METHODS: Data from routine case investigations were compiled from all four states. We conducted a descriptive analysis regarding multiple variables, including patient residence, age, sex, illness onset date, report date, presenting clinical description, diagnostic method, health outcome, and possible risk factors.
RESULTS: As of September 30, 2015, a total of 100 tularemia cases were reported among residents of Colorado (n = 43), Nebraska (n = 21), South Dakota (n = 20), and Wyoming (n = 16), representing a significant increase above the baseline annual mean number of four, seven, seven, and two cases, respectively, reported in each state during 2004–2014. Patient age range was 10 months–89 years (median, 56 years); 74 patients were male. Forty-eight hospitalizations were reported; one death occurred in a man aged 85 years. Median days between illness onset and report date was 19 (range, 4–177). Common clinical presentations were respiratory disease (n = 26), skin lesions with lymphadenopathy (n = 26), and febrile illness without localizing signs (n = 25). Cases were diagnosed by using serology (n = 58) and bacterial isolation (n = 42). Reported risk factors included animal contact (n = 51), environmental aerosolizing activities (n = 49), and arthropod bites (n = 34). Specific animal exposures included cats (n = 23), rabbits (n = 22), and rodents (n = 17). Engaging in outdoor labor was reported (n = 27), specifically lawn mowing (n = 14). Environmental exposures included arthropod bites from ticks (n = 11) or flies (n = 9). Although 41 patients reported >2 possible exposures, no predominating shared exposure among cases was identified.
CONCLUSIONS: Lack of predominating risk factors among patients resulted in no clear epidemiologic explanation for the noteworthy regional increase in tularemia cases during 2015. Time from symptom onset to report date and multiple possible exposures complicate efforts to identify a single risk factor. Continued tularemia case monitoring and assessments of specific exposures are needed.