BACKGROUND: Blastomycosis, a potentially serious illness caused by Blastomyces infection, is endemic in the Mississippi Valley region, including Wisconsin, where it has been a reportable disease since 1984. At least 12 outbreaks occurring in Wisconsin have been reported since 1984. Outbreaks of blastomycosis are usually attributed to epi-linked environmental point sources. Sporadic cases are non-outbreak related cases reported to the public health system; the sources of infection for sporadic cases remain elusive. Understanding the epidemiology of sporadic blastomycosis can inform public health efforts to increase disease awareness and prevention.
METHODS: A standardized blastomycosis case worksheet was maintained in the Wisconsin Electronic Disease Surveillance System and used by local health departments to document patient demographics, symptoms, medical diagnosis and treatment, comorbidities, and potential exposures. Data for patients meeting the Wisconsin blastomycosis laboratory and clinical case criteria with onset reported between January 1, 2011 and December 31, 2015 were extracted for epidemiologic description and analysis of sporadic blastomycosis.
RESULTS: During 2011-2015, a total of 534 blastomycosis cases were reported; including 89 outbreak-related and 455 sporadic cases. This represented a statewide average of 91 sporadic cases per year, and a mean annual incidence rate of 1.61 cases per 100,000 persons. Mean annual incidence rates by race were 1.24, 2.10, 3.73 and 6.36 cases per 100,000 white, black, Asian and Native American persons respectively. Three-hundred and nine (67%) were male, and the median age was 48 years (range, 3-97 years). Sixty-five percent (290) of patients were hospitalized, and the average annual case-fatality rate was 10.65%. Immune-compromising comorbidities were associated with an increased risk of death (OR=8, p<0.01). Nearly 69% of cases were classified with acute illness. While 69% of infections were confined to the pulmonary system, skin lesions were reported in 83% of extra-pulmonary infections. The most common reported exposures were camping-fishing-hiking (35.4%), gardening-mulching (33.8%), and clearing brush (30.9%). County incidence rates (range, 0-48 cases/100,000 persons) were highest in counties located in the northcentral and southeastern regions of Wisconsin.
CONCLUSIONS: This study was unique in differentiating and focusing on sporadic cases. Blastomycosis incidence increased at least 22% from 1.32 outbreak and sporadic cases/100,000 persons reported during 1985-1994 to 1.61 sporadic cases/100,000 persons during 2011-2015. The findings of this study highlight the importance of early disease recognition, especially among immune-compromised individuals, to reduce hospitalization, chronicity, and mortality rates. The disparities of incidence by gender and race likely represent differences in outdoor exposures to conidia-releasing activities that we hope to discern through improved surveillance efforts.