OBJECTIVES : Human babesiosis, caused by intraerythrocytic protozoan parasites of the genus Babesia, can be an asymptomatic or mild-to-severe disease that may be fatal. The study objective was to assess incident babesiosis occurrence among the U.S. elderly Medicare beneficiaries, ages 65 and older, during 2006-2017.

METHODS : Our retrospective claims-based study utilized large Medicare administrative databases. Babesiosis cases were identified based on the recorded diagnosis codes. The study assessed babesiosis occurrence rates (per 100,000 elderly Medicare beneficiaries) overall and by year, diagnosis month, demographics, state and county of residence.

RESULTS : During a 12-year study period, 19,469 elderly Medicare beneficiaries had a babesiosis diagnosis recorded, for an overall national rate of about 6 per 100,000 person-years. The study results showed a significant increase in babesiosis occurrence over time (p<0.05): from 4 per 100,000 in 2006 to 9 per 100,000 in 2017. The highest babesiosis rates by state (per 100,000) were identified in: Massachusetts (62), Rhode Island (61), Connecticut (51), New York (30), New Jersey (19), New Hampshire (12), Maine (11), and Vermont (10). The highest rates by county were identified in: Nantucket, MA (1,089); Dukes, MA (236); Barnstable, MA (213); Dutchess, NY (205); Washington, RI (187); Windham, CT (177); Plymouth, MA (177); and Ulster, NY (151). Seventy-four percent of all cases were diagnosed from May through October. Babesiosis occurrence declined with advancing age and was significantly higher among males vs. females and whites vs. non-whites.

CONCLUSIONS : Our study shows significantly increasing babesiosis occurrence among the U.S. elderly during 2006-2017, with highest rates in the babesiosis-endemic counties and states. The study also shows variation in babesiosis occurrence by age, gender, race, and diagnosis months likely related to tick exposure, and a potential spread to states previously considered non-endemic. Overall, our study highlights the importance of large administrative databases in assessing occurrence of emerging infections in the United States.