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  • History of tick bite or exposure to ticks.

  • Fever, flu-like symptoms, anemia.

  • Intraerythrocytic parasites on Giemsa-stained blood smears.

  • Positive serologic tests.


Babesiosis is an uncommon intraerythrocytic infection caused mainly by two Babesia species and transmitted by Ixodes ticks. In the United States, hundreds of cases of babesiosis have been reported, and infection is caused by Babesia microti, which also infects wild mammals. Most babesiosis in the United States occurs in the coastal northeast, with some cases also in the upper Midwest, following the geographic range of the vector Ixodes scapularis, and Lyme disease and anaplasmosis, which are spread by the same vector. The incidence of the disease appears to be increasing in some areas. Babesiosis is caused by Babesia divergens in Europe and by Babesia venatorum in China. Babesiosis due to Babesia duncani and other Babesia-like organisms have been reported uncommonly from the western United States. Babesiosis can also be transmitted by blood transfusion, but blood supplies are not screened. A survey of a large set of blood samples from endemic regions of the United States identified approximately 0.4% as potentially infectious for B microti.


A. Symptoms and Signs

Serosurveys suggest that asymptomatic infections are common in endemic areas. With B microti infections, symptoms appear one to several weeks after a tick bite; parasitemia is evident after 2–4 weeks. Patients usually do not recall the tick bite. The typical flu-like illness develops gradually and is characterized by fever, malaise, fatigue, headache, anorexia, and myalgia. Other findings may include nausea, vomiting, abdominal pain, arthralgia, sore throat, depression, emotional lability, anemia, thrombocytopenia, elevated transaminases, and splenomegaly. Parasitemia may continue for months to years, with or without symptoms, and the disease is usually self-limited. Severe complications are most likely to occur in older persons or in those who have had splenectomy. Serious complications include respiratory failure, hemolytic anemia, disseminated intravascular coagulation, heart failure, and AKI. In a study of hospitalized patients, the mortality rate was 6.5%. Most recognized B divergens infections in Europe have been in patients who have had splenectomy. These infections progress rapidly with high fever, severe hemolytic anemia, jaundice, hemoglobinuria, and AKI, with death rates over 40%.

B. Laboratory Findings

Identification of the intraerythrocytic parasite on Giemsa-stained blood smears establishes the diagnosis (eFigure 35–13). These can be confused with malaria parasites, but the morphology is distinctive. Repeated smears are often necessary because well under 1% of erythrocytes may be infected, especially early in infection, although parasitemias can exceed 10%. Diagnosis can also be made by PCR, which is more sensitive than blood smear. An indirect immunofluorescent antibody test for B microti is available from the CDC; antibody is detectable within 2–4 weeks after the onset of symptoms and persists ...

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