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Primary diagnosis of FMD commonly involves recognition of typical clinical signs in affected
animals. Before an outbreak is declared it is usual to demonstrate FMD virus or antigen using
laboratory tests. FMD virus can be grown on a variety of cell cultures of bovine, ovine,
caprine or porcine origin, and also BHK21 cells, but these different systems exhibit
different sensitivities to the virus. The most sensitive tissue culture is primary bovine
thyroid cells (BTY), although some strains of FMD virus derived from pigs may grow
preferentially on pig cell cultures, such as pig kidney (IBRS-2).
Samples of epithelial tissue received for diagnosis are prepared and inoculated onto tissue
cultures. At the same time this material is used in a direct antigen detection enzyme-linked
immunosorbent assay (ELISA), which, if sufficient FMD virus antigen is present in the sample,
will both provide a diagnosis of FMD and serotype the sample. Additional rows on the ELISA
plate can be used for differential diagnosis of swine vesicular disease (SVD). The ELISA can
detect FMD virus antigen even if there is no viable virus present in the sample. The majority
of samples which are positive by ELISA are also positive in tissue culture. All cultures
showing cytopathic effect (CPE) are tested in the antigen detection ELISA to confirm the
presence of FMD virus and to determine the serotype involved.
Samples in which there is insufficient antigen to be positive by ELISA using original
suspension may still contain live virus which can be amplified by tissue culture. This
emphasises the importance of using the most sensitive culture system. If a cytopathic effect
is seen in the culture within 48 hours of inoculation, the supernatant can be used in the FMD
virus antigen detection ELISA. If no cytopathic effect is seen after 48 hours, the culture
may be frozen and thawed and the centrifuged supernatant used to inoculate further tissue
culture (blind passage). When very low concentrations of virus are present in the original
sample, serial blind passage may be required for sufficient virus to produce an observable
cytopathic effect.
New techniques are being applied for primary diagnosis, such as polymerase chain reaction
(PCR), using FMD virus group specific or serotype specific primers. Used in addition to ELISA
and cell cultures, the number of positive samples is increased. However, some samples are
tissue culture/ELISA positive and PCR negative, and therefore PCR should not be used in
isolation.
The presence of specific FMD virus antibody in a serum sample indicates that the animal from
which the sample was collected has had contact with FMD virus or antigen. Serological tests
cannot yet reliably distinguish between antibody resulting from infection with FMD virus or
that following vaccination with inactivated virus, but a number of tests which measure
antibody to the non-structural proteins of the virus have been described. A diagnosis of FMD
is not usual on serological evidence alone, but if FMD virus antibody is found in animals
with no history of vaccination, the presence of FMD in the area is considered likely.
Successful virus isolation is usually followed by virus characterisation.
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