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PBAB
81147
Babesia microti, Molecular Detection, PCR, Blood
Clinical Information: Babesiosis is an emergent zoonosis caused by an intraerythrocytic protozoa in
the genus Babesia. Babesia microti is responsible for the vast majority of human cases in the United
States, while Babesia divergens causes the majority of cases in Europe. Recently, cases of Babesia
duncani infection have been reported in Washington and California states, and an unnamed Babesia
species (currently called MO-1) has been isolated from human infections in Missouri. In the United
States, typical "hot spots" of disease include the Northeast coast (eg, Marthaâ€s Vineyard, Long Island,
Nantucket), and Midwest states, although the distribution of disease is spreading. Other important foci
now include the Atlantic and South-Central states, and cases have recently been described from Missouri,
Kentucky, Washington, and California. 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 often prudent to test for all 3 diseases concurrently.
The majority of 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. Symptoms are easily confused with those of Lyme disease. The high frequency of Lyme
seropositivity in patients with babesiosis may provide misleading evidence of active infection with
Borrelia burgdorferi. Failure to respond to antimicrobials that should eradicate Borrelia burgdorferi might
otherwise be interpreted as a Lyme disease treatment failure, when in fact, concurrent infection with
Babesia microti may be responsible for the persistence of symptoms. 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. Recent evidence suggests that babesial infections
have an immunosuppressive effect. This immunosuppression may be most evident in a patient infected
with both Babesia microti and Borrelia burgdorferi. Diagnosis of this infection with traditional methods is
often difficult because in both early acute and persistent disease, parasitemia is often slight and organisms
are often difficult to visualize on peripheral blood smears. Antibody testing is useful for confirmation of
babesiosis, but is not widely available and also may be negative in the early phase of illness. Antibody
testing also is not useful for monitoring the effectiveness of antibabesial therapy. The definitive laboratory
diagnosis of babesiosis rests on the demonstration of Babesia microti characteristic intraerythrocytic
inclusions in Giemsa stained thin blood films. At some stages, these may closely resemble the ring forms
of Plasmodium falciparum, which has resulted in malaria and babesiosis each having been mistakenly
diagnosed as the other infection. The relative scarcity of parasites during early disease, and in many stages
of disease in normosplenic hosts, renders the latter test relatively insensitive. Serologic testing for
babesiosis may fail to detect antibodies during the acute phase of the infection.
Useful For: An initial screening method for suspected babesiosis during the initial flu-like stage of
infection in patients from endemic areas, especially when the Giemsa-stained peripheral blood smear does
not reveal any organisms
Interpretation: A positive result indicates presence of Babesia microti 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 microti in the specimen, but does not always rule out
ongoing babesiosis in a seropositive person, since the parasitemia may be present at a low level or may be
sporadic. Other tests to consider in the evaluation of a patient presenting with a flu-like illness following
tick exposure include serologic tests for Lyme disease (Borrelia burgdorferi), babesiosis and
ehrlichiosis/anaplasmosis. For patients who are past the acute stage of infection, serologic tests for these
organisms should be ordered prior to PCR testing.
Reference Values:
Negative
Clinical References: 1. Anderson JF, Mintz ED, Gadbaw JJ, et al: Babesia microti, human
babesiosis and Borrelia burgdorferi in Connecticut. J Clin Microbiol 1991 Dec;29(12):2779-2783 2.
Homer MJ, Aguilar-Delfin I, Telford SR, et al: Babesiosis. Clin Microbiol Rev 2000 July;13(3):451-469
3. Thompson C, Spielman A, Krause PJ: Coinfecting deer-associated zoonoses: Lyme disease, babesiosis,
and ehrlichiosis. Clin Infect Dis 2001 Sept 1;33(5):676-685 4. Garcia LS: Diagnostic medical
parasitology. 5th edition, Edited by LS Garcia, Washington, DC, ASM Press, 2007 5. Herwaldt BL, de
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