Human babesiosis: Recent discoveries

Abstract

Babesiosis is caused by intraerythrocytic parasites of the genus Babesia, which is a common animal infection worldwide. This protozoa requires both a competent vertebrate and a nonvertebrate host (Ixodes sp. etc.) to maintain the transmission cycle. Human babesiosis is predominantly caused by Babesia microti (rodent-borne piroplasm, an emerging zoonosis in humans in North America) and by Babesia divergens (bovine pathogen, in Europe). Occasionally, infection in America is caused also by a newly recognized species, so-called WA1 piroplasm. The spectrum of human babesiosis in the USA is broad, and ranges from an apparently silent infection to a fulminant. In Europe, babesiosis is considerably rarer, but more lethal (42% mortality rate in Europe and 5% in the USA, for clinically apparent infections) and mostly in splenectomized patients. Various determinants are involved in the severity of infection, such as age, immunocompetence and coinfection with other pathogens (Borrelia burgdorferi). B. microti antigens can trigger specific activation of T-cells and the infection can be effectively controlled by a Th1-dominant CD4+ T-cell response. The diagnosis of babesiosis should include examination of blood smears stained by Giemsa, as well as serologic evaluation with indirect immunofluorescent antibody tests and possibly PCR. The treatment of babesiosis depends on severity of cases; if it is mild it resolves spontaneously, whereas very severe cases with B. divergens require prompt treatment that includes erythrocyte exchange transfuision along with intravenous clindamycin and oral quinine to arrest hemolysis and prevent renalfailure. This paper offers an overview of recent developments in the investigation of Babesia sp. and babesiosis.