Babesia microti

  • Diagnosis
  • Background
  • Lab Tests
  • References
  • Related Topics

Indications for Testing

  • Strong clinical suspicion in patient with febrile illness after tick exposure or with a high-risk occupation

Laboratory Testing

  • CDC diagnosis recommendations for babesiosis
  • Nonspecific – CBC may demonstrate hemolysis in severe disease
  • Giemsa-stained blood films – in patients from endemic areas
    • Diagnostic, if parasites noted
    • Relatively insensitive due to low parasite level in most healthy patients
    • Thick smears of hemolyzed blood are most useful for screening purposes in cases with low-level parasitemia; thin smears are used for parasite classification
  • Serologic (IFA) testing
    • High sensitivity and specificity in Babesia detection
    • Rises 2-4 weeks after infection and wanes at 6-12 months
    • Strain MO-1 (found in Missouri) and B. duncani (found in Pacific Northwest) will not be detected by B. microti serology; separate B. duncani test is available
  • PCR – highly sensitive and specific but relatively expensive
  • In areas of coinfection, consider concurrent testing for Lyme disease and human granulocytic anaplasmosis (HGA)

Differential Diagnosis

Babesiosis is a zoonosis caused by an intraerythrocytic parasite of the genus Babesia.


  • Prevalence
    • Found in the same geographic regions as agents responsible for Lyme disease and human granulocytic anaplasmosis (HGA)
      • Most common in Northeast and Midwest U.S.
      • Coinfections of babesiosis, Lyme disease, and HGA occur in 10% of patients
  • Transmission
    • Tick bite – Ixodes spp
      • Vector – Ixodes scapularis (formerly Ixodes dammini)
        • More commonly known as a blacklegged or deer tick (in U.S.)
    • Blood transfusion – rare
    • For more information on causal agents, life cycle, and geographic distribution, see CDC's information on Babesiosis


  • Protozoal parasite – obligate parasite of red blood cells
  • >100 known species – most common type in U.S. is B. microti

Risk Factors for Severe Disease

  • Splenectomy
  • Immunosuppression
  • Coinfection

Clinical Presentation

  • Most cases of human babesiosis in healthy adults are asymptomatic
  • Symptomatic patients experience fever, headache, nausea, sweats, rigors
    • Symptoms occur 1-6 weeks following tick bite
  • Resemblance to a fulminant malaria-like infection may complicate initial diagnosis
  • Hemolytic anemia and thrombocytopenia may occur in severe disease
    • Parasites fragment red blood cells
      • Results in capillary blockage in the spleen, liver, kidneys, central nervous system
  • Immunocompromised and asplenic patients may experience persistent or relapsing disease
  • Complications


  • Protective clothing (long sleeves and pants) in tick-prone areas
  • DEET on exposed skin

Tests generally appear in the order most useful for common clinical situations. Click on number for test-specific information in the ARUP Laboratory Test Directory.

Parasites Smear (Giemsa Stain), Blood 0049025
Method: Stain


False-negative results may occur due to low parasite levels


Babesia Species by PCR 2008665
Method: Qualitative Polymerase Chain Reaction

CBC with Platelet Count and Automated Differential 0040003
Method: Automated Cell Count/Differential

Babesia microti Antibodies, IgG and IgM by IFA 0093048
Method: Semi-Quantitative Indirect Fluorescent Antibody

Follow Up

If test results are equivocal repeat testing in 10-14 days

Related Tests

General References

Gubernot DM, Lucey CT, Lee KC, Conley GB, Holness LG, Wise RP. Babesia infection through blood transfusions: reports received by the US Food and Drug Administration, 1997-2007. Clin Infect Dis. 2009; 48(1): 25-30. PubMed

Parasites - Babesiosis. Centers for Disease Control and Prevention. Atlanta, GA [Last updated Feb 2014; Accessed: Nov 2015]

Rosenblatt JE. Laboratory diagnosis of infections due to blood and tissue parasites. Clin Infect Dis. 2009; 49(7): 1103-8. PubMed

Sanchez E, Vannier E, Wormser GP, Hu LT. Diagnosis, Treatment, and Prevention of Lyme Disease, Human Granulocytic Anaplasmosis, and Babesiosis: A Review JAMA. 2016; 315(16): 1767-77. PubMed

Vannier E, Krause PJ. Human babesiosis. N Engl J Med. 2012; 366(25): 2397-407. PubMed

Vannier EG, Diuk-Wasser MA, Ben Mamoun C, Krause PJ. Babesiosis Infect Dis Clin North Am. 2015; 29(2): 357-70. PubMed

Medical Reviewers

Last Update: April 2016