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Epidemiological monitoring is an essential part of stamping out and involves examination
of animals which might potentially have had either direct or indirect contact with an IP.
This includes
• farms in the protection and surveillance zone
• farms which have received animals, animal products, vehicles or people from an IP
• farms under a virus plume where windborne dispersal could have occurred.
The frequency of surveillance depends on the perceived level of risk and the manpower
resources available. Surveillance should start at the earliest date on which clinical
signs could become apparent. This is usually the minimum incubation period (usually 2 to
4 days depending on the type of possible transmission) after the event that placed the
premises at risk.
Surveillance for FMD should be by regular and thorough clinical inspection for typical
signs of the disease. In the case of sheep and goats, clinical signs of FMD can be mild.
Particular attention should be paid to reports of lameness and detailed examination of
the feet should be performed. Likewise for pigs, particularly in the early stages, close,
physical examination of the feet is necessary.
Serological surveillance (serosurveillance) is not generally useful during the initial
phase of eradication for the following reasons:
• a large number of animals must be sampled to obtain a statistically reliable indication of the possible presence of seroconversion (e.g., 95% probability of detecting a 5% prevalence of infection)
• serosurveillance is time consuming and expensive compared to physical examination.
• the delay in obtaining laboratory results almost inevitably means that if disease is present it will have shown itself in the form of clinical signs by the time that laboratory results are available.
• restrictions must be maintained until the 'all clear' is given based on serology. This can take several weeks if repeat sampling is required.
• serological results are often not clear-cut due to the presence of non-specific reactions or vaccinated animals. This can lead to difficulties in making the decision to certify a farm as 'free of FMD'.
• conventional tests for antibody to FMD cannot distinguish antibody due to infection from that due to vaccination. This can cause difficulties in the interpretation of serosurveillance. Assays which measure antibody to the non-structural proteins of FMD virus offer the prospect of identifying infected animals, even if they have also been vaccinated.
Serosurveillance is useful in situations where uncontrolled movement of animals has
occurred, particularly in sheep and goats. In these cases, where infection is suspected
after the period when clinical signs are likely to have occurred, serosurveillance for
antibody to FMD virus may be the only method of detecting flocks exposed to FMD virus
which might still contain carrier animals. The significance of carrier sheep in the
overall epidemiology of FMD is unclear but they have at least the potential to act as
focuses of recrudescence of disease.
Prospects for improved surveillance
Two improvements in laboratory diagnosis offer the prospect of improved surveillance.
Further details of both are given in the 'Laboratory Diagnosis' module.
• measurement of antibody to the non-structural proteins of FMD virus.
New serological tests offer the prospect of identifying infected animals, even if they have also been vaccinated.
• the Polymerase Chain Reaction.
This is an exquisitely sensitive technique for detecting virus-specific nucleic acids. It holds the prospect of detecting animals
• during the incubation period of the disease
• during the carrier state
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