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Natural variation is seen in the clinical response to field and experimental
infection in fully susceptible animals. It is probable that physiological factors
influence the response to infection, such as:
Level and stage of lactation
Cows in heavy lactation appear to be more susceptible to clinical disease than
low-yielding individuals.
Shipping stress
Animals are moved from their farm of origin to new farms or feedlots, are subjected
to stressful situations and frequently succumb to infections and develop disease. It
has not been possible to demonstrate any immunological change which might indicate
increased susceptibility to diseases such as FMD, but outbreaks of disease have been
reported following movement of animals. This could be associated with the stress due
to transport over long distances by road or rail, social stress as the animals mix with
others from different origins, and disease stress as the close proximity of the animals
being moved and mixed allows rapid transmission of pathogens.
Breed 'lines'
Breed 'lines' within breeds may also show reduced or increased susceptibility to FMD
virus infection and disease.
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