Serological diagnosis of heartwater in Zimbabwe. Problems and perspectives

Abstract

We investigated the potential value of immunoblotting (Western blotting) for serodiagnosis of heartwater in Zimbabwe using cultured Cowdria ruminantium as antigen. In experimental infections of cattle, the dominant antibody response against Zimbabwe isolates of C. ruminantium was to a polypeptide of approximately 32 kDa. Florida cattle and sheep sera were uniformly negative to this polypeptide, and sera from heartwater-endemic areas of Zimbabwe were positive, suggesting possible use of this reaction in diagnosis. However, on testing cattle and sheep sera from known Amblyomma and heartwater-free regions of Zimbabwe a large number of positive immunoblot reactions were obtained. These positive sera could not be distinguished from sera from heartwater-endemic regions by dilution. Sheep from heartwater-free regions showing this reaction were negative for C. ruminantium by PCR, did not transmit C. ruminantium to ticks and were fully susceptible to heartwater on challenge. It is likely, therefore, that these are false positive cross-reactions caused by a related organism present in heartwater-free areas of Zimbabwe.