In a review of the parasites of Microtus, Timm (1985) lists no protozoan endoparasites whatsoever for this genus. The role of parasitism, whether macro- or microparasites, and whether endo- or ectoparasites, in the demographic machinery of microtines is poorly understood. Timm (1985) astutely observes that one of the most challenging and fruitful directions of future research with Microtus will be the statistical quantification of the cost of parasitism. In addition, since humans in the Grand Teton National Park may encounter protozoon parasites that are potentially pathogenic to humans, it is useful to characterize the occurrence and biology of such organisms in the parko Babesia microti, a parasitic protozoon, is transmitted by a tick vector and reproduces in the erythrocytes of its mammalian host. Initially, Babesia was thought to be restricted to small mammals; however, in 1970 the first human cases were diagnosed in residents of Nantucket Island, Massachusetts (Western et al, 1970). Over 200 cases of human babesiosis have been documented worldwide. In the United States, human babesiosis is caused by B. microti. Most of these cases have occurred in the eastern United States. The earliest report of an organism that fits the description of Babesia in human erythrocytes is that of Wilson et al. (1904), who found an unknown organism in human erythrocytes while investigating the cause of Rocky Mountain Spotted fever. Documented cases of babesiosis in many areas of the United States are increasing (Steketee et al, 1985). As humans insert themselves into places where they have historically been present only occasionally, they often contract new diseases. A second protozoon parasite, Hepatozoon sp., which is widespread in small mammals in Europe, is also found in reptiles throughout the world. The record of Hepatozoon in North American small mammals is not extensive. Fewer than 10 species of mammals have been shown to harbor Hepatozoon parasites. Like Babesia, Hepatozoon is a two-host parasite. Unlike Babesia, for which the intermediate host is always a tick, the intermediate host in the Hepatozoon may be a tick, a mite, a flea, or a mosquito. The method of transmission by the vector also differs in the two parasites. Babesia is transmitted in saliva when the tick bites, whereas Hepatozoon infection requires the vertebrate host to swallow the vector. In our 1994 studies, we sought to extend knowledge of these two parasites. The specific objectives for 1994 were: to sample specific populations of M. molltanus, in which we have previously documented Hepatozoon infections, to determine whether there are differences in the infection rates at different study sites in the park; to search for the vector of Hepatozoon sp. infections in M. montanus by examining ectoparasites; to collect and rear ticks from M. montanus for use in R microti transmission studies; and to complete a comparison of the spleen histology of babesiosis in infected and uninfected laboratory animals, with inclusion of data from wild animals, as available. Our long-term objectives are to document the effects and cost of parasitism on vole populations and to determine the potential of small mammals of Grand Teton National Park to serve as reservoirs of human parasites.
The Global Emergence of Human Babesiosis
