Babesioses are tick-transmitted infections of great veterinary importance. They are caused by protozoa of about 100 different species in the genus Babesia. Two species are known to cause human disease, that is, B. microti transmitted by Ixodes scapularis (I. dammini) in north-eastern US and B. divergens transmitted by I. ricinus in Europe.

The natural host of B. microti in the US is the white-footed mouse, Peromyscus leucopus. Infection with B. microti in humans is usually self-limiting. Many people have an asymptomatic infection, and the severity of the disease symptoms usually increases with age. Severe disease due to B. microti is usually seen only in the elderly or other people with a weakened immune defence. In young people the infection usually only causes sero-conversion without any disease symptoms. B. microti also occurs in Europe where it is transmitted mainly by I. trianguliceps among rodents. Since I. trianguliceps is relatively host specific and (almost) never feeds on humans the risk for B. microti infection is almost non-existent in Europe ( Jaenson 1988).

Most patients who have contracted human babesiosis in Europe had been splenectomized some time before the appearance of symptoms of this disease. The course of the disease resembles that of malignant malaria. The mortality rate has been about 50%. The causative organisms in all cases of European human babesiosis seems to have been B. divergens which is naturally transmitted by I. ricinus among cattle.

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