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.


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 coinfection, 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.
      • Coinfections 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 coinfection
    • 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
  • Coinfection

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

ARUP Laboratory Tests

Primary Tests

Screen for and detect spirochetes and blood parasites, including microfilaria, Babesia, Trypanosoma, and Plasmodium species

Patient’s travel history is necessary to aid in test interpretation

Time sensitive

Useful if Giemsa stain is negative but high suspicion of babesiosis exists

Does not detect B. duncani or strain MO-1

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

First-line test for diagnosing acute babesiosis

Detects nucleic acid from B. microti and detects but does not differentiate between B. duncani, B. divergens, strain MO-1, and strain EU-1

Blood smears are also appropriate for diagnosing and monitoring babesiosis disease; refer to parasites smear (Giemsa stain), blood

Related Tests

Screen to help differentiate between bacterial and viral disease

Refer to Babesia microti antibodies, IgG and IgM panel, which includes IgG and IgM by IFA

Refer to Babesia microti antibodies, IgG and IgM panel, which includes IgG and IgM by IFA

Medical Experts



Marc Roger Couturier, PhD, D(ABMM)
Associate Professor of Clinical Pathology, University of Utah
Medical Director, Parasitology/Fecal Testing, Infectious Disease Antigen Testing, Bacteriology, and Molecular Amplified Detection, ARUP Laboratories


Additional Resources
  • 25999229

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