Babesia microti: from Mice to Ticks to an Increasing Number of Highly Susceptible Humans

Abstract

Babesia microti, a zoonotic intraerythrocytic parasite, is the primary etiological agent of human babesiosis in the United States. Human infections range from subclinical illness to severe disease resulting in death, with symptoms being related to host immune status. Despite advances in our understanding and management of B. microti, the incidence of infection in the United States has increased. Therefore, research focused on eradicating disease and optimizing clinical management is essential. Here we review this remarkable organism, with emphasis on the clinical, diagnostic, and therapeutic aspects of human disease.

Keywords: Babesia microti; United States; clinical significance; diagnosis; epidemiology; immunosuppression; infection; life cycle; prevention; relapse; transmission; treatment.

FIG 1

Life cycles of Ixodes scapularis and Babesia microti. Ixodes scapularis has a four-stage life cycle that generally lasts 2 years and includes vertebrate hosts (including rodents, deer, or humans), with each mobile tick stage having a blood meal on a different individual host. During the first year, mated adult female ticks detach from their vertebrate host to overwinter (1), and they lay eggs in the spring. Eggs hatch in the spring (2), and larvae attach to their first vertebrate host (usually small rodents or birds) (3). The six-legged larvae feed on the first host, generally in late summer, and may become infected with B. microti while taking a blood meal. Engorged larvae leave the host, overwinter, and molt into nymphs (4). In the spring of the second year, nymphs attach to a second vertebrate host (e.g., rodents, deer, or humans) (5); in the fall, they leave the second host to molt into adults and attach to a third vertebrate host (such as deer or humans) (6). Babesia microti has a two-host life cycle, including I. scapularis as the definitive host and a vertebrate intermediate host. In the tick (7), gametes ingested during a blood meal undergo sexual reproduction, leading to the eventual formation of infectious sporozoites. In the mammalian host (e.g., rodents or humans) (8), sporozoites initiate the erythrocytic cycle, resulting in the formation of trophozoites that divide asexually by budding. Subsequently, trophozoites transform into merozoites and perpetuate the erythrocytic cycle or gametes are ingested by ticks during a blood meal to initiate the sexual cycle. All mobile stages of I. scapularis may acquire B. microti parasites from infected first or second vertebrate hosts, but only nymphs and adults transmit parasites.

FIG 2

Photomicrographic images of Babesia and P. falciparum in thin blood films. (A) Images of Babesia showing small, pleomorphic, vacuolated rings. (B) Babesia organisms exhibiting pleomorphic rings and characteristic tetrad (“Maltese cross”) forms. (C) Extracellular forms of Babesia parasites. (D) Ring-form trophozoites of P. falciparum, for comparison, showing multiply infected RBCs, appliqué forms, and thin cytoplasm with double-chromatin dots. Original magnification for all images, ×1,000, with oil immersion.