Blastomycosis, caused by the fungus Blastomyces, is a rare but potentially serious infection of people and animals. The fungus lives in moist, organic soil and decaying wood and leaves, in Wisconsin — which has the highest incidence in the country, about 2.1 cases per 100,000 a year, and 20–40 in some counties — and in other midwestern, south-central and southeastern states.
- People and animals are infected by breathing in spores; it doesn't spread between people and animals through the air.
- It usually starts with mild respiratory symptoms that often clear up, but can become severe, and occasionally fatal, without antifungal treatment.
The cluster
In February 2022, a veterinarian in St. Croix County alerted the Wisconsin Department of Agriculture, Trade and Consumer Protection and the Department of Health Services to four dogs with blastomycosis, all living within 1 mile of each other. Surveillance showed two human cases in the same area within 3 weeks. St. Croix County usually reports only 1–5 human cases a year.
The response: alerts to physicians and veterinarians in nearby counties, stressing prompt diagnosis and treatment — blastomycosis is reportable in people but not in animals, so vets were urged to report possible cases — and a letter from St. Croix County Public Health asking residents with symptoms to seek care.
January–March 2022: four people and five dogs were diagnosed.
| People (4) | Dogs (5) | |
|---|---|---|
| Symptoms | all had cough, fever and shortness of breath, beginning early October 2021 to early February 2022 | four: cough, breathing difficulty, lethargy, fever, poor appetite; one: only a lump under the skin with Blastomyces yeast |
| Testing | two presumptive, two confirmed | urine antigen positive in all |
| Severity | two hospitalized; one adult died | mild to moderate at first |
| Treatment | antifungals | antifungals |
Two of the people were diagnosed only after the cluster was announced. In the 10 years before January 2022, no cases had been reported among the neighborhood's residents, though one dog was said to have died of blastomycosis the previous year.
Where it came from
Investigators found a river and unpaved paths through the neighborhoods, and construction over the past decade may have stirred up spores. A fuller study of environmental sources was under way.
Lessons
- Clusters have mostly followed recreation along waterways or excavation.
- Clinicians should consider blastomycosis in patients with compatible symptoms who live in or have visited endemic areas — especially respiratory symptoms that don't improve with antibiotics.
- Tests include fungal culture, cytologic smear, histopathology, polymerase chain reaction or DNA probe, antigen tests and antibody tests.
- One Health in action: an alert veterinarian, reporting sick dogs, uncovered a human outbreak.
Sources
Based on Segaloff HE, Wu K, Shaw S, et al., "Notes from the Field: Cluster of Blastomycosis Among Neighborhood Residents — St. Croix County, Wisconsin, 2022," MMWR Morbidity and Mortality Weekly Report, volume 72, Centers for Disease Control and Prevention; a work of the United States government in the public domain.
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