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Synonym(s) -
Prevalence <1 / 1 000 000
Inheritance Not applicable
Age of onset All ages
  • B60.0
  • C0004576
  • D001404
  • 10003965


Disease definition

Babesiosis is an infectious disease caused by protozoa of the genus Babesia and characterized by a febrile illness and hemolytic anemia but with manifestations ranging from an asymptomatic infection to a fulminating illness that can result in death.


The worldwide prevalence is unknown but it is most frequently reported in the U.S. In 2011, 1,124 cases were reported in the U.S. with 92% of those cases occurring in northeastern and upper midwestern states. Sporadic cases have also been reported in Europe, Asia, Africa, Australia and South America.

Clinical description

Most immunocompetent patients experience mild to moderate illness or are asymptomatic. Symptoms usually begin 1-4 weeks after being bitten by an infected tick, or 1 week to 6 months after a transfusion with contaminated blood. The most common symptoms are fever (that can reach up to 40.9 °C and may be accompanied by splenomegaly), malaise and fatigue. Other common manifestations include chills, sweats, headache, myalgia, arthralgia, nonproductive cough, nausea, and anorexia. Less frequently, some may experience sore throat, photophobia, vomiting, weight loss and depression. Symptoms can last 1-2 weeks but fatigue may persist for several months. In the immunocompomised or the elderly, more severe and prolonged infections with relapses can occur that require hospital admission. B. divergens and B. duncani infections are generally severe and often occur in people who lack a spleen. Complications occur in about half of hospitalized patients and include disseminated intravascular coagulopathy and acute respiratory distress syndrome and less frequently liver, renal or congestive heart failure, coma and death.


Babesiosis is caused by infections with one of several Babesia species that infect humans, including: B. microti (most common, in the U.S.), B. duncani, B. divergens (most common in Europe), and B. venatorum. Most patients are infected by a bite from an infected nymphal Ixodes scapularis (in the U.S), or Ixodes ricinus (in Europe) tick during early summer to late fall. The parasite can also be transmitted to humans through transfusion of contaminated blood or blood products. A few cases of transplacental transmission have been reported.

Diagnostic methods

Babesiosis should be considered in patients with an unexplained febrile illness who reside in or have recently traveled to a Babesia endemic area or who have received a blood transfusion within the previous 6 months. Laboratory tests usually reveal hemolytic anemia, thrombocytopenia and a normal or slightly decreased leukocyte count. Giemsa or Wright stains of blood smears identify the parasites within host erythrocytes that appear round, pear-shaped or oval with a blue cytoplasm with red chromatin. Polymerase chain reaction (PCR) can confirm diagnosis by detecting babesia DNA in a patient's blood. Serological testing can also be useful in confirming the diagnosis.

Differential diagnosis

Differential diagnoses include other infectious diseases such as malaria, Lyme disease, ehrlichiosis, Rocky Mountain spotted fever, typhoid (see these terms) and infectious mononucleosis.

Management and treatment

Treatment for mild to moderate babesiosis consists of the administration of antimicrobial agents, atovaquone and azithromycin, for 7-10 days. Oral quinine and intravenous clindamycin are recommended for those with a severe babesiosis, but patients must be more closely monitored due to higher frequency of adverse effects. Severe disease is treated with partial or complete exchange transfusion. Severely immunocompromised patients may require 6 weeks of antimicrobial therapy if infection persists or recurs. Preventive measures include avoidance of areas where ticks, mice and deer thrive and the use of tick checks, protective clothing and tick repellents.


Prognosis depends on the species involved and the health of the patient, but is excellent in most immunocompetent patients.

Expert reviewer(s)

  • Dr Peter KRAUSE

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