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Antigenic Nature of FMDV
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The first step in antigenic characterisation of isolates of FMD virus is to identify the serotype involved. The antigenic nature of the virus is then examined in more detail using either polyclonal antisera in the strain characterisation ELISA or cross-neutralisation assay, or using monoclonal antisera in MAB profiling assays.
Antigenic characterisation involves the recognition of viruses by antisera, either polyclonal or monoclonal. Strains of FMD virus are characterised antigenically to select suitable vaccine strains. At present, characterisation is performed mainly using polyclonal sera as reactivity of field viruses with post-vaccinal bovine reference sera provides the best predicative indicator of protection.
Predicting protection
Antigenic characterisation is only one of the factors that need to be taken into account when choosing a suitable vaccine. Other important factors to consider are:
• vaccine potency, i.e., ability to elicit a good
antibody response
• vaccine formulation (oil, oil in water, alum)
• vaccination regime
• manufacturer.
Optimising the choice of vaccine is beyond the scope of this module.
Strain characterisation can be carried out by ELISA or virus neutralisation. The results generally correlate well between the two systems and choice of which system to use depends on the time and facilities available. Previously, strains of FMD virus within each serotype were classified into subtypes according to their reactivity with mono-specific antisera raised against defined reference strains of that type, (e.g., A5, A22, A24). However, the antigenic nature of FMD virus is more complex than this simplistic classification and strains are now characterised according to their relationship to frequently used vaccine strains.
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