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Blastomycosis Cluster in Wisconsin

By Amesh A. Adalja, MD, FACP, July 12, 2013

Blastomycosis is a systemic infectious disease, with both pulmonary and extra-pulmonary manifestations, that is caused by infection with the fungus Blastomyces dermatidis. In the United States, the disease is restricted geographically primarily to regions in the Ohio and Mississippi River basins. In 2010, an unusual uptick in blastomycosis cases in Marathon County in north-central Wisconsin prompted an investigation. Wisconsin is 1 of 6 states where the infection is reportable. Results of the investigation were recently published in Clinical Infectious Diseases.1

Outbreak Case Characteristics

Using the Wisconsin Electronic Disease Surveillance System (WEDSS), investigators surveyed blastomycosis cases in Marathon County from 2000 until 2010. The outbreak began in September 2009 and ran through June 2010, during which time 55 cases were documented. The median age of cases was 31; 65% were male; 70% were hospitalized; and 5% experienced fatal infection. Remarkably, 45% of the cases were in people identified as Asian. The majority of patients reported camping, fishing, and backpacking activities. More than 50% of the cases (30) were associated with 5 residential clusters, defined as 3 or more cases in one neighborhood.1

Dramatically Increased Incidence in Asians

Although the investigators found an overall increase in incidence of blastomycosis in the county since 2005, this increase has disproportionately affected Asians—78% of whom are Hmong. Compared to non-Asians, Marathon County Asians have experienced a 586% increased incidence in this disease since 2005.
When outbreak patients were compared with historical cases, they were found to be less likely to smoke, more likely to be Asian, and less likely to have traveled prior to infection. Asian case patients were also found to be younger and less likely to report potential environmental exposures when compared with non-Asian cases. However, 21% of Asian patients reported another household member with blastomycosis compared to 2% of non-Asians. When analysis was confined to residential clusters, Hmong ethnicity yielded a 10.64 odds ratio for association with a cluster.1

Why an Increased Risk?

The most striking feature of this investigation is the dramatically increased incidence among Hmong residents in Marathon County. The report’s authors indicate that this area in Wisconsin is home to the largest Hmong population in the state, the result of immigration after the Vietnam War.

As the authors note, the lack of natural immunity to Blastomyces in those from Vietnam cannot account for the increased incidence, as many of the case patients had been in the state for more than a decade, and some were born in Wisconsin. Instead, what seems most likely, in light of the fact that Hmong patients were less likely to engage in known risk-conferring behavior, is the presence of a genetic polymorphism that renders the Hmong more susceptible to symptomatic Blastomyces infection. This is similar to the increased vulnerability of Filipinos to the fungal disease coccidioidomycosis.

Understanding why the Hmong population was disproportionately affected by the outbreak will help elucidate the pathophysiology of the infection and, hopefully, refine treatment and prevention strategies.


Roy M, Benedict K, Deak E, et al. A large community outbreak of blastomycosis in Wisconsin with geographic and ethnic clustering. Clin Infect Dis 2013. Accessed June 16, 2013.