Volume 22 • Issue 02 • 2018
Increased exposure for example can move the
cut-off point to the right, meaning that a larger
part of the population becomes susceptible to a
specific disease.
Cut off point
An increase in exposure can have a dramatic
effect on the extra number of animals that will
become susceptible.
In this example it increased from 240 to 370
animals, due to an increase in exposure to the
disease-causing organisms.
Cumulative
number of
susceptible
animals
Low
High
Cumulative number of susceptible animals
Impact of Increased Exposure or
Decreased Resistance
An example of how this occurs in practice is if
a few animals in an unvaccinated herd become
sick, the disease-causing organisms multiply in
these animals and cause a very high exposure to
the rest of the animals in the herd.
The number of animals that become sick can to
a very large extent be influenced by two factors:
1. Increase in the number of disease-causing
organisms that the animal is exposed to.
2. A decrease in the general resistance due to
factors like underfeeding, especially during
critical stages in the production cycle like
just after calving.
The level of general disease resistance will play
a larger (e.g. “dirty cows” = endometritis) or
smaller (e.g. lumpy skin disease) role depending
on the type of disease-causing organism or
factor involved.
Susceptible
animals
Increase in
cumulative
number of
susceptible
animals
Increase in cumulative number of susceptible animals
Resistant
animals
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