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Disease Epidemiology #2 > Strategies for directed action against disease at the population level > Flashcards

Strategies for directed action against disease at the population level Flashcards

1
Q

Disease control strategies

A
  1. depopulation
  2. selective slaughter
  3. quarantine
  4. reduction of contact
  5. mass treatment
  6. mass immunization
  7. education
  8. environmental control
  9. applied ecology
  10. genetic improvement
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2
Q

Depopulation

A

Whole population including non-infected individuals is sacrificed (ex. Foot and mouth)

  • pre-emptive slaughter. Slaughter individuals that may have been exposed to infection and at risk of developing disease
  • often followed by disinfection and destruction of carcasses

Blunt instrument

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3
Q

When is depopulation used?

A
  • when diagnostic tests are not easily applied or they are inaccurate
  • population is inaccessible
  • rapidly spreading disease
  • terminal stages of eradication programs
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4
Q

Selective slaughter

A
  • Test and slaughter- deliberate killing of a minority of infected animals to protect healthy majority
  • usually involves a method of case finding
  • more expensive method as cases become rarer
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5
Q

Disease examples where we have used selective slaughter

A

Brucellosis
TB

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6
Q

When is selective slaughter used?

A
  • works well early in disease outbreaks
  • slowly spreading disease
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7
Q

Quarantine

A
  • isolation of animals that are either infected or suspected of being so
    -ALSO includes restraining movement of exposed or infected animals or items that may be contaminated
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8
Q

Quarantine and Exclusion Biosecurity

A

Preventing entry of infected animals to premises or country… isolating the non-infected

  • national, regional, herd, or farm level

Specific Pathogen Free (SPF) barns, importation quarantines

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9
Q

What does the period of quarantine depend on?

A
  • incubation period of the agent
  • time taken for infection to be confirmed
  • time taken for an animal to become non-infectious
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10
Q

Reduction of contact

A

Reduce or prevent contact (physical or aerosol) between infected and non-infected animals
ex. COVID social distancing

  1. separate in time (All in and All out systems, or milking staph cows last, etc.)
  2. Separate physically (dairy calf hutches, creep areas for beef calves, rotational grazing for parasites)
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11
Q

Mass Treatment

A

Treating all animals (sick and well).
- combats diseases occurring at very high prevalence where depopulation and slaughter are not economical or viable

*Needs to be safe, cheap, effective therapy.
*includes parasite control programs, dry cow therapy, metaphylaxis in feedlots, heartworm in dogs

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12
Q

Mass immunization

A

Creating immunity in population to limit the spread and impact of disease
- can decrease prevalence to a point where other techniques can be used for eradication
- may interfere with screening tests (antibody tests, because vaccines will show positive antibodies)
- difficult in wildlife populations (but can try and use bait vaccines)

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13
Q

Advantages for mass immunization

A
  • successful for many vet diseases (rinderpest, rabies, distemper, parvo, etc.)
  • resistant individuals can freely move and don’t need frequent dosages
  • can be used for eradication
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14
Q

Immunization considerations

A
  1. importance of disease in question (impact of infection, prevalence)
  2. efficacy of vaccine (few vaccines to provide immunity)
  3. duration of immunity (is there need for additional dosages?)
  4. accessibility of animal population
  5. dynamics of population (births, additions, movement of animals)
  6. cost of vaccine relative to other control strategies
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15
Q

Three immunization strategies

A
  1. ring vaccination
  2. barrier vaccination
  3. suppressive vaccination
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16
Q

Ring vaccination

A

vaccinating animals in the geographic area around the outbreak to limit further spread

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17
Q

Barrier vaccination

A

vaccinating animals in a particular zone to prevent spread into other countries

18
Q

Suppressive vaccination

A

vaccinating within an outbreak and in the areas surrounding it

19
Q

Education

A
  • programs which educate the public regarding disease control
  • often used in combination with other techniques (hand washing, internal temp recommendations for meat etc.)
20
Q

Somatic cell count penalty programs

A
  • test numbers, if high then affect on product and penalties to farm (only allowed so many)
21
Q

Hydatid disease in New Zealand

A
  • Parasites cycle between sheep and canines and then into humans
  • Best way to reduce spread was to increase education on how it is spread to humans through dogs. Ex. posters- dont feed raw meat to dogs
22
Q

Environmental control

A

Host/Agent/Environment Triad

  • management, environmental control, feeding, husbandry
    ex. laminitis control, ventilation management, disinfection of fomites, mastitis control
  • many public health programs began with environmental hygiene initiatives
23
Q

Applied ecology

A
  • knowledge about the ecology of the disease is utilized to prevent or control disease
  • biological control mechanisms or Niche filling
  • may include vector control

ex. salmonella control in chicks of poultry industry

ex. screwworm disease in S. USA- released pathogen free flies because they know these flies only reproduce once and will reproduce with pathogen free and reduce numbers

24
Q

Genetic improvement

A
  • genetic screening to identify susceptibility to specific diseases. Then remove susceptible individuals from the breeding population

Ex. sire selection
Ex. breeding for disease resistance

25
Q

Scrapie susceptibility in Sheep (gene improvement)

A
  • Promote ARR gene- more resistant
  • remove VRQ gene- most susceptible
26
Q

Bovine TB Spread with Humans

A
  • mycobacterium bovis
  • zoonotic potential (humans infected by drinking raw milk)
  • federal reportable disease: CFIA and listed under OIE terrestrial animal health code
  • control measures began in many developed countries (1920s) because it was a major disease of domestic animals
27
Q

Bovine TB transmission characteristics

A
  • primary host in domestic cattle, but huge host range (other mammals)
  • long incubation period so latent infections make control and detection difficult
  • transmission through direct contact but also indirect (feed and fomites)
28
Q

Gold standard for Bovine TB

A

culture at necropsy

29
Q

Eradication program in Canada of TB

A
  • began in 1924, achieved freedom in 1985
  • several billions of dollars
30
Q

where is bovine TB still found

A
  • many countries have reduced or eliminated TB from cattle populations
  • pockets of infection in wildlife in Canada (Riding mountain, wood buffalo), UK, USA, New Zealand
  • common in Latin America, carribean countries

-endemic in africa and asia

31
Q

Wildlife reservoirs of TB

A

England/Ireland: Badgers

New zealand: brush tailed possums

Spain: wild boar

N. America: mule deer, white tailed deer, elk, bison

Australia, south africa: Water buffalo

32
Q

Countries with active TB control programs

A

infected herds of cattle have very low prevalence suggesting low rates of cattle-cattle transmission

33
Q

Management activities that likely contributed to control success in Riding Mountain National Park (TB)

A
  • Elk are reservoir
  • adaptive management approach using research and annual surveillance results
  • identified scope of outbreak and core area to focus the resources
    1. barrier hay fences- reduce direct and indirect contact
    2. Deer/elk density reduction- hunting, culling, opportunistic.
    3. Removal of blood test positive individuals by selective culling
    4. Removal of bovine TB infected cattle herds
34
Q

Diagnostics of Bovine TB

A
  • Single intradermal tuberculin test (caudal fold test)
  • measure intradermal skin reaction 72 hours post injection

**Imperfect specificity (97-99%)- false positives occur even in completely negative populations. Usually linked to cross reactions with similar bacteria.

**Test has even poorer sensitivity (80-93%)- so more false negatives

35
Q

What happens when animal tests positive for TB?

A
  • herd is quarantined. Nothing is allowed to leave the property without CFIA permission
  • animal humanely slaughtered and lymph nodes and other tissues are collected and sent for PCR and histopathology. If these are negative, then they are cultured (8-12 weeks) and resubmitted for more PCR tests.
    *If negative again then considered a false positive. If positive, then entire herd is humanely depopulated
36
Q

CFIA investigations

A

They will want to see:
- detailed description of management practices and biosecurity plan
- vet records/labs
- animal movement records
- site map of farm
- contact info with farm vet
- CFIA will begin trace in/trace out process

37
Q

Length of quarantine

A
  • diagnostic process will take up to 14 weeks (because cultures)
  • Quarantine will remain in place (humane slaughter, contaminated products that cant be cleaned will need to be destroyed, farm responsible for cleaning)
  • compensation process based on market value of loss
  • 45 days after cleaning and decontamination, quarantine can be removed
38
Q

Alberta outbreak

A

5 yr old cow found with TB lesions at slaughter.
- moved through minnesota, and saskatchewan
- tests confirmed TB infection and genetics confirmed from strain in central Mexico
- linked to large herd that spends time grazing in community pasture (bulls used for breeding at other farms as well)
- community pasture led to all herds being quarantined and tested.

39
Q

Alberta outbreak final results

A

**6 confirmed cases (all from index herd); over 70 trace out herds and another ~70 trace in herds; thousands tested
** no source of infection identified

  • 11,500 animals destroyed, $39 million paid to producers in compensation; $16.7 million paid for quarantines
  • continued elk surveillance in southern Alberta
40
Q

4 categories of CFIA investigation

A
  1. direct herd contacts
    - low risk (fence line contacts)
    - high risk (commingling on community pasture)
  2. trace-out herds
    - animals that left farm. Terminal feedlots and other farms.
    - feedlots: Usually destroyed and post mortem exams
    - other farms: tested and reactors destroyed
  3. trace-in herds
    - test and destroy reactors
  4. buffer zone herds
    - herds within 5km of index herd are subjected to herd testing
    - also wildlife surveillance

Disease Epidemiology #2 (11 decks)

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