Babesia microti and Borrelia burgdorferi follow independent courses of infection in mice.

Reports of coinfection with Borrelia burgdorferi and Babesia microti in tick vectors, reservoir hosts, and patients have led to increased concern that synergism between the 2 organisms may result in illness more severe than that caused by either infection alone. In the present study, we investigated the impact of simultaneous Lyme disease and babesiosis in the mouse model. Young immunocompetent, young asplenic, and aged C3H/HeN mice, as well as young BALB/c mice, were coinfected with B. burgdorferi and B. microti, and disease severity was compared with that in singly infected and uninfected control mice. Babesiosis followed its normal course of infection in coinfected mice, without evidence for increased severity, as reflected by percentage of parasitemia, spleen weights, and hematologic and clinical chemistry parameters. Likewise, Lyme disease followed its established course and severity in coinfected mice, as reflected by the degrees of spirochete dissemination and arthritis. This study demonstrates that, in the mouse model, these 2 infections proceed independently.