Abstract
Summary In direct complement-fixation tests, the quantitative behaviour of FMD viral antigens with hyperimmune guinea-pig sera was found to be similar to that noted in a number of viral antigen-antibody systems previously studied. The relative complement-fixing capacity of several FMD antigens with different guinea-pig antisera was estimated from the slope of the complement-antigen regression lines. Such a comparison showed a much closer resemblance in the complement-fixing behaviour of the strain of A type FMD virus isolated during the recent Canadian outbreak and an A type strain isolated in England at about the same time, than either displayed toward a stock A strain, No. 119, that had been maintained for many years by cattle passage. Sera of only two of the convalescent cattle tested, exhibited fixation of guinea-pig complement with FMD virus antigen. The relatively rapid development of inhibitory or neutralizing activity in the sera of cattle infected with FMD virus could be detected, however, by an indirect complement-fixation method with FMD antigens; pre-infection bleedings from the same animals did not show this activity. In lower dilutions, these convalescent sera blocked or neutralized the complement-fixing activities of FMD viral antigens of heterologous as well as of the homologous type. In higher serum dilutions the inhibitory activity extended only to virus of the homologous type. At the present time, there is an evident need for a satisfactory in vitro method of demonstrating antibody in the sera of cattle convalescent from foot-and-mouth disease. The direct complement-fixation test has proved of limited value for this purpose since only a small proportion of cattle develop complement-fixing activity with FMD antigens. The preliminary studies presented here suggest that the indirect complement-fixation test, particularly if used in parallel with one of the direct form, merits further investigation from this practical standpoint.
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