Babesiosis Surveillance - United States, 2011-2015.

Problem/condition: Babesiosis is caused by parasites of the genus Babesia, which are transmitted in nature by the bite of an infected tick. Babesiosis can be life threatening, particularly for persons who are asplenic, immunocompromised, or elderly.

Period covered: 2011-2015.

Description of system: CDC has conducted surveillance for babesiosis in the United States since January 2011, when babesiosis became a nationally notifiable condition. Health departments in states in which babesiosis is reportable voluntarily notify CDC of cases through the National Notifiable Diseases Surveillance System (NNDSS) and submit supplemental case information by using a babesiosis-specific case report form (CRF). As of 2015, babesiosis was a reportable condition in 33 states compared with 22 states in 2011.

Results: For the 2011-2015 surveillance period, CDC was notified of 7,612 cases of babesiosis (6,277 confirmed [82.5%] and 1,335 probable [17.5%]). Case counts varied from year to year (1,126 cases for 2011, 909 for 2012, 1,761 for 2013, 1,742 for 2014, and 2,074 for 2015). Cases were reported among residents of 27 states. However, 7,194 cases (94.5%) occurred among residents of seven states with well-documented foci of tickborne transmission (i.e., Connecticut, Massachusetts, Minnesota, New Jersey, New York, Rhode Island, and Wisconsin). Maine (152 cases) and New Hampshire (149 cases) were the only other states that reported >100 cases for the 5-year period, and both states also reported increasing numbers of cases over time. The median age of the 7,173 patients with available information was 63 years (range: <1-99 years; interquartile range: 51-73 years); 4,156 (57.9%) were aged ≥60 years, and 15 (<1%) were aged <1 year. The proportion of patients with symptom onset during June-August was >70% for each of the 5 surveillance years. Approximately half (3,004 of 6,404 [46.9%]) of the patients with available data were hospitalized at least overnight. Hospitalization rates ranged from 16.0% among patients aged 10-19 years (16 of 100) to 72.6% among those aged ≥80 years (552 of 760). Hospitalizations were reported significantly more often among patients who were asplenic than among patients who were not (106 of 126 [84.1%] versus 643 of 1,396 [46.1%]). Fifty-one cases of babesiosis among recipients of blood transfusions were classified by the reporting health department as transfusion associated. The median intervals from the earliest date associated with each case of babesiosis to the initial report via NNDSS and submission of supplemental CRF data to CDC were approximately 3 months and 1 year, respectively.

Interpretation: For the first 5 years of babesiosis surveillance, the reported cases occurred most frequently during June-August in the Northeast and upper Midwest. Maine and New Hampshire reported increasing numbers of cases over time, which suggests that foci of transmission might be expanding. Hospitalizations were common, particularly among patients who were asplenic or elderly.

Public health action: Persons who live in or travel to regions where babesiosis is endemic should avoid tick-infested areas, apply repellent to skin and clothing, conduct full-body inspections for ticks after being outdoors, and remove attached ticks with fine-tipped tweezers as soon as possible. Prevention measures are especially important for persons at risk for severe babesiosis. Increases in the number and geographic range of reported cases warrant investigation to identify contributory factors (e.g., changes in tick density or in testing or surveillance methods). Complete and timely submission of risk factor data could facilitate assessments of the geographic ranges and transmission routes of Babesia parasites. Efforts to allow for electronic submission of CRF data are under way at CDC; electronic submission is expected to improve the timeliness, uniformity, and completeness of the data.