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  • 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).

  • Cases reported to the CDC appear to be on the rise from 911 in 2012 to 1,744 in 2014.
  • 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 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 protection (1).
  • 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 (1). Coinfection rates may be as high as ~27%.
  • Coinfection with Ehrlichia (1)

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