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Evidence to show that an agent that cross-reacts serologically with Cowdria ruminantium in Zimbabwe is transmitted by
D T Savadye1, P J Kelly, S M Mahan
1University of Florida/USAID/SADC Heartwater Research Project, Veterinary Research Laboratory Diagnostic and Research Branch, Harare, Zimbabwe.
Abstract:
The serological diagnosis of heartwater based on reactions to the immunodominant Cowdria ruminantium major antigen protein-1 (MAP-1) is impaired by the detection of false-positive reactions. In this study, the prevalence of false-positive reactions on seven heartwater-free farms in Zimbabwe was determined to be 8-94% by immunoblotting against C. ruminantium antigens. The highest prevalence of false-positives on Spring Valley Farm correlated with the presence of Rhipicephalus evertsi evertsi ticks. The other tick species found on these seven farms were Hyalomma truncatum and Hyalomma marginatum rufipes. Rhipicephalus evertsi evertsi ticks collected from Spring Valley Farm and fed on seronegative sheep caused seroconversion in one of two sheep. This sheep developed a mild febrile reaction and C. ruminantium MAP-1 antigen reactive antibodies 3 weeks after the ticks started feeding. Polymerase chain reactions (PCRs), conducted using C. ruminantium-specific primers on ticks collected from the seven farms and on some of the R. e. evertsi ticks that had caused seroconversion in one sheep, were negative. However, some of these ticks gave positive PCRs with DNA primers which amplify a 350 bp DNA fragment of the 16s rRNA gene from all ehrlichial agents indicating the presence of infection with one or more Ehrlichia species. Although attempts to isolate the cross-reacting agent from the sheep were unsuccessful, this study demonstrates that false-positive reactions with the MAP-1 C. ruminantium antigen are associated with agents transmitted by ticks.
Insights
False-positive heartwater diagnoses occur due to tick-borne agents, not Cowdria ruminantium. This study links Rhipicephalus evertsi evertsi ticks to these misleading serological reactions in sheep.
Area of Science:
- Veterinary Immunology
- Molecular Diagnostics
- Parasitology
Background:
- Serological diagnosis of heartwater, a significant livestock disease, relies on detecting antibodies to Cowdria ruminantium major antigen protein-1 (MAP-1).
- False-positive reactions in MAP-1 based serology can impair accurate diagnosis and disease management.
- Understanding the causes of these false positives is crucial for improving diagnostic specificity.
Purpose of the Study:
- To investigate the prevalence and potential causes of false-positive serological reactions to Cowdria ruminantium MAP-1 antigen in heartwater-free regions of Zimbabwe.
- To identify potential tick vectors associated with these false-positive reactions.
Main Methods:
- Prevalence of false-positive reactions was determined using immunoblotting against C. ruminantium antigens on farms in Zimbabwe.
- Tick species on affected farms were identified.
- Ticks, specifically Rhipicephalus evertsi evertsi, were fed on seronegative sheep to observe serological and clinical responses.
- Polymerase chain reaction (PCR) was used to detect C. ruminantium and other Ehrlichia species in ticks.
Main Results:
- A high prevalence (8-94%) of false-positive reactions to C. ruminantium MAP-1 was observed on heartwater-free farms.
- The highest prevalence of false positives correlated with the presence of Rhipicephalus evertsi evertsi ticks.
- Sheep exposed to R. e. evertsi ticks showed seroconversion to C. ruminantium MAP-1 antigen and mild febrile reactions.
- PCR analysis of ticks did not detect C. ruminantium but indicated the presence of other Ehrlichia species.
Conclusions:
- False-positive serological reactions for heartwater using the C. ruminantium MAP-1 antigen are associated with infection by other tick-borne agents, likely Ehrlichia species.
- Rhipicephalus evertsi evertsi ticks may transmit these cross-reacting agents.
- Diagnostic assays need to account for cross-reactivity to ensure accurate heartwater diagnosis.