In addition to currently used serological tests for the occurrence of foot-and-mouth disease virus (FMDV), a specific “passive” hemagglutination-inhibition (HAI) test has been developed as a supplement. Serial twofold dilutions of antiserum (0.05 ml) were mixed with 0.05 ml of a constant concentration of FMDV. After incubating for 30 min at 37 C, agglutinating antibodies were determined by adding 0.1 ml of 2.5% virus-sensitized erythrocytes. The minimum concentration of antiserum required to agglutinate the erythrocytes defined the inhibition in the HAI test. Similar tests using different concentrations of virus to inhibit antibodies were carried out in parallel fashion. The relationship between the logarithm of the HAI titer and the concentration of inhibiting virus was nearly first order (P > 0.25). The slope was used as a measure of the relative specificities of the antigen-antibody interaction and was independent of concentration. The HAI test was type-, subtype-, strain-, and variant-specific with the viral antigens used. In particular, typing was performed directly on bovine antisera.
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