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Babesiosis is an emergent zoonosis caused by an intraerythrocytic protozoan in the genus Babesia. Babesia microti is responsible for the vast majority of human cases in the United States, with "hot spots" of disease along the Northeast Coast (eg, Martha’s Vineyard, Long Island, and Nantucket) and midwest states, although the distribution of disease is spreading. In addition, a small number of cases of Babesia duncani and Babesia duncani-like human infection (WA and CA strains) have been reported along Pacific Coast states from Washington to northern California, and Babesia divergens/Babesia divergens-like strains have been isolated from humans in Missouri (MO-1 strain), Kentucky, and Washington. At this time, only Babesia microti is a nationally notifiable disease.
Babesia microti shares a tick vector with Borrelia burgdorferi and Anaplasma phagocytophilum, the causative agents of Lyme disease and human granulocytic anaplasmosis (HGA), respectively. Recent studies suggest that exposure to Babesia microti is quite common in areas endemic for Lyme disease and anaplasmosis, so it is prudent to consider testing for all 3 diseases concurrently. Less commonly, babesiosis may be acquired through blood transfusion, and therefore donor units are tested for this parasite in some endemic areas.
Most patients with babesiosis have a mild illness or are asymptomatic, but some develop a severe illness that may result in death. Patient symptoms may include fever, chills, extreme fatigue, and severe anemia. The most severe cases occur in asplenic individuals and those over 50 years of age. Rare cases of chronic parasitemia, usually in immunocompromised patients, have been described.
The definitive laboratory diagnosis of babesiosis rests on the demonstration of Babesia species characteristic intraerythrocytic parasites in Giemsa-stained thick and thin blood films. This method is capable of detecting (but not differentiating) human-infective Babesia species.
However, parasites may be difficult to identify in cases of low parasitemia, and the ring forms of Babesia may closely resemble those of Plasmodium falciparum. Antibody testing is useful for confirmation of babesiosis, but is not widely available and may be negative in the early phase of illness. The Mayo Clinic real-time PCR assay provides a rapid and sensitive alternative to blood film examination and enables the detection and differentiation of Babesia microti, Babesia duncani/Babesia duncani-like and Babesia divergens/ Babesia divergens-like parasites. It does not cross-react with malaria parasites.
An initial screening method for suspected babesiosis during the acute febrile stage of infection in patients from endemic areas, especially when Giemsa-stained peripheral blood smears do not reveal any organisms or the organism morphology is inconclusive.
A positive result indicates the presence of Babesia species DNA and is consistent with active or recent infection. While positive results are highly specific indicators of disease, they should be correlated with blood smear microscopy, serological results and clinical findings.
A negative result indicates absence of detectable DNA from Babesia species in the specimen, but does not always rule out ongoing babesiosis in a seropositive person, since the parasitemia may be present at a very low level or may be sporadic.
Other tests to consider in the evaluation of a patient presenting with an acute febrile illness following tick exposure include serologic tests for Lyme disease (Borrelia burgdorferi), and molecular detection (PCR) for ehrlichiosis/anaplasmosis. For patients who are past the acute stage of infection, serologic tests for these organisms should be ordered prior to PCR testing.
While this assay is designed to detect symptomatic infection with Babesia microti, Babesia duncani and Babesia divergens/MO-1, it may detect low-grade asymptomatic parasitemia in individuals in babesiosis-endemic areas. Thus, it should used for patients with a clinical history and symptoms consistent with babesiosis.
Inhibitory substances may cause false-negative results.
Inadequate specimen collection or improper storage may invalidate test results.
This is a qualitative assay and the results are reported either as negative or positive for targeted Babesia species DNA.
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