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Virus Isolation in Tissue Culture
Antigen Detection ELISA
Polymerase Chain Reaction
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Early recognition in the field coupled with rapid laboratory diagnosis of the viral agent is of prime importance in limiting the spread of any FMD infection. Definitive diagnosis of FMD is based on the demonstration of FMD virus or antigen in tissue samples. Consequently, laboratory assays need to be highly sensitive and specific.
The complement fixation test (CFT) had long been the test of choice for FMD virus antigen detection but the CFT has many disadvantages: pro- and anti-complementary factors commonly invalidate the assay; test results are read subjectively and can vary between test operators; the CFT is complex, reagents have a short shelf-life and must be stringently maintained, and, most importantly, the CFT is relatively insensitive.
An indirect, sandwich antigen detection ELISA was subsequently developed, its superiority for routine FMD virus diagnosis demonstrated, and the assay adopted for regular use. The particular advantage of the indirect, sandwich ELISA is that the rabbit trapping type-specific antibodies will select the FMD virus antigen from other competing protein in sample suspensions and ensure that its amount is semi-concentrated to increase the chances of its detection. Other advantages of the ELISA include: increased sensitivity over the CFT; ability to serotype degraded preparations of FMDV efficiently; pro- and anti-complementary activities are of no importance; ELISA plates can be read spectrophotometrically, i.e., objectively, giving greater accuracy to results; it is rapid, highly reproducable and simple to perform, the reagents have a long shelf-life and can be readily standardised.
Ideally, the ELISA should be used in combination with virus isolation in tissue culture in susceptible cell cultures to amplify low amounts of antigen in poor quality specimens to a concentration detectable by ELISA. In the absence of cell cultures more than one sample should be submitted from the herd to increase the chance of detection.
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