Babesiosis

Jeffrey A Gelfand and Michael V Callahan, Department of Medicine, Tufts University School of Medicine, Boston, Massachusetts, USA

Babesiosis is a malaria-like disease transmitted by parasitized ticks of the genus Ixodes during blood feeding. There are over 100 species of Babesia which, with Theileria, comprise the family Piroplasmorida. Both adult and nymph stages of the Ixodes ticks (I. scapularis, I. pacificus, I. ricinus) transmit the disease from reservoir competent-species, principally rodents, to humans (Figure 1). The rodent strain, Babesia microti (in the United States), and the cattle strains, B. divergens and B. bovis (in Europe), are responsible for the majority of human cases. Conditions favorable for human infection include presence of disease reservoir species (rodents), of suitable hosts for blood feeding (deer, cattle) and proximity to human populations. Disease transmission through blood products and transplacental and possible perinatal infection have been described. In the United States, the majority of cases occur in the Northeast, upper Midwest and Pacific Northwest. Recently, a new species of Babesia, designated WA-1, has been implicated in human babesiosis in Washington and northern California. Phylogenetic analysis of DNA isolated from this species demonstrates greater similarity with B. gibsoni, a canine form, and to theileria species than to species known to infect humans. A fatal case of babesiosis has also been reported from Missouri. The isolate, designated MO-1, was found to have antigenic similarities and ribosomal DNA

sequences similar to B. divergens, a strain that causes disease in cattle and humans in Europe.

Infection with the murine strain, B. microti, is typically a mild illness in healthy people; however, in the asplenic or immunocompromised patient, such as those with AIDS, lymphoma or transplant patient, the disease may be overwhelming. Infection with bovine strains B. divergens and B. bovis, as seen in Europe, occurs in asplenic patients and is often fatal. Initial symptoms of babesiosis are nonspecific and include fever, chills, nausea, vomiting, arthralgia, myalgia, headache, fatigue and dark urine. The presence of a rash similar to erythema chronicum migrans probably reflects simultaneous infection with Borrelia burgdorferi, which is transmitted by the same vector. Laboratory studies may show decreased haptoglobin, elevated reticulocytes and parasitized red cells on blood films. Leukocytosis is rare; however, thrombocytopenia is common. Patients often have a positive direct Coombs test, and urine studies reveal proteinuria and hemoglobinuria. Liver function tests often show mild elevations of alkaline phosphatase, serum bilirubin, aspartate transaminase, alanine transaminase and lactate dehydrogenase. Successful treatment involves combination drug therapy with clindamycin plus quinine, or atovaquone plus azithromycin. Emergence of resistent strains has been documented following monotherapy. Life-

Babesia lile cycle in tick lick E<*> lile cycle

Augusl-Septerber hosts lor tick

Babesia life cycle in host lick E<*> lile cycle

Augusl-Septerber

Figure 1 Life cycle of Babesia. The parasite requires the presence of Ixodes ticks, a reservoir-competent host animal such as rodent species, and large mammals for nymph and adult stages of the tick. Human infection occurs following the bite of infected nymph or adult ticks.

Erythrocytic cycle (

Erythrocytic cycle (

Figure 1 Life cycle of Babesia. The parasite requires the presence of Ixodes ticks, a reservoir-competent host animal such as rodent species, and large mammals for nymph and adult stages of the tick. Human infection occurs following the bite of infected nymph or adult ticks.

threatening cases may benefit from exchange blood transfusion.

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