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Babesiosis is a zoonotic infection caused by the protozoan parasite Babesia microti. The infection is acquired by contact with Ixodes ticks carrying the parasite. The deer mouse is the animal reservoir and, overall, the epidemiology of this infection is much like that of Lyme disease. Babesiosis is most prevalent in the Northeast, Upper Midwest, and Pacific Coast of the United States.
Infectious forms (sporozoites) are injected during tick bites and the organism enters the vascular system where it infects RBCs. In this intraerythrocytic stage it becomes disseminated throughout the reticuloendothelial system. Asexual reproduction occurs in RBCs, and daughter cells (merozoites) are formed which are liberated on rupture (hemolysis) of the RBC.
Most cases of babesiosis are probably subclinical or mild, but the infection can be severe and life threatening, especially in older or asplenic patients. Fever, fatigue, malaise, headache, and other flu-like symptoms occur most commonly. In the most severe cases, hemolysis, acute respiratory distress syndrome, and shock may develop. Patients may have hepatomegaly and splenomegaly.
A serologic test can be used as an adjunct in the diagnosis of babesiosis or in seroepidemiologic surveys of the prevalence of the infection in certain populations. Babesiosis is usually diagnosed by observing the organisms in infected RBCs on Giemsa-stained thin blood films of smeared peripheral blood.
Serology may be useful if the parasitemia is too low to detect or if the infection has cleared naturally or following treatment.
Serology may also be useful in the follow-up of documented cases of babesiosis or if chronic or persistent infection is suspected.
A positive result of an indirect fluorescent antibody test (titer > or =1:64) suggests current or previous infection with Babesia microti. In general, the higher the titer, the more likely it is that the patient has an active infection. Patients with documented infections have usually had titers ranging from 1:320 to 1:2,560.
Previous episodes of babesiosis may produce a positive serologic result.
In selected cases, documentation of infection may be attempted by animal inoculation or PCR methods (PBAB / Babesia microti, Molecular Detection, PCR, Blood)
Performance characteristics have not been established for the following specimen characteristics:
Spach DH, Liles WC, Campbell GL, et al: Tick-borne diseases in the United States. N Engl J Med 1993;329:936-947