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Interpretive Handbook

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Test 81147 :
Babesia microti, Molecular Detection, PCR, Blood

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

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, Martha’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 age 50. 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 Suggests clinical disorders or settings where the test may be helpful

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 Provides information to assist in interpretation of the test results

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.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

While this assay is designed to detect only Babesia microti infection of clinical significance, it may detect low grade asymptomatic parasitemia in individuals exposed to babesiosis-endemic areas. Thus, it should used for patients with a clinical history and symptoms consistent with babesiosis. This test should not be used to screen asymptomatic patients or blood transfusion units.


Inhibitory substances may cause false-negative results.


Inadequate specimen collection or improper storage may invalidate test results.


This test is designed for detection of Babesia microti, but may cross-react with a few other species of Babesia that may be pathogenic for humans, including Babesia duncani. It will not detect Babesia divergens or the newly described MO-1 strain. If these organisms are suspected, blood smears and serologic tests should be considered.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.


Clinical References Provides recommendations for further in-depth reading of a clinical nature

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. Fifth edition, Edited by LS Garcia, Washington, DC, ASM Press, 2007

5. Herwaldt BL, de Bruyn G, Pieniazek NJ, et al: Babesia divergens-like infection, Washington State. Emerg Infect Dis 2004;10(4):622-629

6. Persing DH, Herwaldt BL, Glaser C, et al: Infection with a Babesia-like organism in northern California. N Engl J Med 1995;332(5):298-303

7. Quick RE, Herwaldt BL, Thomford JW, et al: Babesiosis in Washington State: a new species of Babesia? Ann Intern Med 1993;119(4):284-290

8. Herwaldt B, Persing DH, Precigout EA, et al: A fatal case of babesiosis in Missouri: identification of another piroplasm that infects humans. Ann Intern Med 1996;124(7):643-650