Vaccination against heartwater using in vitro attenuated Cowdria ruminantium organisms.

Sequential passage of Cowdria ruminantium (Senegal isolate) in cultures of bovine umbilical endothelial cells has resulted in loss of virulence without loss of immunogenicity, as previously demonstrated. We have carried out further immunization of 39 Dutch sheep using in vitro attenuated rickettsiae of passage 21 and challenged these animals either with the homologous or with heterologous Cowdria stocks. After vaccination several sheep developed elevated rectal temperatures for a maximum of 2 days, but no further clinical response to the vaccine was observed. All sheep developed high titres of antibodies to Cowdria. Challenge of 10 sheep with the homologous virulent stock did not provoke any clinical reaction, demonstrating that these animals were solidly immune. Reactions to heterologous challenge varied from virtually no reaction to fatal heartwater depending on the stock of Cowdria used. These results are discussed in relation to currently available vaccination methods against cowdriosis. In Senegal 30 susceptible sahelian sheep were immunized with attenuated rickettsiae of passage 21. Hyperthermia was seen in 13, the only other clinical symptom was a temporary diarrhoea. The immunized animals are at present exposed, together with 30 controls, to field challenge in the Niayes, the area where the Senegal isolate was originally isolated.