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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.

The role of the spleen and rosette-formation responses was investigated in sheep experimentally infected with Eperythrozoon ovis. Phagocytic activity was observed in the spleen 19 days after primary infection. Phagocytosis of E. ovis-parasitised and non-parasitised erythrocytes by cordal reticular cells occurred. E. ovis organisms seemed to be detached from the erythrocytes by pseudopodia extending from macrophages and cordal reticular cells without causing damage to the plasmalemma of the erythrocyte. No phagocytic activity was observed in spleens removed 74 and 146 days after infection. Antigen-specific lymphoid cell responsiveness, assessed by rosette formation, indicated that 2.8, 15.4, 8.0 and 6.0% of lymphoid cells in the spleens of the four E. ovis-infected sheep, respectively, formed antigen-specific rosettes. Rosette formation did not occur when splenic lymphocytes from E. ovis-infected sheep were mixed with non-infected erythrocytes or when splenic lymphocytes from an uninfected sheep were used.

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.