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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. The disease is named after a European microbiologist, Victor Babes, who discovered the causative microorganism. Babesia species are parasites of mammals and birds that are currently classified in the subphylum Apicomplexa, together with those organisms that cause malaria (Plasmodium species) and toxoplasmosis (Toxoplasma gondii) (Fig. 337-1). Only a few of the more than 90 species of Babesia that have been described cause disease in humans, including Babesia microti from the United States, Asia, and Europe, Babesia duncani (WA1) from California and Washington state, MO1 from Missouri, Babesia divergens and Babesia venatorum from Asia and Europe, and KO1 from Korea.

Figure 337-1

Human erythrocytes infected by Babesia microti on a thin blood film (1000×).


The pathogenesis of babesiosis in humans is not well understood. Similar to the Plasmodium species, Babesia species gain entry into the red blood cell, the organism multiplies into 4 daughter cells (merozoites), and these are released in an asynchronous fashion infecting new red blood cells. Red blood cell lysis is associated with many of the clinical manifestations associated with this disease. Babesia are usually found inside the red blood cells but can be demonstrated outside of the red blood cells in heavily parasitized patients. Disease probably results from excessive release of proinflammatory cytokines (eg, tumor necrosis factor) similar to patients with malaria. The spleen plays an important role in protection against Babesia species as it has long been recognized that patients without spleens have more severe disease. Most disease occurs in adults, although children seem to be equally susceptible. Almost all cases of disease in children have been reported in neonates, probably as a result of transplacental transmission of the organism or via blood transfusion.

Babesiosis has long been recognized as an economically important disease in livestock, but the first human case was not described until 1957. Over the past 60 years, the epidemiology of the disease has changed from a few isolated cases to the establishment of endemic areas in the northeastern and upper midwestern United States (Fig. 337-2) and in northeastern China, as well as reports from a wide geographic range in America, Africa, Asia, Australia, and Europe. The incidence of babesial infection is similar in children and adults. 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). 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 ...

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