Babesiosis among Elderly Medicare Beneficiaries, United States, 2006–2008

We used administrative databases to assess babesiosis among elderly persons in the United States by year, sex, age, race, state of residence, and diagnosis months during 2006–2008. The highest babesiosis rates were in Connecticut, Rhode Island, New York, and Massachusetts, and findings suggested babesiosis expansion to other states.

H uman babesiosis is a zoonotic disease caused by intraerythrocytic protozoan parasites of Babesia species. In the United States, Babesia microti is the primary etiologic agent of human babesiosis and is usually transmitted through the bite of Ixodes scapularis, the principal tick vector for this species (1)(2)(3). Human B. microti infections are regional, endemic to Northeastern (Connecticut, Rhode Island, Massachusetts, New York, New Jersey) and Midwestern (Minnesota, Wisconsin) states, and the geographic range is believed to be expanding (1,2,4,5). Babesiosis is characteristically seasonal, with peak transmission from May through September (1,2,6). In younger persons, babesiosis is more likely to be a mild or asymptomatic disease that may persist for months or even years undetected (1,2,7,8). Elderly, splenectomized, and other immunocompromised persons tend to be symptomatic (e.g., fever, chills, fatigue) and at risk for complications, including hemolytic anemia, acute respiratory failure, renal failure, and death (1)(2)(3)9).
Recently, there has been an increase in the number of reported clinical and transfusion-transmitted babesiosis cases in the United States (1,2,10

The Study
We used 100% inpatient, outpatient, skilled nursing facility, and carrier standard analytical as well as Medicare enrollment fi les for calendar years 2006-2008 to assess babesiosis among elderly Medicare benefi ciaries ages >65 years of age. Standard analytical fi les are generated to capture medical services rendered and patient diagnoses, and enrollment fi les help to ascertain coverage eligibility. To be eligible for the study, benefi ciaries had to be continuously enrolled in Medicare fee-for-service Parts A and B for >365 days before and including the latest month of continuous enrollment in the calendar year. We identifi ed likely new babesiosis cases on the basis of the fi rst recording of the International Classifi cation of Diseases, 9th Revision, Clinical Modifi cation diagnosis code 088.82 during the calendar year, with no recorded babesiosis infection in the preceding 365 days.
We assessed annual babesiosis rates by estimating the number of cases recorded per 100,000 benefi ciaries per calendar year, overall and by sex, age, race, and state of residence. Seasonal occurrence was analyzed by using the number of cases in each month and number of benefi ciaries continuously enrolled in Medicare fee-for-service within 365 days of each month. Cases were assigned age on the basis of diagnosis date, and persons without babesiosis had age assessed at the beginning of the latest enrollment month in the year. Benefi ciaries with babesiosis were excluded from denominators of subsequent calendar years or diagnosis months. We performed χ 2 tests comparing babesiosis rates by using Epi Info version 3. 5

Conclusions
We report a national population-based study of babesiosis among the US elderly, which used large administrative databases. The study found variations in the number of babesiosis cases by year, state, race, sex, age, and diagnosis month. Overall, our 3-year study suggests that there were more cases of babesiosis in 2008 compared with previous years. Northeastern and Mid-Atlantic States accounted for most newly diagnosed cases among the US elderly, with state-specifi c rates up to 10× higher than national annual rates. Our results show highest babesiosis rates in known babesiosis-endemic states of Connecticut, Rhode Island, New York, and Massachusetts and suggest possible expansion of human babesiosis to Maryland, Virginia, and other states. Human encroachment into tick and deer habitat, growth of deer population, climatic effects, and travel to disease-endemic areas may be responsible for variations in number of babesiosis cases and spread of the infection to non-disease-endemic states (1)(2)(3)9,11). Our fi ndings show babesiosis trends similar to surveillance results in Rhode Island and New York State over the same period, with fewer cases reported in 2007, and more in 2008 (12,13). Data from the Connecticut Department of Public Health demonstrate broad annual variation in numbers of reported babesiosis cases in 2000-2008, with an increasing trend over time (14). Our fi nding of higher babesiosis rates among men versus women and among younger elderly women versus older elderly women are generally consistent with state surveillance data (12,15). Similarly to the literature (1,2,6,12,15), our study shows that most babesiosis cases are diagnosed during May through October. These fi ndings are likely related to life cycle and activity of the tick vector and to activity of human and other mammalian hosts (1,2,9).
Study limitations are related to the use of administrative databases and include diffi culty in identifying incident versus prevalent cases, possible misdiagnosis, and lack of clinical detail for diagnosis verifi cation and transfusiontransmitted babesiosis case identifi cation, as well as inability to differentiate Babesia species. Medical record review is needed to address above-mentioned limitations. Choosing a different continuous enrollment period could produce slightly different rates. Although Medicare data do not provide population-wide information on babesiosis among persons <65 years of age, younger persons are more likely to remain asymptomatic and less likely to get a diagnosis (1,2,7).
Our nationwide large medical database study is an additional tool to better understand regional, seasonal, and other babesiosis transmission patterns, by year and demographic characteristics, among the US elderly. Because the elderly are also known to use the majority of transfused blood, studies are needed to evaluate transfusiontransmitted babesiosis in this group. Overall, our study suggests that large administrative databases can be useful for assessing emerging infections in the United States.