zones to test for TB
-currently accept heads from deer, moose, elk
-no associated cases of bovine TB in SK wildlife to date
