Geographic Regions with High Endemicity

  • United States-New England
    • Nantucket Island
    • Southeastern Massachusetts/Cape Cod/Western Boston Suburbs
    • Northeastern Massachusetts
    • Southwest Rhode Island
    • Eastern Connecticut
    • Counties East of the Hudson River/Lower Hudson River Valley
    • Southern/Central New Jersey Counties
    • Southern New York
  • United States-Upper Midwest
    • Minnesota
    • Wisconsin

Geographic Regions with Low Endemicity

  • United States-New England
    • Southern Coastal Maine/New Hampshire Counties
    • Northern/Central Connecticut Counties
    • Western Long Island
    • Northern/Southern New Jersey Counties
    • Delaware and Maryland Counties

Other Geographic Regions

  • Indiana: few reported cases

Risk Factors

  • Asplenia (see Asplenia, [[Asplenia]])
    • Babesia is an Encapsulated Organism
  • Blood Transfusion (see Packed Red Blood Cells, [[Packed Red Blood Cells]])
  • Rituximab (see Rituximab, [[Rituximab]]): possible risk factor


Routes of Transmission

  • Tick Bite (see Tick-Borne Illnesses, [[Tick-Borne Illnesses]]): main route of transmission
  • Transmission Via Blood Transfusion (see Packed Red Blood Cells, [[Packed Red Blood Cells]]): less common
    • Platelet Transfusion (see Platelet Transfusion, [[Platelet Transfusion]]): few cases have been reported (platelets were contaminated with red blood cells)
  • Transplacental Transmission: case reports

Babesia Species

Babesia Microti

  • Geography
    • Northeast US
    • Upper Midwest US
  • Tick Vector: Ixodes Scapularis (see Ixodes Ticks, [[Ixodes Ticks]])
    • Reservoir: white-tailed deer and mice
    • Incubation Period: 5-33 days

Babesia Divergens

  • Geography
    • Europe
  • Tick Vector: Ixodes Ricinus (this is also the presumed tick vector responsible for European cases of Human Granulocytic Anaplasmosis, HGA) (see Anaplasmosis)
    • Reservoir: cattle

Other Babesia Species

  • Babesia Duncani
    • Sporadic Outbreaks Along the Pacific Coast of the US (from California to Washington)
  • Babesia Divergens-Like Organisms: Midwestern US
    • Midwest US (Kentucky, Missouri) and Northwest US (Washington)
  • Babesia Divergens-Like Organisms: Canary Islands
  • Babesia Venatorum: Europe
  • Babesia Venatorum: Northeastern China
  • Babesia Microti-Like Organisms: Japan
  • Babesia KO1: South Korea


Complete Blood Count (CBC) (see Complete Blood Count, [[Complete Blood Count]])

Blood Culture (see Blood Culture, [[Blood Culture]])

  • Useful

Peripheral (Thin) Blood Smear (see Peripheral Blood Smear, [[Peripheral Blood Smear]])

  • Thin Blood Smear is the Preferred Diagnostic Test
  • Technique
    • Giemsa or Wright Stain
    • Thick Blood Smears are Not Recommended, as Babesia are Small (<3 μm) and May Be Missed
  • Findings
    • Trophozoites within Red Blood Cells
      • Rings with 1 Chromatin Dot: classic ring form
      • Maltese Cross (4 Merozoites Arranged in a Tetrad): seen with Babesia Microti, Babesia Duncani, and *Babesia Divergens

PCR for Babesia Microti DNA

  • Alternative Test
    • More Sensitive Than Thin Blood Smear: may be useul early in the course of disease (when the organism burden is low)
    • Do Not Amplify DNA from Plasmodium Species (Which Cause Malaria)
    • Do Not Cross-React with Babesia Duncani
    • Less Useful to Follow the Course of Disease, Since DNA May Be Detected for Weeks After Parasites are No Longer Visualized on the Thin Blood Smear


  • Indirect Immunofluorescence Assay (IFA)
    • Serologic Testing for IgM/IgG Antibodies by IFA Can Be Performed, But Seropositivity Per Se Does Not Indicate Active Infection
    • Antibodies May Be Absent Early in the Course of Disease

Clinical Manifestations

General Comments

  • Variable Severity of Disease: ranges from asymptomatic disease to rapidly fatal disease
    • Most Immunocompetent Patients Have Mild-Subclinical Disease
  • Risk Factors for Severe Babesiosis
  • Coinfection: may occur in some cases

Constitutional Manifestations

  • Chills/Sweats (see Chills, [[Chills]])
  • High Fever (see Fever, [[Fever]])
  • Malaise

Gastrointestinal Manifestations

Hematologic Manifestations

Neurologic Manifestations

Rheumatologic Manifestations


General Comments

  • Therapy is Recommended for Symptomatic Infection Only

Clindamycin + Quinine (see Clindamycin, [[Clindamycin]] and Quinine, [[Quinine]])

  • Clindamycin (IV) + Quinine (PO) is the Classical Regimen for Babesiosis: recommended by the Infectious Diseases Society of America (IDSA) for severe disease
  • Administration
    • Usual Course: 7-10 days
    • Course for Highly Immunocompromised Patients: at least 6 wks (with negative thin blood smears for at least 2 wks prior to discontinuation of therapy)
  • Clindamycin + Quinine Regimen Has a High Incidence of Adverse Effects: adverse effects occur in 72% of cases

Atovaquone + Azithromycin (see Atovaquone, [[Atovaquone]] and Azithromycin, [[Azithromycin]])

  • Indications
    • Mild-Moderate Disease
  • Administration: oral atovaquone + oral azithromycin
    • Usual Course: 7-10 days
    • Course for Highly Immunocompromised Patients: at least 6 wks (with negative thin blood smears for at least 2 wks prior to discontinuation of therapy)
  • Adverse Effects: lower incidence of adverse effects (only 15% of cases) than with the classical clindamycin + quinine regimen
  • Clinical Efficacy
    • Atovaquone + Azithromycin is as Effective Clindamycin + Quinine Regimen in Mild-Moderate Babesiosis (N Engl J Med, 2000) [MEDLINE]
    • Resistance: has been reported in immunocompromised hosts (Clin Infect Dis, 2010) [MEDLINE]

Exchange Transfusion (see Exchange Transfusion, [[Exchange Transfusion]])

  • Indications
    • High-Grade Parasitemia (≥10%)
    • Severe Hemolysis (see Hemolytic Anemia, [[Hemolytic Anemia]]): most common indication for exchange transfusion
    • Pulmonary/Renal/Hepatic Compromise
  • Technique
    • Unclear Superiority of Red Blood Cell vs Whole Blood vs Plasma Exchange


  • Risk Factors for Persistent/Relapsing Babesiosis
    • B-Cell Lymphoma Treated with Rituximab (Rituxan) Therapy (see Rituximab, [[Rituximab]])
    • HIV with Low CD4 Count (see Human Immunodeficiency Virus, [[Human Immunodeficiency Virus]])
    • Immunosuppression for Solid Organ/Hematopoietic Stem Cell Transplant


  • Atovaquone and azithromycin for the treatment of babesiosis. N Engl J Med. 2000 Nov 16;343(20):1454-8 [MEDLINE]
  • Emergence of resistance to azithromycin-atovaquone in immunocompromised patients with Babesia microti infection. Clin Infect Dis. 2010 Feb 1;50(3):381-6. doi: 10.1086/649859 [MEDLINE]
  • Human babesiosis.  N Engl J Med.  2012;366:2397–2407 [MEDLINE]
  • Diagnosis, treatment and prevention of Lyme disease, human granulocytic anaplasmosis, and babesiosis.  JAMA.  2016;315:1767–1777 [MEDLINE]
  • CDC Tickborne Diseases of the United States (Accessed 7/17) [LINK]