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Babesiosis is a malaria-like illness caused by intraerythrocytic protozoa that are transmitted by the bite of the same hard-bodied ticks (Ixodid) that transmit Lyme disease and human granulocytic anaplasmosis.1-2 Babesia species are parasites of mammals and birds that are currently classified in the subphylum Apicomplexa, together with those organisms that cause malaria (Plasmodium sp.) and toxoplasmosis (Toxoplasma gondii). Only a few of the more than 90 species of Babesia cause disease in humans including, Babesia microti and Babesia microti-like species from the United States, Asia, and Europe, Babesia duncani (WA1) from California and Washington State, Babesia divergens and Babesia divergens-like species from Europe and the United States, and Babesia venatorum (EU1) from Europe.2-3

Although babesiosis has long been recognized as an economically important disease in livestock, the first human case was not described until 1957. Over the past 30 years, the epidemiology of the disease has changed from a few isolated cases to the establishment of endemic areas in southern New England, New York, and the upper Midwest, and reports come from a wide geographic range in America, Africa, Asia, and Europe.1-2 The incidence of babesial infection is similar in children and adults (eFig. 342.1).4 Human babesiosis is transmitted in the northeastern United States by deer ticks (Ixodes scapularis) that feed from infected animal reservoirs (primarily the white-footed mouse, Peromyscus leucopus).5 Nymphal ticks feed in the late spring and summer, and those that are infected transmit B. microti to rodents or man. Consequently, most human cases of babesiosis occur in the summer. The white-tailed deer is an important host of the deer tick. The recent increase in the deer population is thought to be a major cause of the increased incidence of human babesiosis, human granulocytic anaplasmosis, and Lyme borreliosis. Rarely, babesiosis is acquired through blood transfusion or transplacental-perinatal transmission.6-8

eFigure 342.1.

Age distribution of B. microti and B. burgdorferi infection. The pink bars depict the age distribution of subjects seropositive for B. microti, and the blue bars depict the age distribution of subjects seropositive for B. burgdorferi. There was no significant difference in the percentage of subjects seropositive for either B. microti or B. burgdorferi.

(From Krause PJ, Telford SR, Pollack RJ, et al. Babesiosis: an underdiagnosed disease of children. Pediatrics. 1992;89:1045-1048.)

Clinical Manifestations

The clinical manifestations of babesiosis range from subclinical illness to fulminating disease resulting in death or prolonged convalescence.1-3 Symptoms begin after an incubation period of 1 to 6 weeks from the beginning of tick feeding. The unengorged I. scapularis deer tick is only about 2 mm in length, so there is often no recollection of a tick bite. Typical symptoms include intermittent temperature to as high as 40°C (104°F) and one or more of the following: chills, sweats, ...

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