HUMAN BABESIOSIS
Tick Vectors
Transmission of Babesia microti from one host to another generally takes place through direct tick-host contact. In the northeastern United States, the enzootic cycle is maintained principally by the interactions between immature blacklegged ticks and white-footed mice. In general, larvae acquire the infection when feeding on Babesia microti-infected mice. Engorged larvae overwinter and pass the parasites transstadially to the next developmental stage, the nymph, in the following spring. Nymphs infected in the larval stage pass the infection to susceptible animals upon feeding. In areas where Babesia microti is endemic, infection rates of nymphal ticks with the pathogen typically range from 5 to 40%. Concurrent infections of Babesia microti and Borrelia burgdorferi are commonly found in ticks collected in endemic areas. The prevalence of Borrelia burgdorferi in nympal blacklegged deer ticks is usually twice that of Babesia microti. The role of adult ticks in transmission of the infection appears to be minimal. Adults feed predominantly on white-tailed deer, which is not a competent reservoir for Babesia microti. Transovarial transmission (from parents to offsprings through the egg) of Babesia microti infection in the tick has not been documented.The ability to transmit Babesia microti in some other tick species has been determined. Laboratory experiments have demonstrated that Babesia microti can be successfully transmitted by Ixodes pacificus and Ixodes trianguliceps. However, similar studies have shown that attempts to transmit this parasite with Dermacentor andersoni, D. variabilis, Rhipicephalus appendiculatus, as well as Hyalomma dromedarii and Hyalomma anatolicum failed. Babesia microti infection in arthropods other than ticks has never been documented.
Reservoir Hosts
Plasmodium (Malaria) and Babesia species are the most ubiquitous intraerythrocytic parasites that affect humans and animals. The infection occurs wherever certain hosts are parasitized by ticks. Babesia microti, in particular, has been recognized as the zoonotic agent infecting rodents in various regions of the world. This parasite has been detected in animals including the white-footed mouse, the meadow vole, the eastern chipmunk, the Norway rat, the cottontail rabbit, and the short-tailed shrew. However, the white-footed mouse appears to serve as the primary reservoir host for the infection. In areas endemic for Babesia microti, the infection rates in mouse populations can reach as high as 60%. Laboratory experiments also demonstrated that nearly all mice with the infection produce Babesia microti-infected ticks, suggesting that transmission of Babesia microti between mice and ticks can be intensive in endemic foci. More interestingly, mice collected in endemic areas appeared to be universally infected with both Babesia microti and Borrelia burgdorferi, indicating that the enzootic cycle of these two pathogens may be similar in nature. There is no evidence incriminating the white-tailed deer as the competent reservoir for Babesia microti. However, deer are an essential host for all trophic stages of the blacklegged deer tick.
HUMAN BABESIOSIS
EPIDEMIOLOGY
SYMPTOMS, DIAGNOSIS, AND
TREATMENT
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