Establishment of a continuous culture of in human erythrocytes reveals unusually high tolerance to recommended therapies.

Human babesiosis is an emerging tick-borne disease caused by apicomplexan parasites of the genus Clinical cases caused by have been associated with high parasite burden, severe pathology, and death. In both mice and hamsters, the parasite causes uncontrolled fulminant infections, which ultimately lead to death. Resolving these infections requires knowledge of biology, virulence, and susceptibility to anti-infectives, but little is known and further research is hindered by a lack of relevant model systems. Here, we report the first continuous culture of in human red blood cells. We show that during its asexual cycle within human erythrocytes, develops and divides to form four daughter parasites with parasitemia doubling every ∼22 h. Using this culture assay, we found that has low susceptibility to the four drugs recommended for treatment of human babesiosis, atovaquone, azithromycin, clindamycin, and quinine, with IC values ranging between 500 nm and 20 μm These data suggest that current practices are of limited effect in treating the disease. We anticipate this new disease model will set the stage for a better understanding of the biology of this parasite and will help guide better therapeutic strategies to treat associated babesiosis.