Effects of season and weather on blastomycosis in dogs: Northern Wisconsin, USA

Dogs and humans acquire Blastomyces dermatitidis infections from the same incompletely defined habitat. Studies of blastomycosis cases have not consistently demonstrated seasonality or significant antecedent climate effects. To determine the distribution of disease by season, we studied over 18 years 219 dogs with blastomycosis from a single veterinary practice in Northern Wisconsin. The 202 Vilas County dog addresses were compared to 200 random-number selected addresses from the practice registry. Street addresses were geocoded and mapped using ArcGIS, including ratio of cases/random addresses to construct a control chart. Stepwise and linear regression was used to model case counts by season and by 6 month warm (April-September) and cold periods, using lagged local weather data. The geographic distribution of cases was found to be similar regardless of season and time period, and no season exceeded control chart limits. Seasonal distribution of cases was; winter (n = 53, 24%), spring (39, 18%), summer (79, 36%), fall (48, 22%), p = 0.002. When cases were considered by 6-month warm/cold periods, 67% of variation is explained by the total precipitation which occurred two periods prior, and lower average temperature, but higher maximum temperature one period prior (p = 0.000). Weather parameters along with fixed and variable environmental factors likely determine the occurrence of B. dermatitidis, perhaps as part of a 'grow and tolerate change' model.