Small ruminant babesiosis
Babesiosis is a disease complex caused by unicellular Babesia parasites, which are transmitted by ticks and invade and proliferate in the red blood cells of vertebrate hosts. They cause a febrile disease of domestic and wild animals characterised by extensive erythrocytic lysis leading to anaemia, icterus and haemoglobinuria, which can be fatal. In contrast to Theileria, Babesia parasites do not have a pre-erythrocytic stage in the vertebrate host. Babesia parasites are among the most widely distributed blood parasites and have a considerable economic, veterinary and medical impact worldwide. A number of species are gaining interest as emerging zoonosis in humans. For example, approximately 60 human cases mostly due to infection with B. divergens have been recorded in Europe .
Live attenuated vaccines for B. bovis and B. bigemina have been applied in many countries such as Australia, Argentina, South Africa, Brazil and Israel. It is postulated that attenuation through passage in culture results in enrichment of less virulent parasite populations or in down-regulation of virulence . To date, no vaccine has been developed against small ruminant babesiosis. However, due to great similarities between the bovine pathogen B. bovis and B. ovis, the techniques developed for cultivation and attenuation of bovine babesiosis are likely to be applicable for establishing culture systems for Babesia of small ruminant and to achieve their attenuation.
Both cellular and humoral immune responses are activated during Babesia infection. Thus, the resolution of acute B. bovis infection in naive animals is largely achieved by activation of the innate immune response, involving activation of macrophages via IFN-gamma , and resulting in killing of the organisms by phagocytosis and production of toxic macrophage metabolites, including nitric oxide (NO) . On the other hand, the protection against clinical disease in persistently infected or vaccinated cattle relies upon the activation of memory and effector CD4+ T cells that secrete IFN-gamma . Activated macrophages also secrete other products that include, TNF-alpha , IL-18 and IL-12, the latter activates natural killer (NK) cells to produce enhanced levels of IFN-gamma and activated effector function . During Babesia infection, antibodies can reach high titres shortly after the infection and can neutralise sporozoites or merozoites at the extracellular stage . The mechanisms of the immune response against small ruminant Babesia have not been adequately investigated. Due to the similarities between bovine and ovine Babesia species, particularly between B. bovis and B. ovis, it is possible that similar immune reactions are induced in sheep after an infection with B. ovis.