Herd-level testing and immunity

Positive for a disease or condition Negative for a disease or condition

The objective of herd-level testing is to classify the herd (population) as what or what?

An evaluation of a sample of (or all) animals from a herd and the application of decision rules that classify the herd as positive or negative based on the test results from individual animals

What is a herd-level test?

It is important for things like:

- Specific-pathogen-free (SPF) programs (e.g., a herd owner wants to have the herd classified as free of one or more disease)

- Herd certification programs (e.g. , Johne's disease)

- Disease eradication programs (e.g., cull whole herds as compared to individual animals)

- Selling disease-free animals (e.g., BVD)

- Improving production

Why would we want to classify the health status of a herd as opposed to individuals?

- Apply to individual first: select a cutoff value for the individual

- Aggregate across results for all sampled animals in the herd: selecting a cutoff for the herd-level interpretation

What are steps to herd-level interpretation?

the herd will be positive if one individual animal tests positive.

When interpreting results for a herd - what would the cutoff be for a test that has perfect specificity?

Probability that a positive herd yields a

positive herd-level test result

What is herd-level sensitivity?

Probability that a negative herd yields a negative herd-level test result

What is herd-level specificity?

- The number of animals tested (this can be a sample or the entire herd)

- The true within herd prevalence

- The herd cutoff value for test positive animals used to declare a herd positive

- Diagnostic sensitivity (Se)

- Diagnostic specificity (Sp)

Herd level sensitivity and specificity is dependent on what 5 things?

◦ Probability that a test positive herd is truly a disease positive herd

What is herd-level positive predictive value?

◦ Probability that a test negative herd is truly a disease negative herd

What is herd level negative predictive value?

o Herd-level sensitivity

o Herd-level specificity

o The true prevalence of positive herds (herd-level prevalence)

Herd level predictive values are dependent on what three things?

Bovine Tb testing example in class -

if positive - perform multiple tests in series (to get specificity as close to perfect as possible)

if there was one positive animal - this results in a positive herd

Understand how individual animal test performance affects herd-level test performance

it describes the collective immunologic status of a population of hosts, as opposed to an individual organism, with respect to a given infectious agent

What is herd immunity?

◦ Many animals have been immunized

◦ Many animals have recovered from infection with immunity

◦ Lower probability of contact between infected and susceptible animals

What three things might contribute to a high level of herd immunity?

R values

What is relied upon in order to determine herd immunity?

Basic

The _______ Reproductive Number (R0) is the expected number of secondary cases produced by a typical primary case in an entirely susceptible population

1

if R(0) < ___ the infection will die out.

implies the condition will spread (without control measures) ... higher numbers are more likely to cause epidemics.

What does an R(0) equal to or > 1 imply?

infectiousness

R0 gives a measure of the _____________ of the organism per se.

nah - it tends to be relatively fixed

Is R(0) affected much by the uptake of vaccine or immunity from previous epidemics of the disease?

decreases

As there are fewer susceptible individuals in the population the number of new/introduced cases ___________

Effective

The ___________ Reproductive Number (R) represents the average number of secondary cases which are attributable to a single case of infection

it represents the average number of secondary cases which are attributable to a single case of infection

What is effective reproductive number?

smaller

The effective reproductive number (R) is typically ___________ than the value of the basic reproductive number (R0)

the effective reproductive number

Which reproductive number reflects the impact of control measures and depletion of susceptible animals by the infection?

the fraction of the population that is susceptible to infection

R = R0x, where x is what?

decreases

Relating to the equation, R = R0x

As an epidemic spreads, and animals die or become immune to the disease, x __________, and can eventually becomes small enough that R drops below 1

1) The infectiousness of the agent given the exposure status and route of infection

2) The number of adequate contacts per time period between infectious and susceptible animals

3) Duration of the infectious period

4) Susceptible animals need to be present

What four factors affect R (effective reproductive number)?

decrease

As transmission of a disease proceeds through the herd and infect animals become immune - what happens to the number of susceptible animals in the herd - does it increase or decrease?

the likelihood that an infected animal will come in contact with a susceptible animal decreases

The R value declines

As the number of susceptible animals declines, what happens to the likelihood that an infected animals will come in contact with a susceptible animal? Consequently, what happens to the value of R?

when the number of susceptible animals becomes so small that the effective reproductive number will decline to R = 1

What is the endemic state?

1 - (1/R0)

How can we estimate the number of animals needed to protect against the transmission of the disease?

reduce

By vaccinating the herd, we __________ the number of susceptible animals and hence possibly transmission (R

more

The greater the R(0) value, the __________ vaccine coverage is required in order to attain herd immunity.

◦ If new susceptible or infected animals are added to the herd, hence the proportion of immune animals decrease

◦ If the contact pattern changes

◦ A new strain with different virulence enters the population

What are three things that might change R (effective reproductive number)?