CLINICAL, SEROLOGICAL AND MOLECULAR ASPECTS OF THE TRANSMISSION CYCLE OF THE BLOOD PARASITE BABESIA OVIS

The blood parasite Babesia ovis (B. ovis, order Piroplasmida, family Babesiidae) is a tick-borne pathogen causing ovine babesiosis, or “tick fever”, in small ruminants, present in Asia, the Mediterranean basin and North Africa. Babesiosis is characterized by fever, loss of appetite, hemoglobinuria, icterus, and death. In this study, the efficacy of B. ovis transmission and the ovine host immune response were studied, by infecting lambs with different stages of its vector, the tick Rhipicephalus bursa (R. bursa) and by infected blood injection. Clinical and serological monitoring showed that infestation of lambs with immature tick stages was largely insufficient, injection of infected blood inflicted babesiosis only in immuno-compromised animals, but adult tick infestation caused severe babesiosis. qPCR analysis of the ovine blood showed presence of B. ovis DNA up to 180 days post-infection, long after the parasite was undetectable microscopically. The presence of B. ovis DNA in the ovine blood was accompanied by high specific antibody response (1:4000 positive titter), detectable at least 180 days post-infection. Examination of B. ovis propagation in unfed adult ticks showed that the relative parasite load increased rapidly upon completion of the tick pre-feeding period, when the tick is ready to feed. These results demonstrate that the major route of B. ovis transmission is via the adult tick stage. The data also suggest that the parasite is circulating in the host several months after the disappearance of clinical signs, and that this is reflected in the host specific antibody response. The sharp increase in the relative parasite load in unfed adult ticks may suggest a synchronization of the parasite infectious state with the tick vector feeding behavior.