The experiments here described were undertaken in order to determine (a) the duration in sheep of immunity to heartwater and whether immunologically different strains exist, and (b) how the immunity is maintained. Among ten strains from different parts of South Africa no immunological difference was observed. During the observation a new virus was encountered to which the name " virus A " was given. The mortality from this virus was less than 2 % and it appeared to be harboured by some of the passage sheep. Unlike the causal agent of heartwater, this virus produces a mild reaction in horses. The duration of immunity was solid in all cases up to six months and in varying periods up to 58 months after recovery. In a few cases from 7-84 months after recovery, febrile reactions due to the test injection occurred. It is suggested that these partially immune animals may act as main-tainers of heartwater in treks where fully susceptible animals are absent. Splenectomy in recovered cases did not cause a relapse of heartwater. Rickettsia ruminantium could not be demonstrated in recovered sheep at periods longer than 60 days after inoculation
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