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in the USA, whereas the cattle strains B. divergens and B. bovis have been associated with human disease in Europe (Garnham,
1980). Previously unknown Babesia strains or Babesia-like organisms infecting humans have been described recently (Quick et al., 1993; Persing et al., 1995; Herwaldt et al., 1996; Shih et al., 1997; Thomford et al., 1994).
DESCRIPTION OF THE ORGANISM
The taxonomy of babesias was recently described in a review by Telford et al. (1993) as phylum Apicomplexa, class Aconoidasida, order Piroplas-midora, family Babesdiidae and genus Babesia. A listing of the many species and their hosts is included in the review. However, only a few species are known to cause disease in humans. The organisms are intracellular parasites which are piriform, round or oval, depending upon the species. B. microti, normally found in rodents, measures 2.0 x 1.5 pm. Of species found in cattle which infect humans, B. bovis measures
2.4 x 1.5 pm and B. divergens measures
1.5 x0.4 pm. These organisms are frequently mistaken for Plasmodium falciparum, one of the agents that causes malaria, because of their intracellular ring forms and the peripheral location of the parasite in the erythrocyte. However, in contrast to the appearance of the developing intraerythrocytic Plasmodium, intraerythrocytic Babesia contain no
Principles and Practice of Clinical Parasitology
Edited by Stephen Gillespie and Richard D. Pearson © 2001 John Wiley & Sons Ltd
PRINCIPLES AND PRACTICE OF CLINICAL PARASITOLOGY
Fig. 4.1 (A) Wright-Giemsa stained blood smear of a patient with B. microti. Numerous merozoite ring forms are seen (arrow)
within erythrocytes. The ring forms resemble P. falciparum, but are large, clear and devoid of the brown (hemozoin) pigment seen with P. falciparum. The absence of gametocytes and schizonts further distinguishes B. microti from P. falciparum. Courtesy of Philip R. Daoust MD. (B) Wright-Giemsa stained human blood smear. Ring form merozoites are seen (arrows) but one erythrocyte contains five immature merozoites, characterized by sparse cytoplasm and small nucleus. As these develop, they form rings. The presence of parasites at different stages is consistent with the asynchronous schizogeny that characterizes babesial infection. Smear courtesy of Philip R. Daoust MD
hemoglobin-derived pigment. The appearance of the tetrad form of B. microti, the result of division by budding rather than schizogeny, is diagnostic of babesiosis (Figure 4.1A,B).
PATHOGENESIS The Tick Vector
Babesiosis, a zoonotic disease, requires transmission from an animal reservoir to the human host via a tick vector. The cattle tick Ixodes ricinus, in its larval form, is the vector for B. divergens (Donnelly and Peirce, 1975). I. ricinus is widely distributed across the countryside of the UK. The prevalence of infection of I. ricinus by B. divergens is thought to be low, with estimates that 1 in 500 or fewer ticks are infective for cattle (Donnelly, 1980). The tick Boophilus microplus, which also feeds upon cattle, is the major vector of B. bovis (Potgieter et al., 1976; Potgieter and Els, 1976). Spielman (1976) described studies identifying the tick Ixodes dammini, the northern deer tick, as the vector of babesiosis on Nantucket Island. I. dammini is thought to be the same as I. scapularis, which is found in the southern USA, based on genetic, life-cycle and mating studies (Oliver et al., 1993; Wesson et al., 1993), and this name is used here.
Three developmental forms of ticks exist, the larval, nymph and adult forms. Most information on the life-cycle of ticks which harbor Babesia pertains to I. scapularis. The larval and nymph forms of I. scapularis feed mainly on Peromyscus leucopus, the white-footed deer mouse (Healy et al., 1976), but have also been found on other hosts, such as rats, other mice, rabbits, deer, dogs and man (Piesman and Spielman, 1979; Spielman et al., 1979). The adult forms feed mainly on deer (Piesman et al., 1979). Interestingly, deer do not become infected with B. microti. It is thought that the reintroduction of the deer to Nantucket Island in the 1930s after decimation of herds due to hunting, with the subsequent growth of the deer population, is responsible for the spread of I. scapularis (Spielman et al., 1985). The tick requires a blood meal to progress to the next developmental stage. While feeding on the deer, the adult female tick becomes impregnated and produces up to 20 000 eggs.
Almost 80% of white-footed deer mice sampled during a 1976 survey on Nantucket Island were infected with B. microti. While feeding on an infected mouse, the tick larvae become infected with B. microti. The organism is transmitted from the larval to the nymphal forms via trans-stadial transmission. There is no evidence of transovarian transmission of B. microti by I. scapularis (Oliveira and Kreier, 1979; Telford et al., 1993). After infection of the nymphal form, the nymph obtains another blood meal and, in the process, infects the host. The host is usually a rodent, although humans also serve as hosts. Infestation of a human by a nymph is difficult to detect, since the nymph is small (1.5-2.5mm in length) (Telford et al., 1993). The three development forms of I. scapularis feed on humans but the nymph is the main vector of babesiosis. The three forms also feed on deer, which do not become infected. Thus, deer are an important link in the life cycle of B. microti, since they sustain the adult form of the arthropod vector. A convergence of all three organisms— deer, mouse and tick—is necessary to create the conditions favoring the infection of humans, as incidental hosts, with B. microti. For B. divergens and B. bovis, the convergence of cattle and ticks is necessary to create conditions favoring the infection of humans in Europe (Donnelly, 1980).