We obtained the number of human cases of babesiosis and Lyme disease cases per year and county during1995–2011, from the Maine Center for Disease Control (Table 1) and obtained census data (http://quickfacts.census.gov/qfd/states/23000lk.html) for the years 2000 and 2010. We calculated incidence (cases/100,000 population) using the census for the year 2000 for 1995–2004 and the 2010 census for 2005–2011 (Table 1). Early (2001–2004) babesiosis cases occurred in the 2 southernmost coastal counties, but since 2005, cases have been reported in 8 additional counties, including 2 noncoastal counties (Figure 1). In addition to treatment with antiparasitic drugs, several severely ill patients underwent exchange transfusion. No deaths were reported.

For the period of 1995–2001, we reviewed published and unpublished data regarding presence of Babesia spp. in ticks tested by endpoint PCR (n = 1,433) (7–9) (Table 2). We examined data from all towns sampled, but to minimize spatial bias, we also examined data only from the town of Wells, from a site sampled in each study. Host-seeking adult ticks made up 66% of the samples, and fed nymphs made up the remainder (8) (Table 2). We tested for Babesia spp. in ticks using Feulgen stain (7–9), PCR (7–9,11), or both (Table 2). In Maine, I. scapularis ticks are known to harbor 2 Babesia spp: B. odocoilei (a deer parasite, presumed to be nonpathogenic to humans) and B. microti (7). However, 18s rRNA sequences (GenBank accession nos. AF028346, AF028343, respectively) to differentiate the species were difficult to obtain because of low DNA concentrations in some samples. Thus, for each study, we calculated the proportion of B. microti to B. odocoilei as the number of B. microti–positive ticks divided by the number of ticks positive for either B. microti or B. odocoilei (Table 2). Analyzing data from all towns sampled or only Wells, we observed an apparent increase in the ratio of B. microti to B. odocoilei over time that corresponded with the 2001 appearance of and subsequent increase in reported cases of babesiosis (Table 1). B. microti was documented in ticks only from the 2 southernmost counties.

We performed a longitudinal review of the abundance of I. scapularis adult ticks and prevalence of B. burgdorferi infection. Questing adult I. scapularis ticks were collected annually in the fall as previously described (12) at 2 long-term study sites in southern coastal Maine and episodically at other sites (Figure 2). A subset of these ticks was tested for B. burgdorferi infection by direct fluorescent microscopy (12). Table 1 shows upward trends in abundance of adult I. scapularis ticks, prevalence of B. burgdorferi infection, and incidence of Lyme disease and babesiosis.

Mather et al. (5) used logistic regression to demonstrate that abundance of questing nymphal I. scapularis ticks (nymphs per hour) of 19–135/hour in wooded areas predicted a ≥20% probability of human babesiosis cases in Rhode Island. Using SAS 9.2 (SAS Institute Inc., Cary, NC, USA), we examined the 20% probability of a babesiosis case as a function of adult ticks collected per hour, B. burgdorferi infection prevalence in adult ticks, or Lyme disease incidence.

We categorized babesiosis cases as present or absent and calculated the number of adult ticks collected per hour, B. burgdorferi infection prevalence, and Lyme disease incidence by county and year. We evaluated each univariate model by using the Wald χ²_W and assuming models to be significant at χ²_W p≤0.05; and the Hosmer and Lemeshow goodness-of-fit test χ²_HL, assuming suitable fit at χ²_HL p≥0.10. For the models, all χ²_W≥8.4 and p≤0.004, and all χ²_HL≤11.3 and p≥0.13. A 20% probability of >1 babesiosis case was predicted during years in which abundance of adult ticks collected exceeded 17 per hour (n = 83; 95% CI 12.1%–31.7% for a 20% probability), when B. burgdorferi infection prevalence among adult ticks exceeded 34% (n = 74; 95% CI 11.6%–33.6%), or when Lyme disease incidence exceeded 58 cases/100,000 population (n = 272; 95% CI 13.6%–27.9%).

With approval from the Maine Medical Center Institutional Review Board, and in collaboration with Coral Blood Services, Inc., (Scarborough, ME, USA) blood samples from healthy donors were de-identified and screened for Babesia antibodies during July–December 2010. Using postal codes, we selected 311 donors from the 2 southernmost Maine counties (Cumberland and York) where I. scapularis ticks are more abundant than elsewhere in the state (13). Samples were tested via indirect immunofluorescent antibody (IFA) tests as per Krause et al. (14) by using serum diluted 1:64. Of 311 blood samples, 10 (3.2%) tested positive for Babesia antibody (IgG), which is in the middle of the range for blood donors in the northeastern United States (0.2% in areas where babesiosis is not endemic to 7.3% in highly disease-endemic areas [15]). When serum samples were IFA-positive, DNA was extracted from the corresponding whole blood sample at the Yale University School of Public Health laboratory for real-time PCR (Peter Krause, proprietary protocol). B. microti DNA was not detected in any of the 10 antibody-positive serum specimens.

Early studies revealed a higher ratio of presumed nonpathogenic B. odocolei to B. microti in areas where these species co-exist. The observed temporal shift to B. microti in questing adult and fed nymphal I. scapularis ticks in this review could be related to ecologic change, or to sampling bias (7). Adult I. scapularis tick abundance, B. burgdorferi infection prevalence among these ticks, and Lyme disease incidence may assist in the prediction of human babesiosis risk. The association observed between number of babesiosis cases and adult I. scapularis tick abundance was on a scale similar to that found by Mather et al. (5), although that study used nymphal tick abundance. IFA positivity for Babesia spp. IgG antibody at >1:64 has a high specificity (90%–100%) (14), but its predictive value is uncertain because disease prevalence is unknown for healthy blood donors. As human babesiosis emerges in Maine, refined models correlating babesiosis cases with tick abundance or Lyme incidence, or both, may help track geographic risk and permit targeted screening of the blood supply.