• Rare tick-borne hemolytic disease caused by intraerythrocytic protozoan parasites of the genus Babesia
  • Infrequently reported outside the United States
    • Sporadic cases have been reported from:
      • France, Italy, the United Kingdom, Ireland, the former Soviet Union, Mexico (1)
      • China, Italy, and Turkey have reported a reemergence of cases.
    • In the United States, infections have been reported in many states. The most endemic areas are:
      • Islands off the coast of Massachusetts (including Nantucket and Martha’s Vineyard)
      • New York (including Long Island, Shelter Island, and Fire Island), Connecticut
      • Asymptomatic infection common in these areas (1)
  • Incubation period varies from 5 to 33 days:
    • Most patients do not recall specific tick exposure.
    • After transfusion of infected blood, the incubation period can be up to 9 weeks (1).
  • System(s) affected: cardiovascular, gastrointestinal, hemic/lymphatic/immunologic, musculoskeletal, nervous, pulmonary, renal/urologic

Pediatric Considerations
Transplacental and perinatal transmission rarely reported (1),(2)

Geriatric Considerations

  • Morbidity and mortality higher in patients >60 years
  • Cases more common in patients >70 years who have medical comorbidities.


  • Babesiosis affects patients of all ages. Most patients present in their 40s or 50s (1).
  • Coinfection with other tick-borne illness is increasingly recognized.


  • Cases reported to the Centers for Disease Control and Prevention appear to be on the rise from 911 in 2012 peaking at 2,368 in 2017 before falling slightly to 2,101 in 2018.
  • Prevalence is difficult to estimate due to lack of surveillance and asymptomatic infections.
  • Transfusion-associated babesiosis and transplacental/perinatal transmission have been reported (1).
  • In patients at high risk for tick-borne diseases, seroconversion data show antibodies to Babesia microti in 7 of 671 individuals (1%) (1).

Etiology and Pathophysiology

  • B. microti (in the United States) and Babesia divergens and Babesia bovis (in Europe) cause most human infections (1). B. divergens and a new strain Babesia duncani appear to be more virulent. Other species identified in case reports. All share morphologic, antigenic, and genetic characteristics (1).
  • Ixodid (hard-bodied) ticks, particularly Ixodes dammini (Ixodes scapularis: deer tick) and Ixodes ricinus, are the primary vectors.
  • The white-footed deer mouse is the primary reservoir.
  • Infection is passed to humans through the saliva of a nymphal-stage tick during a blood meal. Sporozoites introduced at the time of the bite enter red blood cells and form merozoites through binary fission (classic morphology on blood smear). Humans are a dead-end host for B. microti.

Risk Factors

  • Residing in endemic areas
  • Asplenia
  • Immunocompromised state

General Prevention

  • Avoid endemic regions during the peak transmission months of May to September (1).
  • Appropriate insect repellent is advised during outdoor activities, especially in wooded or grassy areas:
    • 10–35% N,N-diethyl-meta-toluamide (DEET) provides adequate skin protection (1).
    • Acaricides, such as permethrin, provide impregnated clothing with even further protection.
  • Early removal of ticks—daily skin checks
  • Examine pets for ticks; flea/tick control for pets

Commonly Associated Conditions

  • Coinfection with Borrelia burgdorferi and B. microti, particularly in endemic areas. Coinfection rates may be as high as ~27%.
  • Coinfection with Ehrlichia (1)

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