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Circulating antibody was detected by an indirect fluorescent antibody test (IFAT) in the serum of sheep infected experimentally with Eperythrozoon ovis. Antibodies were first detected 15 to 32 days after infection with E ovis and titres peaked at 41 days. This antibody may be associated, at least in part, with protection against infection with E ovis since the initial increase in antibody titre coincided with a fall in the primary parasitaemia. A role for antibody is suggested further by the fact that the prepatent period of infection was prolonged by one day and the parasitaemia initially remained at low levels in infected sheep protected by passively transferred hyperimmune serum. Moreover, following primary infection, acquired immunity was manifest by a lack of parasitaemia following challenge infections while increased IFA titres were observed. No evidence of opsonic activity was observed in an in vitro erythrophagocytosis test in that neither mouse macrophages nor sheep monocytes phagocytosed E ovis infected or uninfected erythrocytes sensitised with hyperimmune serum.

Non-splenectomised Cheviot and Soay sheep experimentally infected with Eperythrozoon ovis were treated subcutaneously with two doses of 4 mg/kg of imidocarb dipropionate 24 hours apart. A rapid reduction in E. ovis parasitaemias following the first injection of imidocarb was observed and E. ovis organisms were not demonstrable 48 hours after the first treatment. However, recrudescences of infection were observed 14 and 28 days after treatment in the Soay and Cheviot sheep respectively. Side-effects associated with treatment were registered in the Cheviot sheep but not in the Soay sheep.