Babesiosis is emerging as a disease of public health significance in the U.S.A. with increased reports of clinical, even fatal, cases in areas where the risk of infection with Babesia was not recognized previously. The potential importance of these hemoprotozoal parasites is also evidenced by the increased risk of tick-transmitted Babesia in Lyme disease endemic areas and the enhanced susceptibility of patients with human immunodeficiency virus infections to severe babesiosis. Until recently, human babesial infections in this country have been attributed to B. microti, derived from rodents. The proposed project is designed to investigate two hypotheses. The preliminary hypothesis is that human babesiosis in the U.S.A. may be caused by babesial parasites that are antigenically and genotypically distinct from B. microti. To investigate this possibility, Babesia will be isolated from asymptomatic carriers, primarily in Lyme disease endemic areas, and from patients with clinical babesiosis. The isolates will be characterized and compared molecularly to morphologically similar Babesia isolates from humans and animals in other geographic locations. The emphasis of our studies will focus on parasites that represent genotypically distinct isolates, are particularly virulent in humans and/or were isolated from a patient after recovery from clinical disease or from an asymptomatic carrier. Isolates selected by these criteria will be utilized to test the primary hypothesis, which is that small babesial parasites, such as B. microti and other human babesial isolates in the U.S.A., may have an exoerythrocytic developmental stage, which if like the intralymphocytic schizont stage of Theileria, would be capable of inducing host cell transformation and proliferation. To evaluate this possibility, tissues from babesiosis patients and hamsters infected by tick-transmission of human babesial isolates will be examined for the presence of exoerythrocytic parasite stages. Selected tissues with lymphoproliferative foci and/or potential intracellular parasites will be evaluated further by in situ hybridization of fluorescent-labelled, Babesia-specific ss-rDNA probes. In vitro cultivation techniques developed for the maintenance of Theileria schizont-infected lymphoblastoid cell will be employed to isolate similar stages in the blood and/or lymphoid tissues of humans and hamsters with tick-derived Babesia infections. In addition, the infectivity of tick-derived babesial sporozoites for human mononuclear cells in vitro will be determined. The proposed study will provide valuable information about the basic biology of these Babesia parasites and methodology which can be applied to improve the diagnosis and control of human babesiosis.
