By Stan Deresinski, MD, FACP, FIDSA

Clinical Professor of Medicine, Stanford University

SYNOPSIS: Babesiosis has been increasing in frequency and geographic reach in the United States among individuals ≥ 65 years of age. The Infectious Diseases Society of America has released a new updated guideline assisting clinicians in the diagnosis and management of the potentially lethal infection.

SOURCES: Menis M, Whitaker BI, Wernecke M, et al. Babesiosis occurrence among United States Medicare beneficiaries, ages 65 and older, during 2006-2017: Overall and by state and county of residence. Open Forum Infect Dis 2020;8:ofaa608.

Krause PJ, Auwaerter PG, Bannuru RR, et al. Clinical Practice Guidelines by the Infectious Diseases Society of America (IDSA): 2020 Guideline on Diagnosis and Management of Babesiosis. Clin Infect Dis 2021;72:185-189.

Menis and colleagues examined large Medicare administrative databases to identify recorded United States cases of babesiosis in Medicare recipients ages > 65 years from 2006-2017. They identified 19,649 cases, with a national incidence of six per 100,000 beneficiary years. The rate per 100,000 more than doubled over this time, increasing from four in 2006 to nine in 2017. 

This increase was even more dramatic in the seven states (Connecticut, Massachusetts, Rhode Island, New Jersey, New York, Minnesota, and Wisconsin) considered to have well-established endemicity and accounting for 77.7% of all cases, where the rate increased from 17 to 42 per 100,000. Several-fold increases were seen in New Hampshire, Maine, Vermont, Pennsylvania, and Delaware, and increases were seen in a number of counties in states not generally considered to be endemic. The overall rate during the study period in the top three states was 51 per 100,000 in Connecticut, 61 in Rhode Island, and 62 in Massachusetts. The incidence varied greatly by county, with Nantucket, MA, being the champion at 1,089 cases per 100,000. Half of the cases were diagnosed during July through August, and three-fourths were diagnosed from May through October.

The Infectious Diseases Society of America recently published their guideline for the diagnosis and management of babesiosis. The following is a summary of the recommendations. The parenthetical comments indicate the strength of the recommendation followed by the quality of the evidence.

• The diagnosis should be confirmed by either examination of a peripheral blood smear or polymerase chain reaction (PCR). (Strong, Moderate)

• A single positive antibody test is not diagnostic of active infection. (Strong, Moderate)

• The recommended treatment is atovaquone plus azithromycin or clindamycin plus quinine (strong, moderate), with the former regimen preferred. Treatment is continued for seven to 10 days in immunocompetent patients.

Exchange transfusion may be considered in some patients with severe disease. (Weak, Low quality) These include selected patients with high-grade (> 10%) parasitemia or moderate-to-high grade parasitemia and related complications.

The response in immunocompetent patients should be monitored during treatment by serial examination of peripheral blood smears with no further testing after symptoms have resolved. (Strong, Moderate) In immunocompromised patients, examination of smears should continue until they are negative, but if symptoms persist, PCR testing should be performed. (Weak, Moderate)


Although limited to individuals 65 years of age, the report by Menis and colleagues gives an excellent idea of the extent of babesiosis in the United States. However, the disease is unevenly distributed, with some places seeing amazingly large numbers of cases — e.g., 1,089 cases per 100,000 Medicare beneficiary years in the county of Nantucket, MA. For comparison, although the incidence of tuberculosis in the state was 2.6 per 100,000 residents in 2019, Nantucket is not listed among the counties with cases.1 (The tuberculosis rate in 2018 in Santa Clara County, where I live and work, was 8.6 per 100,000 population.2 (I have never personally cared for a patient with babesiosis. Although cases are seen in California, these infections have mostly been acquired elsewhere.) The guidelines for diagnosis and treatment of babesiosis are straightforward in the abstract, but I am certain that many cases must present problems in both realms. An important issue in the microscopic diagnosis is differentiation of Babesia parasites from plasmodia. 


  1. Commonwealth of Massachusetts. Tuberculosis data and statistics.
  2. Santa Clara County Public Health. TB fact sheet.