Background: The incidence of tickborne disease has recently increased in the United States, as longer summers and milder winters have led to a surge in tick populations (1). The impact has been most marked in New England, where temperatures are rising faster than the global average, particularly in winter (2). Babesiosis is a tickborne illness of special concern, given its high fatality rates in older and immunocompromised patients. Once restricted to coastal islands, babesiosis has become hyperendemic in New England. Between 2011 and 2019, the incidence of babesiosis increased by 193% in Massachusetts, 338% in Connecticut, 1422% in Maine, and 1602% in Vermont (3). We examined 31 years of data at our institutions to see if more patients are developing babesiosis outside of the peak summer months, which might lead to delayed diagnosis and worse outcomes.
Methods: We performed a retrospective chart review of adults (age ≥18 years) presenting with babesiosis to 3 teaching hospitals in Boston, Massachusetts (Brigham and Women’s Hospital, Brigham and Women’s Faulkner Hospital, and Massachusetts General Hospital) between May 1, 1993 and May 1, 2024. Charts were identified via the Research Patient Data Registry (RPDR), a data warehouse containing inpatient and outpatient records from multiple from multiple hospital systems, using International Classification of Diseases, Ninth Revision (ICD-9) codes 088.82 and Tenth Revision (ICD-10) codes B60.0. For the diagnosis of babesiosis, we required either a positive blood parasite smear or serum PCR; we excluded cases diagnosed by serology alone, as antibody testing does not reliably distinguish between active and resolved Babesia infection. The study was approved by the Partners Institutional Review Board (protocol 2024P000553). Informed consent was waived due to the lack of use of identifiable health information and the logistical difficulties in obtaining informed consent in a retrospective chart review study.
Results: 1975 patient records were reviewed. Of these, 845 cases were rejected, either due to inadequate documentation, negative babesiosis testing, use of serology or other non-standard testing, possible disease acquisition outside of New England, age < 18 years at disease onset, or coding errors. Of the 1130 cases, 986 were acquired in Massachusetts, 43 in New Hampshire, 25 in Rhode Island, 8 in Maine, 6 in Connecticut, and 1 in Vermont. 61 patients reported recent travel to or residence in 2 or more New England states. The proportion of cases with symptom onset during the peak months of June, July, and August was similar in 1993-2013 (230/294 cases, 78%) and 2014-2024 (667/836 cases, 80%). However, the proportion of cases with symptom onset in late winter and early spring increased in recent years. Between 1993 and 2013, 10 of 294 cases (3.4%) had symptom onset during February-May, compared to 56 of 836 cases (6.7%) diagnosed between 2014 and 2024 (p=.04 by chi-square).
Conclusions: Our results suggest that the rise in babesiosis cases in New England has been accompanied by a spike in cases presenting in late winter and early spring, presumably due to increased tick survival and earlier activity. In endemic areas, babesiosis should be suspected in all patients presenting with fever and anemia, even during the winter months.