This was a naturally occuring outbreak of tularemia involving 14 patients with exposures along the western side of Utah Lake. Results of the cohort study were consistent with transmission by deer flies, a known vector for tularemia in Utah. This finding was supported by local reports and other cohort study results that both deer-fly and rabbit populations were high throughout the outbreak area during June and July 2007. Evidence of a tularemia epizootic among rabbits and a consequent rabbit die-off was gathered from an environmental study, as previously reported.10
Eleven of 12 (92%) collected rabbit carcasses tested positive for F. tularensis
, including two F. tularensis
subsp. holarctica (type B) and nine F. tularensis
subsp. tularensis (type A; four type A1 and one type A2). Subtyping of isolates and PCR-positive materials from 11 patients revealed that all were infected with F. tularensis
type A (six type A1 and five type A2).13
This is the first known documentation of multiple subspecies and clades in a localized outbreak, contrasting with previous findings of the geographic characteristics and associated vectors of F. tularensis
subspecies and clades.1,10
The deer flies, which were collected three weeks after the outbreak, did not test positive for F. tularensis
, but this was not a surprising finding because long-term maintenance of F. tularensis
has not been shown to occur.
Tularemia is endemic in Utah, and an outbreak in Utah linked to an epizootic among rabbits has been previously reported.6
During a three-month period in 1971, a total of 39 tularemia cases were reported in Utah. As in the 2007 outbreak, the majority of patients (72%) were presumably infected through deer-fly bites. A major difference between the 1971 and 2007 Utah outbreaks was the location of the infectious exposure. The 1971 outbreak involved exposures within 11 counties in Utah. The 2007 outbreak was localized to a particular area on the western side of Utah Lake.
Two risk factors for disease in this outbreak were having been bitten by a deer fly and having worn a hat. In a univariate analysis, having worn insect repellent, including DEET, was a risk factor. Neither wearing a hat nor wearing insect repellent was a protective factor, as might be expected. Approximately half of all cohort study respondents reported having worn a hat, and more than half reported use of DEET. People who wore DEET were possibly protected against tularemia transmitted by tick bites. If used correctly, DEET can be effective against certain arthropods that transmit tularemia (i.e., mosquitoes and ticks), but it has not been reported to be effective against deer flies. This likely explains, at least in part, why wearing insect repellent was not a protective factor. Another potential explanation of these risk factors might be differential recall (i.e., recall bias); ill respondents might have been more likely to recall having used insect repellent or wearing a hat, which are both considered optimal behaviors. Finally, these factors might simply be markers of exposure, reflecting greater repellent use and hat wearing among people who spent more time outdoors where deer flies were active.
This outbreak highlighted the importance of collaboration among public health agencies with knowledge both of tularemia and of the occurrence of other cases, physicians examining patients with compatible symptoms, and the Utah Public Health Laboratories, which, as a Laboratory Reference Network laboratory, possessed the expertise to confirm diagnoses. Expertise in entomology and wildlife ecology provided by the Utah Department of Natural Resources/Division of Wildlife Resources and CDC were also needed to characterize the outbreak.
Tularemia is a disease of low prevalence, which might mask risk and contribute to delayed diagnosis and misdiagnosis. Four weeks passed from the first patient's illness onset to public health notification. After two physicians recognized unusual clinical manifestations among patients and reported the cases to public health, personnel at state and local health departments were able to recognize the outbreak and, consequently, respond with public health messaging to find additional cases and help prevent future cases. Thus, the importance of public health agency involvement was further illustrated. Increased awareness among health-care providers of the endemic nature of tularemia in Utah might aid in earlier recognition, reporting, and, consequently, outbreak detection. Establishing ongoing communication between wildlife resources workers with knowledge of animal die-offs and vector density and public health workers likely to learn of cases can also improve ability to recognize a tularemia outbreak and allow earlier implementation of prevention measures. Effective collaboration between health-care providers and public health agencies is essential.
Prevention is the key to stopping the spread of infectious diseases. The general public should be educated to avoid bites of ticks, mosquitoes, and flies, including correct use of insect repellent (including the fact that DEET might not protect against deer-fly bites), wearing light-colored long-sleeved shirts and long pants while outdoors, and use of mosquito netting while sleeping outdoors or in an unscreened structure. Education for leaders of groups planning to spend time in areas endemic to tularemia is also appropriate. Educational materials should be distributed to leaders of outdoor groups (e.g., scouting groups, summer camps, wilderness survival and recreational groups, and church groups) and to outdoor retailers. Group events are teachable moments, and group leaders should inform group members of disease risks and how to minimize those risks and recognize disease.
Limitations of the cohort study included missing information and potential recall bias associated with the long period that elapsed between respondents' visits to the lodge and questionnaire completion. This time delay was caused, in part, by delayed diagnosis of certain patients and, consequently, delayed outbreak detection. Another limitation was a low response rate among one group, which was not included in the cohort study analysis. Finally, we only recruited participants from groups that had at least one confirmed, probable, or suspect case. If exposure also occurred during the visits of those groups not included, their exclusion might have limited the ability to detect factors contributing to the absence of tularemia patients among those groups. However, we do not anticipate that members of groups who were not recruited would have differed significantly from cohort participants with regard to demographic variables or risk factors, because they were all members of groups affiliated with the same church and engaged in similar activities (i.e., hiking and camping). Cohort study exclusions did not affect calculation of attack rates because total attendance numbers by week were available.