Mycopathologia 153: 113–120, 2001. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 113 Cluster of pulmonary blastomycosis in a rural community: Evidence for multiple high-risk environmental foci following a sustained period of diminished precipitation Mary E. Proctor 1 , Bruce S. Klein 2 , Jeffrey M. Jones 3 & Jeffrey P. Davis 1 1 Wisconsin Department of Health and Family Services, Division of Public Health, Bureau of Communicable Diseases, Madison, WI, 53701, USA; 2 University of Wisconsin Medical School, Departments of Pediatrics and Medicine, Madison, Wisconsin, USA; 3 The Clinics and Research Service, William S. Middleton Memorial Veterans Hospital, Madison, WI 53705, USA Received 21 August 2001; accepted 7 November 2001 Abstract Much of our understanding of the epidemiologic features of infection with Blastomyces dermatitidis has come from cluster and outbreak investigations which have established the association of human disease with recreational pursuits and the presence of infectious microfoci in areas of moist soil with high organic content. This report describes the clustering of eight cases of pulmonary blastomycosis without an apparent common source exposure which occurred during a 90 day period in a 96 square mile area (population 4,450) within Oconto County, Wis- consin. We conclude that multiple high-risk environmental foci may have existed following a sustained five-year period of diminished precipitation in the cluster area. A case-control study which included family and community controls concluded that multiple earth-disturbing activities engaged in by case-patients was statistically associated with illness. Lymphocyte-proliferation assays of whole blood samples detected previously unrecognized infection with B. dermatitidis among five of 32 family controls. Key words: Blastomyces dermatitidis, cluster investigation, fungal infection, pulmonary disease Introduction On March 16, 1990, the Wisconsin Division of Public Health (WDPH) was notified about three cases of pul- monary blastomycosis among residents of a 96 square mile area (population 4,450) within Oconto County, Wisconsin. By March 20 five additional case-patients with onset of signs and symptoms between December 1, 1989 and March 1, 1990 were identified. Since the reported mean annual incidence of blastomycosis in Oconto County during the years 1981 to 1989 was 1.83 cases per 100,000 population, we evaluated the possibility that Blastomyces dermatitidis infection in this cluster was enhanced by the presence of specific soil types and recent climatic trends in the county which might have been conducive for Blastomyces dermatitidis growth. Potential exposure risk factors * Published in 2002. associated with disease acquisition in this cluster were also examined. Materials and methods Epidemiologic investigation Case definition. An acute case was defined as a posit- ive culture for B. dermatitidis from any body site in a resident of the 96 square mile area with onset of signs and symptoms occurring during December 1, 1989 to March 1, 1990 (the cluster interval). A past case was defined as demonstration of an in vitro lymph- ocyte transformation response to one of the two B. dermatitidis yeast cell wall antigens tested in a person who lived in the geographically defined area, but had no symptomatic evidence of active pulmonary disease during the cluster interval.