False positive serological reactions in bovine brucellosis: evidence of the role of Yersinia enterocolitica serotype O:9 in a field trial

To investigate the epidemiology of false positive serological reactions (FPSR) in bovine brucellosis, 1 259 bovines from 20 herds were sampled on three successive occasions during the winter of 1993-1994 in an area where the herd prevalence rate of FPSR was high. Serum samples were examined by classical brucellosis serological tests (Rose Bengal and complement fixation) and faeces were cultured for the presence of Yersinia enterocolitica O:9. Thirty-nine bovines expressed at least one positive serological reaction during the study. In the herds with FPSR during the 1993-1994 annual brucellosis surveillance campaign, the specificity of the brucellosis serological tests varied significantly from December to March (97.0% to 99.1%). Y enterocolitica O:9 was isolated from 42 bovines but only three of them showed a positive serological response during the study. Y enterocolitica O:9 isolation rates also decreased with time. Young animals and animals having demonstrated FPSR in the past had a greater risk of having a FPSR. Older animals, which rarely showed FPSR, could form a reservoir for Y enterocolitica O:9. While isolation of Y enterocolitica O:9 was not linked to presence of FPSR and conversely, the FPSR phenomenon should be considered, either at the herd level or at the individual level. This work reinforces the link, at least partial, between FPSR and infection by Y enterocolitica O:9.