Babesia microti - Babesiosis

Babesiosis is a zoonosis caused by an intraerythrocytic parasite of the genus Babesia. Laboratory testing for babesiosis includes nonspecific tests, including CBC, and specific tests, such as Giemsa-stained blood films and serology.

  • 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 (2014)
  • Nonspecific testing
    • CBC – may demonstrate hemolysis in severe disease
  • Specific testing
    • Giemsa-stained blood films – in patients from endemic areas
      • Diagnostic, if parasites noted
      • Low parasite level in most healthy patients
        • Low sensitivity, particularly if single slide examined
        • May require viewing of multiple slides
      • 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 (immunofluorescent assay [IFA]) testing
      • High sensitivity and specificity in Babesia detection
      • Rises 2-4 weeks after infection and wanes at 6-12 months
      • B. microti serology does not detect
        • Strain MO-1 (found in Missouri)
        • B. duncani (found in Pacific Northwest); separate B. duncani test is available
    • Polymerase chain reaction (PCR)
      • Highly sensitive and specific
      • Relatively expensive
  • In areas of co-infection, consider concurrent testing for Lyme disease and human granulocytic anaplasmosis (HGA)

Differential Diagnosis


  • 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.
      • Co-infections of babesiosis, Lyme disease, and HGA occur in 10% of patients
  • Transmission
    • Tick bite – Ixodes spp
      • Vector – Ixodes scapularis
        • More commonly known as a blacklegged or deer tick in U.S.
        • Also vector for Borrelia, Anaplasma and Ehrlichia species – can lead to co-infection
    • Blood transfusion – rare
    • For more information on causal agents, life cycle, and geographic distribution, see CDC's information on Babesiosis (2016)


  • Protozoal parasite – obligate parasite of red blood cells
  • >100 known species
    • Only a few cause human disease
    • Most common type in U.S. is B. microti

Risk Factors for Severe Disease

  • Splenectomy
  • Immunosuppression
  • Co-infection

Clinical Presentation

  • Most cases of human babesiosis in healthy adults are asymptomatic
  • Mild to moderate disease
    • Fever, headache, nausea, sweats, rigors, nonproductive cough
    • Symptoms occur 1-4 weeks following tick bite
  • Severe disease
  • Immunocompromised and asplenic patients may experience persistent or relapsing disease
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


Time sensitive

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


Does not detect B. duncani or strain MO-1


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

Babesia Species by PCR 2008665
Method: Qualitative Polymerase Chain Reaction

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: May 2016; Accessed: June 2017]

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: November 2017