A serological technique using electron microscope grids coated with protein A and antiserum was able to detect foot-and- mouth disease virus particles in oesophageal-pharyngeal fluids from infected cattle without the need for prior concentration of the sample. The technique was adapted to differentiate serologically among foot-and-mouth disease virus types A, O and C with antigen-adsorbed sera. When grids were coated with heterotypic antigenadsorbed antisera, the homotypic antigen could be observed in viral specimens containing 10(5) PFU/mL, but the heterotypic antigen was not visualized until its concentration was about tenfold higher. Grids coated with the appropriate antigen-adsorbed antiserum can thus be used to indicate foot-and-mouth disease viral serotypes in specimens containing less than 10(6) PFU/mL.
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