14 - Disease Control in Populations

Question 1

What do we need for an infection to occur?

Answer 1

• Susceptible host
• Effective contact with infectious host
• *probability of contact and likelihood of transmission

Question 2

What does probability of contact with an infectious host depend on?

Answer 2

• Number of contacts with others in population
• Prevalence of infection in that population

Question 3

What does the likelihood of transmission given contact depend on?

Answer 3

• 1. Number of organisms to which animal is exposed
• 2. Characteristics of infectious agent
• 3. Route of transmission: presence of innate resistance or natural barriers

Question 4

‘endemic stability’ concept

Answer 4

• 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)

Question 5

What are the opportunities for control of infection?

Answer 5

1. Remove agent
   a. Effective case finding or depopulation
2. Stop transmission
   a. Education, hygiene, quarantine, vector control
   b. Contact with infected horse, indirect with contaminated environment and vectors
3. Enhance host resistance
   a. Inherent
   b. Acquired: passive and active

Question 6

What makes control programs motivating? (iceberg)

Answer 6

• Bovine influenza
  o Lots under the iceberg!

Question 7

What are the methods to control disease in populations?

Answer 7

• *Selective slaughter
• Depopulation
• Quarantine
• *Mass treatment
• *Mass immunization
• *Environmental control
• Education
• Applied ecology
• Genetic improvement

Question 8

Why less genetic improvement in beef cattle compared to Holsteins?

Answer 8

• not much genetic variation in Holstein’s (poultry, or swine)
• *beef cattle=all over the map!
  o Lots of variability=good thing!

Question 9

Selective slaughter

Answer 9

• “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

Question 10

Mass treatment

Answer 10

• 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

Question 11

Mass immunization

Answer 11

• 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

Question 12

Herd immunity: rabies vaccination example in dogs

Answer 12

• Outbreaks will not propagate if between 39-57% of dogs are vaccinated
• WHO recommends immunization of 70%

Question 13

Basic reproductive ration (R0)

Answer 13

• 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

Question 14

What determines R?

Answer 14

• P=Probability of infection on contact
• C=Rate of contact
• D=Duration of infectiousness
• R0=pc*D

Question 15

What is need for communicable infections to establish in a population?

Answer 15

• On average each infected individual must infect 1 or more susceptible individuals
• If less than 1=outbreak will die out

Question 16

What is effective reproductive ratio (R*)?

Answer 16

• 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)

Question 17

What does effective reproductive ratio depend on?

Answer 17

• Probability of contact
• Probability of transmission given contact
• Duration of infectiousness
• ***% of population that is susceptible

Question 18

What is critical fraction?

Answer 18

• *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

Question 19

Environmental control

Answer 19

• 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

Question 20

What are some environmental factors potentially affecting disease control programs?

Answer 20

• Population density
• Housing
• Environmental conditions

Question 21

Population density: to control disease example

Answer 21

• Deer feeding stations and TB in Michigan

Question 22

Housing: examples to control disease

Answer 22

• Ventilation
• Sanitation
• Bedding quality
• Floor quality
• Overcrowding

Question 23

Environmental conditions: example to control disease

Answer 23

• Temperature
• Humidity
• Wind velocity
• Precipitation
• Climatic changes

Question 24

Examples: rabies control program in Ontario

Answer 24

• 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

Question 25

What methods of disease control were used for the rabies control program in Ontario?

Answer 25

- Depopulation o Since NO test (NOT selective slaughter) o All animals in specific area - Mass immunization - Education

Question 26

TB in Manitoba: Riding Mountain National Park elk and white-tailed deer example

Answer 26

- 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’

Question 27

What methods of disease control were used for TB control in MB?

Answer 27

- *environmental control - Depopulation: cow herd - Quarantine - Education - (selective slaughter: did something with the positive wildlife)

Question 28

Chronic wasting disease (CWD) transmission (prion disease)

Answer 28

- 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

Question 29

CWD in SK

Answer 29

- 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

Question 30

CWD and infecting people?

Answer 30

- Could happen, NOT strong evidence - *test before you eat! - Hunting and environmental perspective (poop in your yard)

Question 31

What methods of disease control could be done for CWD?

Answer 31

- 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)