Babesia and its close relatives are members of a group of organisms called piroplasms, a name which comes from their pear-shaped outlines. Long associated with blood diseases of cattle and other mammals, members of the genus Babesia have been recognized since the 1950s as infectious agents in humans. Species of this protozoan blood parasite that have routinely been isolated from mice B. microti or cattle B. divergens have also been isolated from humans. In addition to these familiar species, new isolates that resist being placed in existing taxonomic categories are the basis for rethinking their phylogenetic relationships based on sequencing data. The parasite represents a threat to the safety of the blood supply in that blood from asymptomatic humans can transmit Babesia to blood recipients. Such transmissions have occurred.

     The development of methods for cultivation of these organisms represents a significant opportunity to study their biology and disease potential. In addition, in vitro cultivation has provided a basis for studying immune responses of mammals to these infectious agents, with the hope of ultimately producing attenuated strains that could be used for immunizing of cattle and, perhaps, humans who live in areas of endemicity. The microaerophilous stationary phase culture technique, which uses a tissue culture medium base supplemented with appropriate serum and erythrocytes, has made it possible to obtain large numbers of parasitized erythrocytes for studying the biology of this parasite. Members of the genus Babesia are better known to those with an interest in animal parasitology than those focused on human disease, but in recent years, there has been a developing interest in this protozoan parasite with recognition of its role as a zoonotic agent of human disease, i.e., a disease communicable from animals to humans. Babesiosis, as the disease is known, is found in a wide variety of mammals but is perhaps most prevalent in rodents, carnivores, and cattle. In excess of 100 species of Babesia have been described from mammals. Like the malaria agent, the parasite attacks and damages host erythrocytes. The disease is transmitted to humans by infected ticks, a route shared with the agents of spotted fever, borreliosis, and ehrlichiosis; human-to-human transmission does not occur with the exception of transfusion-mediated spread. Babesia is placed in the same taxonomic group, the Piroplasmidaas another blood parasite, Theileria. The group name derives from the pear-shaped appearance of the parasites as seen in infected erythrocytes. Theileria, however, has not been reported from humans. The isolation of a new strain of Babesia with features of Theileria and the reexamination of several existing babesias as possible theilerias has introduced uncertainty in the separation of the two genera.

     The babesia organism was named after Viktor Babès, who first recognized it in blood cells of cattle in the late 19th century. Babesiosis was also the first vector-borne disease to be described in the literature by Smith and Kilbourne, as the etiologic agent of Texas cattle fever. Babesia shares a close affinity with malaria parasites in its intraerythrocytic niche in the host, which can and has led to its being mistaken for Plasmodium spp., the malaria agent. It differs significantly from the genus Plasmodium in a number of features. First, there is no exoerythrocytic stage in its development as there is in the case of Plasmodium, although this is now open to question. Mehlhorn and Schein have suggested that B. microti and B. equi initially invade lymphocytes prior to invading erythrocytes, similar to the development of Theileria spp. Subsequently, Mehlhorn and Schein have shown that B. equi indeed initially invades lymphocytes, serving as the basis for their reclassification of B. equi as T. equi. Additionally, Babesia divides by binary fission and not schizogony, develops within the cytoplasm of the host erythrocyte and not in a parasitophorous vacuole as does the malaria parasite, and does not form the characteristic pigment in infected erythrocytes as does Plasmodium. The insect vectors for Plasmodium spp. are anopheline mosquitoes, while Babesia spp. are transmitted by ticks, either transovarially for the large babesias or transstadially for B. microti. A variety of factors are taken into account in diagnosing babesiosis, most importantly patient history and examination of blood smears. Detection of Babesia infections is most often by examination of stained blood smears. There are, however, drawbacks in reliance upon blood smears alone. Identification of the parasites in thin blood films requires a reasonable level of parasitemia, otherwise the parasites might be missed. Some Babesia spp., because of their small size, might not be seen in thick blood smears. Inoculation of hamsters with samples of blood suspected of being infected with B. microti, for example, is also useful, especially when the level of parasitemia is low. This approach, however, is time-consuming, requiring 7 to 10 days before parasites are visible. Gerbils and splenectomized calves have been used for diagnosis of B. divergens infections.Mice, whose erythrocytes had been replaced by human erythrocytes, were used to diagnose babesiosis caused by a B. microti like parasite in an asymptomatic human. Indirect immunofluorescence is also used in diagnosis. However, since cutoff titers vary between species of Babesia and the infected host, it is necessary that cutoff values be carefully established by each testing laboratory. Cross-reactivity of surface antigens may occur between Babesia spp., as well as between Babesia and Plasmodium, making it potentially difficult to identify different species or to clearly distinguish babesiosis from malaria.