The Role of the Deer Tick in Human Babesiosis

Transmission of Babesia microti in humans takes place primarily through direct tick-host contact, although transfusion-acquired human babesiosis cases have been documented on several occasions. Ixodes scapularis becomes infected with B. microti most likely while feeding on infective reservoir hosts. In the northeastern United States, this enzootic cycle is maintained principally between immature I. scapularis and their primary animal host, the white-footed mouse (P. leucopus). Larvae acquire the infection when feeding on B. microti-infected mice. Engorged larvae then overwinter and pass the parasites transstadially to the subsequent nymphal stage the following spring. Research indicates that B. microti can survive in salivary glands of ticks for 9-10 months. Nymphs infected from the larval stage are able to transmit the infection to susceptible animals upon feeding. In areas where B. microti is present, infection rates within tick populations typically range between 5 and 40%. Adults primarily feed on white-tailed deer (O. virginanus), which is not a competent reservoir for B. microti. In addition, attempts to demonstrate B. microti infection in adult Ixodes ticks have failed. Apparently, B. microti parasites acquired by larval I. scapularis do not survive into the adult stage and only about 25% of the adults that derived the infection as nymphs become infectious. Transovarial transmission of the infection in I. scapularis has not been documented. Therefore, it appears that the role of adult I. scapularis in the transmission of B. microti infection is minimal.

The development of B. microti within I. scapularis has been well documented. It has been reported that after larval ticks acquire B. microti, parasites enter the gut lumen from hemolyzed erythrocytes and undergo a complex differentiation, including gametogenesis and fertilization. When an infected tick attaches to a host, sporozoites are readily shed into the saliva and injected into the animal during the feeding process.

The prevalence of Ba. microti and Bo. burgdorferi in I. scapularis populations has been determined in some endemic areas. In Massachusetts, approximately 24% of nymphs and 47% of adults were infected with Bo. burgdorferi while only 11% of nymphs and 14% of adults were infected with Ba. microti. They suggested that spirochetal infection are more abundant than babesial infection in those ticks examined. In the laboratory, similar disproportionate rates of infection have been observed among nymphs, feeding as larvae, on either Ba. microti- or Bo. burgdorferi-infected mice. Concurrent infection by both pathogens in nymphal I. scapularis ticks has also been described. This is consistent with the findings that P. leucopus are sometimes simultaneously infected with both pathogens and immature I. scapularis ticks abundantly feed on this rodent species. Experimentally, ticks fed on mice with concurrent spirochetal and babesial infections exhibited twice the incidence of spirochetal infection than babesial infection, suggesting that acquisition and transstadial transmission of Ba. microti are less efficient than those of Bo. burgdorferi.

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