Babesia microti


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

Clinical Background

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

Indications for Laboratory Testing

  • 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
Test Name and Number Recommended Use Limitations Follow Up
Parasites Smear (Giemsa Stain), Blood 0049025
Method: Stain

Detect blood parasites, including species of Plasmodium, Babesia, microfilaria, and trypanosomes

Travel history required

Use to detect blood spirochetes (e.g. relapsing fever Borrelia spp)

False-negative results may occur due to low parasite levels


Babesia Species by PCR 2008665
Method: Qualitative Polymerase Chain Reaction

Detect presence of B. microti

Nucleic acid from B. duncani, B. divergens, strain MO-1, and strain EU-1 are detected but are not differentiated

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

Screen to help differentiate between bacterial and viral disease

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

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


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

Babesia duncani (WA1) Antibody, IgG by IFA 2008794
Method: Semi-Quantitative Immunofluorescence
May be useful in patients for whom Babesiosis is highly suspected, yet B. microti testing is negative and patient has a recent history of West Coast travel with possible exposure to ticks    
Additional Tests Available
Click the plus sign to expand the table of additional tests.
Test Name and NumberComments
Babesia microti Antibody, IgG by IFA 0093049
Method: Semi-Quantitative Indirect Fluorescent Antibody
Babesia microti Antibody, IgM by IFA 0093050
Method: Semi-Quantitative Indirect Fluorescent Antibody