Lecture 2 Flashcards
Approaches to Disease Control
- Eradication (Globally, Nationally and at farm level)
- Prevention and Exclusion
- Immunization
- Disease Management
What are 5 infectious agents
BACTERIA
VIRUSES
PRIONS
MYCOTIC
PARASITES
Characteristics of GLOBAL disease Eradictation
Characteristics of an eradicable disease:
- No carrier state
- No subclinical infection/short incubation period
- Limited to one species or family (e.g. ruminants)
- Available intervention – good quality vaccine or test
What is carrier state?
No clinical disease but potential to
transmit an infectious agent.
What is carrier state?
No clinical disease but potential to
transmit an infectious agent.
What is Subclinical Infection?
Not visible, but measurable in
some way.
What is Subclinical Infection?
Not visible, but measurable in
some way.
What is incubation period?
The period of time between when infection and signs of disease (or immunity) develop.
NATIONAL Disease Eradication
• Similar difficulties to global eradication
• Canada – free from:
1. Brucellosis – cattle, pigs
2. “TB free status” – except for Manitoba
• Mainly due to test and cull
FARM-LEVEL Disease Eradication
Methods include…
Depopulation and selective removal
FARM-LEVEL Disease Eradication
Methods include…
Depopulation and selective removal
What is depopulation?
• Large proportion or entire population of herd is
removed
• Depending on disease – determine what is done
and where
What is depopulation?
• Large proportion or entire population of herd is
removed
• Depending on disease – determine what is done
and where
What is selective removal?
• Deliberate euthanasia or removal of a minority of
infected animals to protect the well majority
• Need a screening or diagnostic test to be able to
detect disease
Selective removal test must:
a. Detect the disease agent
b. Test for evidence of an immune response to agent
What are the two types of selective removal tests?
Screening Test
• Applied to healthy animals
• Usually before clinical disease evident
Diagnostic Test
• Confirm or classify disease (may be follow up to
positive screening test)
• Applied to “abnormal” or “unhealthy” animals
What are the two types of selective removal tests?
Screening Test
• Applied to healthy animals
• Usually before clinical disease evident
Diagnostic Test
• Confirm or classify disease (may be follow up to
positive screening test)
• Applied to “abnormal” or “unhealthy” animals
FARM-LEVEL Disease Eradication
Selective Removal
We want to use a diagnostic test that we trust
the results from (i.e., an accurate diagnostic
test) = high sensitivity and specificity
• No diagnostic test performs perfectly well - there
will always be some misclassification of disease
FARM-LEVEL Disease Eradication
Selective Removal
We want to use a diagnostic test that we trust
the results from (i.e., an accurate diagnostic
test) = high sensitivity and specificity
• No diagnostic test performs perfectly well - there
will always be some misclassification of disease
What do sensitivity and specificity testing tell us?
– The sensitivity and specificity of a test tell us how
well the test is able to correctly identify diseased
and healthy animals
What is sensitivity testing?
= the ability of a test to identify truly diseased/
infected animals
= the proportion of truly diseased individuals that
the test diagnoses as diseased in the population
What is specificity testing?
= the ability of a test to identify truly healthy or
non-diseased animals
= the proportion of truly healthy animals that the
test calls non-diseased
• If specificity is poor, then the test will call healthy
animals, diseased (↑ false positives)
• If specificity is high, there will be few false positives
Predictive value of a test depends on
1: Characteristics of the test
- test specificity and test sensitivity
2: Prevalence of disease
in the population
Predictive value of a test depends on
1: Characteristics of the test
- test specificity and test sensitivity
2: Prevalence of disease
in the population
