Babesiosis is a tickborne disease caused by intraerythrocytic Babesia parasites. In the United States, most babesiosis cases are caused by Babesia microti, transmitted from bites of blacklegged ticks, Ixodes scapularis, in northeastern and midwestern states. Transmission can also occur through blood transfusions, transplantation of organs from infected donors, or congenital (mother-to-child) transmission (1). Babesia infection can be asymptomatic or cause mild to severe illness that can be fatal. Overall, U.S. tickborne disease cases have increased 25%, from 40,795 reported in 2011 to 50,856 in 2019 (2). Babesiosis trends were assessed in 10 states* where babesiosis was reportable during 2011–2019. Incidence increased significantly in Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, and Vermont (p<0.001), with the largest increases reported in Vermont (1,602%, from two to 34 cases), Maine (1,422%, from nine to 138), New Hampshire (372%, from 13 to 78), and Connecticut (338%, from 74 to 328). Unlike the other seven states, Maine, New Hampshire, and Vermont, were not included as states with endemic disease in previous CDC babesiosis surveillance summaries. These three states should now be considered to have endemic transmission comparable to that in other high-incidence states; they have consistently identified newly acquired cases every year during 2011–2019 and documented presence of Babesia microti in the associated tick vector (3). Because incidence in Northeastern states, including Maine, New Hampshire, and Vermont, is increasing, tick prevention messaging, provider education, and awareness of infection risk among travelers to these states should be emphasized.

Babesiosis can cause illness ranging from asymptomatic or mild to severe; the disease can be fatal, particularly among persons who are immunocompromised or asplenic. Common symptoms include fever, muscle and joint pain, and headache. In certain patients, severe complications can occur, including thrombocytopenia, renal failure, and acute respiratory distress syndrome (1). Babesiosis can be treated using a combination of antimicrobial medications, such as azithromycin and atovaquone (2).

The first case of human babesiosis acquired in the United States was identified in 1969 on Nantucket Island, Massachusetts (4). In 2011, babesiosis became a nationally notifiable condition. Where babesiosis is reportable, cases are reported to CDC by state health departments. Until now, CDC considered babesiosis to be endemic in seven states: Connecticut, Massachusetts, Minnesota, New Jersey, New York, Rhode Island, and Wisconsin (5). In 2019, the U.S. Food and Drug Administration (FDA) recommended screening blood donations for Babesia in states where residents were considered to be at high risk for Babesia infection. As a result, FDA recommended blood donation screening in the following 15 states or jurisdictions: Connecticut, Delaware, Maine, Maryland, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, Virginia, Wisconsin, and the District of Columbia (6).

Previous studies have examined babesiosis transmission and found increasing case counts or rates in particular geographic areas, such as New York (7) in previous years (2011–2015) (4) and among specific populations, such as those enrolled in Medicare (8). The current study identifies trends in babesiosis in the United States during 2011–2019 and highlights establishment of endemic transmission in new geographic areas. Tracking babesiosis transmission over time provides important data to monitor the transmission risk in areas with and without endemic disease.

This analysis used data from the previously described national babesiosis surveillance system (4). These data included reported cases from the 41 states where babesiosis was reportable during 2011–2019 (5); data reported by the state of New York and New York City were merged and are referred to as New York. Trends were tracked over time by including in the analysis all states that met the following criteria: 1) data were submitted for the entire analytic time span (2011–2019), and 2) 10 or more babesiosis cases were reported for ≥2 consecutive years. Using these criteria, case data reported by Connecticut, Maine, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Rhode Island, Vermont, and Wisconsin were included. Yearly incidence and overall percent rate change from 2011 to 2019 were calculated for each state. State babesiosis rates were modeled with Poisson regression. An overall model was fit, controlling for state, with year of diagnosis as a continuous variable. State-level models were also fit, controlling for event year (symptom onset or laboratory diagnosis date) as a continuous variable. The natural logarithm of the state’s census population for each year was used in the offset (a variable used when data are recorded over an observed period) to control for state population. All analyses were conducted using SAS (version 9.4, SAS Institute). This activity was reviewed by CDC and was conducted consistent with applicable federal law and CDC policy.^†

During 2011–2019, a total of 16,456 cases of babesiosis were reported to CDC by 37 states, including 16,174 (98.2%) reported from the 10 states included in this analysis (Figure). New York reported the largest number of cases (4,738 total; average = 526.4 per year), followed by Massachusetts (4,136; 459.6), and Connecticut (2,200; 244.4). The lowest numbers of cases were reported in Vermont (114; 12.7) and New Hampshire (340; 37.8). Incidences ranged from 0.32 per 100,000 population in Vermont in 2011 to 18.0 in Rhode Island in 2015 (Table). The three states with the highest reported incidences were Rhode Island (18.0 per 100,000 population in 2015), Maine (10.3 in 2019), and Massachusetts (9.1 in 2019).

Vermont, Maine, and New Hampshire experienced the largest percent change in incidence between 2011 and 2019. Vermont reported two cases in 2011 (incidence = 0.3 per 100,000 population) and 34 cases in 2019 (5.4), representing a 1,602% increase in incidence. Maine reported nine cases in 2011 (0.7) and 138 cases in 2019 (10.3), a 1,422% rate increase. Reported cases in New Hampshire increased from 13 in 2011 (1.0) to 63 in 2019 (4.6), a 372% rate increase. Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, and Vermont reported significant changes in annual babesiosis incidence. Annual incidence did not change significantly in Minnesota and Wisconsin. Incidence trended upward in Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, and Vermont, whereas incidence in Minnesota and Wisconsin remained stable.