Weather and environmental exposures in human blastomycosis: northern Wisconsin


Department of Family Medicine, Aurora UW Medical Group, Center for Urban Population Health

Presentation Notes

Presented at 2014 Aurora Scientific Day, Milwaukee, WI


Background: Blastomycosis is a potentially fatal fungal infection contracted by inhalation of Blastomyces spores from the environment. Blastomycosis case occurrence in dogs has been associated with antecedent weather, using 6-month warm and cold periods. Paracoccidioidomycosis in South America has been associated with variations in the Southern Oscillation Index (SOI). A comprehensive human case registry has been kept in Vilas County, Wisconsin, since 1979 (mandatory reporting since 1984).

Purpose: Determine the effects of weather parameters on the occurrence of human blastomycosis cases in this highly endemic area; update exposure factors since last report in 1996.

Methods: Database review. Chi-squared test was used for categorical data of exposures, comparing 1979-1996 (N=101) vs. 1997-June 2013 (N=95). Linear regression was used to model lagged local weather station data (available 1990-present; N=126) and SOI, North Atlantic Oscillation Index (NAOI) and Wisconsin River water discharge (WRD)data from the adjacent county (all available for 1984-present; N=174), using pulmonary blastomycosis case counts of known onset by 6-month warm (April-September) and cold (October-March) periods.

Results: The seasonal distribution of pulmonary cases, 1984-2012, was: winter (N=35 [22%]), spring (40 [25%]), summer (45 [28%]), fall (42 [26%]); P=0.9. Environmental exposures, 1997-June 2013 (mean age 45, 59% male), were: residence < 400 m from waterway (76%), excavation (42%), gardening (31%); none different than 1979-1996. Fishing (23% vs. 37%; P=0.09) and hunting (15% vs. 26%; P=0.13) were less common than 1979-1996 but not significantly different. Overall, 69% of cases recalled some prior soil- disturbing activities. Considering the 6-month warm/cold periods, 19% of variation is explained by a direct relationship with total precipitation two periods prior (P=0.005). There was no association of case occurrence with SOI, NAOI or WRD. The estimated annual incidence of blastomycosis, 1997-present, was 27/100,000 compared to 44/100,000 in 1984-1996.

Conclusion: As with dogs, human blastomycosis case occurrence is partially determined by antecedent precipitation (an association that may be more robust with increased cases). It is unclear if recent lower incidence rates are due to less opportunity for exposure, reduced fungal virulence or random variation.

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