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Figure 2 Giemsa-stained thin blood film. Four mature merozoites form the 'tetrad' or 'Maltese cross' prior to schizogony (single arrow). (Courtesy of Philip r. Daoust md.)

Figure 3 Giemsa-stained thin blood film of a patient with B. microti. Parasites often adopt intracellular ring forms that resemble Plasmodium falciparum. (Courtesy of Philip R. Daoust md.)

ium infection, does not occur because schizogony is asynchronous.

Host response to babesia is poorly understood; however, the spleen is known to play a central role in immune defense. Asplenic patients and animals are more susceptible to infection by babesia and have increased parasitemia. Splenectomy and steroid therapy in animals and patients that have recovered from babesiosis may result in recrudescence of disease. As blood percolates through the spleen it is in close contact with splenic endothelium, macrophages and macrophage products. Erythrocytes must squeeze through intraendothelial spaces. Infected, deformed and potentially more rigid infected erythrocytes are more likely to be retained, where ingestion by macrophages or the action of macrophage products may serve to control the infection. Complement-sensitized erythrocytes would likely bind under these circumstances. Furthermore, complement activation by babesia could theoretically lead to the generation of tumor necrosis factor a (TNFa) and interleukin-1 (IL-1), enhancing local defense. Decreased complement levels, presumably secondary to activation, are frequently found in babesiosis. In addition, increased circulating Clq binding activity, presumably due to immune complexes, with a commensurate decrease in C4, C3 and CH50 levels, are seen in patients. The generation of these primarily macro-phage-produced mediators (TNFa and IL-1) in turn could explain many of the clinical features (fever, anorexia, arthralgias, myalgias) of babesiosis, especially the fulminant shock syndrome of bovine babesiosis. It is likely that, in the less fulminant situation, TNFa functions to enhance the killing of Babesia, as with malaria. In addition to macrophage factors, other cellular immune functions appear to be important in the host response to Babesia. Nude mice and mice with greatly reduced T lymphocyte responses due to thymectomy, lethal irradiation or anti-T-lymphocyte serum develop significantly greater parasitemia. The disease itself alters cellular immune function. Patients with acute babesiosis have an increase in T suppressor/cytotoxic lymphocytes and decreased responses to lymphocyte mitogens with a polyclonal hypergammaglobulinemia.

See also: Parasites, immunity to; Ungulate immune systems.

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