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Human infections with Babesia, particularly B. microti, are no longer a novelty. Well over 100 cases of human babesiosis have been reported, with new reports arising frequently (Dammin et al., 1981; Herwaldt et al., 1995; Falagas and Klempner, 1996; Herwaldt et al., 1997). Human infections with Babesia species have been documented from several continents. In Europe, reports of infection, usually with B. divergens, have come from the former Yugoslavia, France, Russia, Ireland, Scotland, Sweden and the Canary Islands (Dammin et al., 1981; Uhnoo et al., 1992; Olmeda et al., 1997). Most European cases have occurred in splenectomized individuals. Additionally, several cases of babesiosis in Asia, Central America and South Africa have been reported (Dammin et al., 1981; Li and Meng, 1984; Bush et al., 1990; Shih et al., 1997).
PRINCIPLES AND PRACTICE OF CLINICAL PARASITOLOGY
Most human cases of babesiosis have been reported from the USA, particularly from the north eastern part of the country (Dammin et al., 1981). B. microti is responsible for almost all of the American cases. However, recent reports document infection by newly identified species of Babesia in California, Missouri and Washington State (Quick et al., 1993; Persing et al., 1995; Herwaldt et al., 1996). The emergence of human babesiosis in the USA, where most epidemiologic studies have been conducted, appears to be related to the increase in the deer population (Spielman, 1994).
Several serological surveys have demonstrated that infection with Babesia is more widespread than case reports would indicate, suggesting that most human babesiosis caused by this organism is subclinical. Over half of 173 men from a northwestern region of Nigeria were infected by Babesia species. Similar results were obtained in a study of persons residing in Mozambique. This has led some to suggest that so-called drug-resistant malaria might actually represent misdiagnosed babesiosis (Telford et al., 1993).
Similar results have been obtained in the USA. In a survey of persons at high risk for infection in New York State, 4.4% were seropositive in June, whereas 6.9% were seropositive in October, resulting in a seroconversion rate of 5.9% (Filstein et al., 1980). None were symptomatic. In a survey of sera from 779 blood donations on Cape Cod, 3.7% of persons had positive B. microti antibodies (Popovsky et al., 1988). Children are as likely as adults to be infected with Babesia without a formal diagnosis of babesiosis being made (Krause et al., 1992).
A group with an increased likelihood of seropositivity for B. microti consists of persons with a history of infection by Borrelia burgdorferi, the causative agent of Lyme disease. Positive
B. microti serologies have been found in 9.5-66% of persons with positive serologies for Borrelia burgdorferi (Benach et al., 1985; Krause et al., 1991, 1992, 1996c). Similar findings occur in children (Krause et al., 1992). The explanation for this is the transmission of both agents by the same vector, I. scapularis. Co-infection with Ehrlichia, which is also transmitted by I. scapu-laris, has been documented in persons infected with Babesia and/or Borrelia (Magnarelli et al., 1995; Mitchell et al., 1996). The phenomenon of
co-infection might be more than a mere curiosity, since there is evidence that persons co-infected with Borrelia burdorferi and Babesia microti exhibit more severe and prolonged symptoms (Krause et al., 1996c).
Although transmission of human babesiosis occurs in most cases through a tick bite, there are other modes of transmission. Infectious parasites have been retrieved from B. microti-infected blood stored at 4°C for up to 21 days, under conditions which are replicated during blood banking (Eberhard et al., 1995). Not surprisingly, acquisition of babesiosis through blood transfusion has been well documented. Almost all reports have concerned the transmission of
B. microti (Wittner et al., 1982; Marcus et al., 1982; Gordon et al., 1984; Rosner et al., 1984; Mintz et al., 1991; Anderson et al., 1991). However, there is a recent case report of transmission of the WA1-type parasite by transfusion in Washington State in the USA (Herwaldt et al., 1997). A comparison between
B. microti-seronegative and -seropositive blood donors in Massachusetts identified no differences that would enable the identification of high-risk donors (Popovsky et al., 1988). Despite this, the risk of acquiring babesiosis from a blood transfusion obtained from a donor residing in an endemic area is very low (Gerber et al., 1994).
Another, less well-documented mode of transmission of babesiosis is vertical transmission. This has been documented in humans and cattle (Esernio-Jenssen et al., 1987; New et al., 1997; De Vos et al., 1976). There is no data on the efficiency of transmission. However, there is a report of a woman who became infected with Babesia during the fifth month of pregnancy. Her illness resolved without chemotherapy and the fetus had no evidence of infection (Raucher et al., 1984).