Clinical Pearls & Morning Reports
Published February 1, 2023
Left untreated, babesiosis can range in severity from asymptomatic disease (observed in approximately 20% of healthy adults) to severe illness that is complicated by acute respiratory distress syndrome and disseminated intravascular coagulation. Read the NEJM Clinical Problem-Solving Article here.
Q: Where do most cases of babesiosis occur?
A: In 2019, a total of 2420 cases of babesiosis were reported to the Centers for Disease Control and Prevention, 93.1% of which occurred in the Northeast region of the United States, with most of the remaining 6.9% of cases scattered across the South Atlantic and upper Midwest regions. Most infections occur between May and September and are caused by the protozoan Babesia microti, an obligate parasite of human erythrocytes that is transmitted to humans through the bite of the Ixodes scapularis tick, with an incubation period of 1 to 4 weeks. As a result of the warming of the North American climate, the range of I. scapularis is extending northward beyond the Canadian border, and temperatures are expected to remain suitable for tick activity during longer periods of the year.
Q: When should babesiosis be suspected?
A: Babesiosis should be suspected in any patient who is living in, or has recently visited, an area where the condition is endemic and is having fever, sweats, chills, headache, or fatigue with hemolytic anemia, thrombocytopenia, and elevated aminotransferase levels, or in patients in whom other infections transmitted by I. scapularis (such as Lyme disease, anaplasmosis, Borrelia miyamotoi infection, and Powassan virus infection) have been diagnosed.
A: Diagnosis of tickborne illness requires a high index of clinical suspicion and familiarity with local epidemiologic factors because many patients will not recall a tick bite. On microscopic examination of a thin blood smear through an oil-immersion lens with Wright-Giemsa staining, B. microti trophozoites appear as round, oval, or pear-shaped ring forms within erythrocytes that are occasionally arranged in pathognomonic tetrads. The sensitivity of microscopy has been reported to range from 59 to 84%, as compared with 95% for PCR testing. A PCR test can be used to establish a diagnosis in cases of low-level parasitemia that is undetectable on a blood smear, when personnel with expertise in microscopy are unavailable, or when species-level identification is desired.
A: The preferred initial treatment regimen for babesiosis, regardless of disease severity, is atovaquone and azithromycin for 7 to 10 days, with longer treatment courses reserved for patients who are highly immunocompromised or who have refractory disease. Clindamycin and quinine sulfate is an alternative treatment regimen; in an unblinded, randomized trial involving 58 immunocompetent patients with babesiosis, this regimen showed efficacy that was similar to that of atovaquone and azithromycin but resulted in a higher incidence of adverse effects. Exchange transfusion is reserved for patients with greater than 10% parasitemia, severe hemolytic anemia, disseminated intravascular coagulation, or respiratory, renal, or hepatic failure. Treatment response can be monitored by the evaluation of serial peripheral-blood smears in immunocompromised patients, but resolution of parasitemia need not be confirmed in immunocompetent patients whose symptoms have resolved; repeat PCR testing should be reserved for patients with persistent symptoms whose blood smears no longer show B. microti. Unlike most tickborne parasites, B. microti is not susceptible to doxycycline, and the use of doxycycline as prophylaxis after a tick bite does not prevent babesiosis.