• Human babesiosis is a tick-borne, malaria-like illness characterized by fever, malaise, and hemolytic anemia.
  • Most infected individuals are asymptomatic.


  • Human babesiosis is a protozoal illness caused by the intraerythrocytic parasite of the Babesia genus.
    • Babesia microti is responsible for most cases of babesiosis in the United States.
    • Babesia duncani has been the causative agent in older men on the Pacific coast of the United States.
    • Babesia divergens is responsible for most European cases.
    • Babesia venatorum has been the causative agent in China.
  • Most cases in the United States occur in the Northeast and Upper Midwest.
    • 94% of reported cases in 2014 occurred in Connecticut, Massachusetts, New Jersey, New York, Rhode Island, Minnesota, and Wisconsin.
  • Human infection most commonly occur in the warm months—late spring to early fall.


  • In 2011, babesiosis became a nationally notifiable disease monitored by the Centers for Disease Control and Prevention (CDC).
  • According to the CDC, 1,744 cases were reported in the United States in 2014. Six of the cases were transfusion related.


Prevalence is difficult to ascertain, as asymptomatic infection appears to be common in endemic areas. For instance, seroprevalence is as high as 9% in some endemic areas of Rhode Island.

Risk Factors

  • Asplenia (functional or anatomic)
  • Extremes of age, especially age >50 years
  • Immunosuppressive medications
  • Malignancy
  • Primary immunodeficiency syndrome


There is no known genetic predisposition.

General Prevention

  • Prevention begins with avoidance of tick bites.
  • Simple measures include wearing long-sleeved shirts and long pants, with pants tucked into the socks in tick-infested areas.
  • Avoid endemic regions during the peak months of May to September.
  • Light clothing will make ticks easier to see.
  • Use N,N-Diethyl-meta-toluamide (DEET)-containing insect repellents during outdoor activities.
  • Spraying one’s clothing with a permethrin tick repellent may be helpful.
  • Children and dogs should be inspected daily for ticks after being outside.
  • Prophylaxis is not recommended after a tick bite.
  • No vaccine is currently available.
  • There is currently no universal laboratory screening of blood products.


  • Babesial infection of the erythrocyte causes membrane damage and lysis, which promotes adherence to the endothelium and microvascular stasis.
  • This process results in a hemolytic anemia.
  • The spleen plays an important role in decreasing the protozoal load through antibody production and filtering of abnormally shaped infected red blood cells.


  • The protozoa is transmitted by a bite from the tick Ixodes scapularis (deer tick), the same vector responsible for transmission of Borrelia burgdorferi (the causative agent in Lyme disease).
  • The white-footed mouse (Peromyscus leucopus) is the primary reservoir host of Babesia.
  • Incubation period
    • Usually 1 to 4 weeks for tick-transmitted disease
    • 1 to 9 weeks for transfusion-associated disease
  • Human-to-human transmission is limited to infection through contaminated blood.
    • Babesiosis is currently the most common transfusion-transmitted infection in the United States.
    • Rare cases of transplacental and perinatal infection have been reported.

Commonly Associated Conditions

  • Two thirds of patients have concurrent Lyme disease.
    • Coinfections with Borrelia account for 80% of tick-borne coinfections.
  • One third may have concurrent human granulocytic anaplasmosis (HGA).
  • Less common coinfection may occur with other tick-borne agents.

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