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Babesia microti

Babesiosis is an emerging zoonotic disease caused primarily by Babesia microti, an intraerythocytic protozoan. Babesia microti is endemic to the northeastern and upper midwestern United States where it is usually transmitted by the blacklegged tick, Ixodes scapularis. This is the same tick vector that spreads Borrelia burgdorferi and Anaplasma phagocytophilum, the causative agents of Lyme disease and human granulocytic anaplasmosis (HGA), respectively. White-tailed deer are the primary hosts for adult blacklegged ticks, and white-footed mice and other small mammals are reservoirs of B. microti. Most human cases of babesiosis result from tick bites that occur during the spring and summer months, but infection by blood transfusion and perinatal transmission have also been reported.

Increases in the prevalence of tickborne illnesses across the United States are likely, given concurrent evidence of blacklegged tick population growth and geographic expansion, a change that might be attributable to changing weather patterns and increasing forest fragmentation

Many infected persons are asymptomatic but the disease can be life-threatening, especially among older and immunocompromised persons. Initial clinical signs and symptoms of babesiosis are nonspecific and include fever, fatigue, chills, and diaphoresis.  Because of this nonspecific presentation diagnosis may be delayed by up to two weeks after symptom onset.The most severe cases occur in asplenic individuals and those over 50 years of age. Rarely, immunocompromised patients can experience chronic parasitemia. Babesiosis is effectively treated with atovaquone and azithromycin.

A confirmed case of babesiosis is defined as the occurrence of fever, anemia, or thrombocytopenia in a patient with supportive laboratory findings. The definitive laboratory diagnosis of babesiosis depends on the demonstration of intraerythrocytic Babesia species in Giemsa-stained thick and thin blood films. Blood smear exam has a lower sensitivity of parasite detection of 100 to 500 parasites/uL of blood.

Babesia may closely resemble those of Plasmodium falciparum. Real-time polymerase chain reaction (PCR) is a more sensitive alternative to blood film examination and can differentiate Babesia microti, Babesia duncani/Babesia duncani-like and Babesia divergens/Babesia divergens-like parasites. It does not cross-react with malaria parasites. PCR can detect as few as 1 to 3 parasites per uL of blood.

Detection of Babesia antibody by immunofluorescence (IFA) is another possibility for patients with low parasitemia. IgG levels greater than or equal to 1:1024 can be detected in acute phase patients with parasites in blood smears. Reference range is a titer of less than 1:64.

References

Vannier EG, Diuk-Wasser MA, Ben Mamoun C, Krause PJ. Babesiosis. Infect Dis Clin North Am 2015;29:357–70

White DJ, Talarico J, Chang HG, Birkhead GS, Heimberger T, Morse DL. Human babesiosis in New York State: Review of 139 hospitalized cases and analysis of prognostic factors. Arch Intern Med 1998;158:2149–54.

Wang G, Wormser GP, Zhuge J, et al. Utilization of a real-time PCR assay for diagnosis of Babesia microti infection in clinical practice. Ticks Tick Borne Dis 2015;6:376–82

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