Studies on the origin of false positive reactions to the complement-fixation test for contagious bovine pleuropneumonia.

Serums of cattle free from contagious bovine pleuropneumonia (CBPP) were tested in complement fixation (CF) tests using 3 antigens; these were the standard antigen (SA) used to test to CBPP in Australia, an ethanol extract antigen (EA) also prepared from Mycoplasma mycoides subsp. mycoides (M. mycoides) and an antigen prepared from a Group 7 bovine mycoplasma isolated from arthritis (AA). The serums included 146 which fixed complement with SA. Eighty percent of these false-positive serums reacted with AA but not with EA; the other 20% were positive at low titre with EA but gave no reaction with AA. Attempts were made to produce false-positive serums experimentally by inoculating 3 Group 7 mycoplasmas and 2 Mycoplasma bovigenitalium strains into cattle. Serums from 3 of 9 cattle inoculated with strain L2917 (Group 7) reacted with SA but differed from the false-positive serums of field cattle by reacting with all 3 antigens. Tests with serums from cattle experimentally infected with CBPP gave similar titres with SA and EA, but the results with AA were mostly negative or less than 10% of the titres obtained with SA and EA. The results of CF tests on serums from the experimental cattle, after absorptions with suspensions of the mycoplasmas, showed that there was a one-way serological relationship between strain L2917 and M. mycoides and between this Group 7 strain and M. bovigenitalium. The CF tests with 3 antigens have assisted in demonstrating the false-positive nature of the reacting field serums encountered in the course of routine CF tests for CBPP in cattle in Australia.