• Rare tick-borne hemolytic disease caused by intraerythrocytic protozoan parasites of the genus Babesia
  • Infrequently reported outside the United States
    • Internationally, sporadic cases have been reported from France, Italy, the United Kingdom, Ireland, the former Soviet Union, and Mexico. China, Italy, and Turkey have also reported cases.
    • In the United States, infections have been reported in many states. Transmission has primarily occurred in the Northeast and upper Midwest, especially in parts of New England, New York, Pennsylvania, New Jersey, Wisconsin, and Minnesota. Asymptomatic infection is common in these areas.
  • 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.
  • System(s) affected: cardiovascular, gastrointestinal, hemic/lymphatic/immunologic, musculoskeletal, nervous, pulmonary, renal/urologic

Pediatric Considerations
Transplacental and perinatal transmission is rare. Atovaquone has been used safely in children who weigh > 5 kg.

Pregnancy Considerations
Safety data on use of atovaquone in pregnant women are limited. However, there are limited safety data available to suggest administration of quinine plus clindamycin during pregnancy may be safe. For this reason, this drug combination rather than atovaquone plus azithromycin is generally recommended for treatment of symptomatic babesiosis during pregnancy.

Geriatric Considerations
Morbidity and mortality are higher in elderly populations, especially with those who have comorbidities.


  • Babesiosis affects patients of all ages. Most patients present in their 40s or 50s.
  • Coinfection with other tick-borne illness (anaplasmosis, ehrlichiosis, Lyme disease) is common.


Prevalence is difficult to estimate due to lack of surveillance and asymptomatic infections.

Etiology and Pathophysiology

  • B. microti (in the United States) and Babesia divergens and Babesia bovis (in Europe) cause most human infections. 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.
  • 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

Those residing in endemic areas are at elevated risk for contracting babesiosis. Complications from the disease are highest in those with asplenia, who are immunocompromised or elderly.

General Prevention

  • Avoid endemic regions during the peak transmission months of May to September.
  • 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.
    • Acaricides, such as permethrin, provide impregnated clothing with even further protection.
  • Daily skin checks to ensure early removal of ticks is essential.

Commonly Associated Conditions

  • Coinfection with Borrelia burgdorferi and B. microti in endemic areas has been reported and rates may be as high as ~27%.
  • Coinfection with Ehrlichia is not uncommon.

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