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Definition of the carrier state in FMD
An animal from which FMD virus can be isolated more than
28 days after infection.
Following the initial, clinical phase of infection in ruminants,
a large proportion of animals will continue to harbour the virus for a
considerable period of time after resolution of lesions has occurred.
Monogastric species (e.g., pigs) appear to clear the infection
and the carrier state is not recognised in these species.
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The site of virus persistence is the oropharynx, particularly
the dorsal soft palate, and the cranial oesophagus.
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Carrier animals represent an inapparent source of infection
once lesions have healed.
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The establishment of the carrier state is independent
of the immune status of the animal at the time of exposure to infection,
i.e., pre-existing immunity to FMD, in the form of circulating antibody,
does not prevent animals becoming carriers.
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The proportion of animals which become carriers following
exposure to infection varies depending on the strain of virus involved
and the innate susceptibility of the animal under investigation.
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Transmission of clinical disease from carrier to susceptible
cattle has never been demonstrated under experimental conditions.
However, there is growing circumstantial evidence from Africa that
transmission can occur from carrier cattle and buffalo to susceptible
cattle with which they have close contact. In situations where it
is not possible to slaughter all infected and in-contact animals,
the carrier animal must therefore be considered a potential source
of virus for future outbreaks and is therefore important in the epidemiology
of the disease.
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