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Molecular Epidemiology
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Ruminant animals that recover from FMD infection, and immune
animals that have contact with live virus, become persistently infected
for a variable period of time and become 'carriers' (Salt,
1993; Woodbury, 1995).
A carrier is defined as an animal from which live virus can be recovered for more than
28 days after infection. The duration of the carrier period varies considerably between
different animals.
Carrier animals Duration of carrier period
Cattle 3 years
Sheep 9 months
Goats 4 months
African buffalo 5 years
Other wild ruminants carry virus for shorter periods.
Pigs do not become carriers.
Virus can be recovered from the pharynx and dorsal soft palate of carrier animals.
Although there is field evidence that carrier animals can cause new outbreaks of disease,
it has not been possible to show experimental transmission of FMD virus from a carrier
to a susceptible in-contact animal.
During the 1980s there was a series of outbreaks of SAT 2 virus in cattle in the FMD-free
zone of Zimbabwe. Nucleotide sequencing of the outbreak strains showed them all to be
closely inter-related. Frequently, outbreaks occurred when quarantine restrictions placed
during previous outbreaks had been lifted, and cattle that had been in contact with disease
were moved to new premises or taken through market.
In 1991 an outbreak of SAT 2 was associated with the movement of cattle from a farm
vaccinated during an outbreak in 1989. Probang samples subsequently collected from the
vaccinated farm showed the presence of carrier cattle and nucleotide sequencing showed
that virus isolated from these cattle was identical to that causing the new outbreak.
However, since vaccination had not been repeated on the farm since 1989, there were now
many fully susceptible cattle present, but no transmission from the carriers on the farm
occurred to the susceptible cattle on the farm. Only when carrier cattle were moved did
they appear to start transmitting virus, and an outbreak occurred.
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