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Thirteen isolates of Cowdria ruminantium were made from eight different Districts of Kenya by four different isolation methods. Feeding adult Amblyomma species ticks derived from nymphs collected in the field and the inoculation of homogenates prepared from adult field ticks had the highest success rate. The reattachment of adult ticks collected in the field was successful on only one of five attempts, and the subinoculation of blood from suspected heartwater carriers was unsuccessful. Seven of the isolates were derived from A variegatum ticks, four from A gemma, one from A lepidum and one from a mixed pool of the last two species. This is the first report of the isolation of C ruminantium from A gemma ticks, and the first report of its transtadial transmission from nymphal to adult A gemma.

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Groups of cattle were immunized with 10(-2) dilutions of sporozoite stabilates of Theileria parva lawrencei derived from African buffaloes either alone or in combination with Theileria parva parva derived from cattle and concomitant treatment with either long or short-acting formulations of oxytetracyline. At 90 or 120 days after infection, uninfected Rhipicephalus appendiculatus nymphal ticks were applied to individual immunized cattle and the resultant adults ticks were applied to individual susceptible cattle. Theilerial infection developed from ticks fed on 6 out of 11 animals investigated for evidence of a carrier state. Two additional animals were shown by cell-culture isolation to have persistent theilerial infections. Nine cattle infected with the parasites from carrier animals were treated with paravaquone and 7 recovered. These recovered cattle were then challenged with the original immunizing stabilates at 10 degrees dilution together with the original immunized and carrier cattle. Six out of 7 cattle which had recovered from carrier-derived infection succumbed to this challenge and died but none of the original immunized cattle showed theilerial reactions. When a carrier-derived sporozoite stabilate was used to challenge cattle immune to the original immunizing parasite, they proved to be immune. Cattle immune to the carrier-derived parasites were all immune to challenge with the original parasite. A monoclonal antibody profile against T. parva schizonts isolated by cell culture from samples of the experimental animals did not appear to be sensitive enough to determine the antigenic differences between the carrier-derived parasite and the original immunizing parasite. Indications are that the carrier state is not likely to produce new antigenic strains which would be dangerous to immunized cattle.