A CDC analysis found that blastomycosis — an infection caused by a fungus found in soil and decaying leaves — occurs in parts of the United States not typically known to have the disease.
Blastomycosis, AKA blasto, is an illness caused by Blastomyces, a fungus found in moist soil and decaying organic matter. People become infected with blasto by breathing in fungal spores, most often when participating in outdoor activities where the fungus is present.
People living in specific areas of the United States, such as Arkansas, Louisiana, Michigan, Minnesota, and Wisconsin, are likely familiar with the disease, as it is mainly found in midwestern, south-central, and southeastern regions of the country close to the Ohio and Mississippi Rivers, Great Lakes, and St. Lawrence Seaway.
Only about 50% of those infected with the spores develop symptoms, which include fever, cough, shortness of breath, and fatigue. These can range from mild to severe and typically appear two to 15 weeks after exposure.
However, because the symptoms of blasto mimic many other respiratory diseases, healthcare providers unfamiliar with Blastomyces infections might not think to check for it.
And the illness can be deadly if not treated with anti-fungal drugs such as itraconazole or amphotericin B. According to the Centers for Disease Control and Prevention (CDC), among people with symptoms, 57 to 69% are eventually hospitalized, and 4 to 22% die from the disease.
Blasto cases can occur in clusters. For example, a 2023 blastomycosis outbreak involving over a dozen paper mill employees in Escanaba, Michigan, had health officials scrambling to identify the source.
Moreover, in 2022, four people and five dogs living in the same St. Croix County, Wisconsin neighborhood contracted blastomycosis — two were hospitalized, and one died.
The spread of blastomycosis
Blasto is a nationally notifiable disease with public health surveillance in only five states — Arkansas, Louisiana, Michigan, Minnesota, and Wisconsin. Wisconsin has the highest annual statewide incidence of blasto, with 2.1 cases per 100,000 residents. However, some parts of the state report 20 to 40 cases per 100,000 people.
Yet, a 2023 study suggests that the incidence of blasto in northeastern regions of the U.S., like Vermont, might be more significant than previously thought.
To find out for sure, the CDC recently analyzed insurance records in Vermont to determine the hospitalization rates, deaths, annual incidence, and geographic distribution of blastomycosis in that state.
The results, published in Emerging Infectious Diseases, show that during some years, the incidence of blastomycosis in Vermont was higher than in states like Wisconsin.
Blasto's prevalence in Vermont
Insurance records showed that 114 Vermont residents were diagnosed with blastomycosis from 2011 to 2020, and the average annual statewide incidence of the illness was 1.8 cases per 100,000 people.
The CDC notes that this incidence rate is greater than the average annual incidences from 1987 to 2017 in four of five states that mandate reporting of blastomycosis, including Arkansas, Louisiana, Michigan, and Minnesota. Wisconsin was the only state with a higher incidence rate.
However, in 2019, the number of people who contracted blastomycosis in Vermont was higher than in all five blasto-reporting states.
Most cases in Vermont occurred in Lamoille, Orleans, and Washington counties. Although these counties represent only 18% of the state population, 49% of all blasto cases and 65% of hospitalized patients resided in those counties.
Overall, 30% of patients were hospitalized due to blasto during the study period, and four people died.
The researchers say that Vermont's acidic spodosol soil — which is similar to soils in regions with higher blastomycosis rates — may promote the growth of the Blastomyces fungus. However, the reasons for the high statewide incidence of blastomycosis are unclear.
Although it's possible that Blastomyces and blastomycosis may have already existed in Vermont and the analysis simply revealed its prevalence, the findings highlight the need for improved awareness by healthcare providers and expanded surveillance in Vermont and other areas of the country that could harbor the fungus.
- Wisconsin Department of Health Services. Blastomycosis.
- CDC. Notes from the field: Cluster of blastomycosis among neighborhood residents — St. Croix County, Wisconsin, 2022.
- CDC. Using insurance claims data to estimate blastomycosis incidence, Vermont, USA, 2011–2020.
- Clinical Infectious Diseases. The geographic distribution of dimorphic mycoses in the United States for the modern era.