Blastomycosis
Annual Summary of Reportable Diseases

Blastomycosis is a fungal infection typically acquired by breathing in spores of the fungi Blastomyces dermatitidis or Blastomyces gilchristii. These fungi can be found in moist soils, particularly in wooded areas and along waterways. Blastomycosis occurs most often in people living in the Midwest, SouthCentral, and Southeastern United States, and in eastern Canada. In Minnesota, blastomycosis is most common in Northeastern counties, but can occur throughout the state (Figure 1). In 2022, 117 blastomycosis cases were reported, a 39% increase over the previous high of 84 cases in 2020. While it is possible that some of the observed increase was due to an increase in testing or reporting, it was more likely a true increase due to local environmental conditions. There was also a substantial increase in blastomycosis cases in animals during the same timeframe, suggesting a true increase in human cases. In Minnesota, some highly endemic areas for Blastomyces experienced significant drought in 2021, followed by a much wetter than normal 2022, possibly leading to proliferation and dissemination of the organism. Additionally, clusters of blastomycosis cases associated with flooding events are known to occur. In 2022, the median age of blastomycosis cases was 46 years (range, 3 to 90 years), and 42 cases (36%) were female. Of the 113 cases for whom race and ethnicity were reported, 75 (66%) cases were white, 18 (16%) were American Indian/ Alaska Native, 8 (7%) were Asian, 6 (5%) were Black/African American, 1 (<1%) was Native Hawaiian/Pacific Islander and 5 (4%) were of another race. Five cases (4%) were Hispanic. When hospitalization status was known, 70 of 113 (62%) cases were hospitalized, for a median of 9 days (range, 1 to 80 days). Fourteen cases (12%) died and blastomycosis was the listed cause of death for nine of these; other causes of death were listed for three cases, and the cause of death was unknown for two cases. When the body system infected by Blastomyces was known, 88 (75%) cases had a pulmonaryonly infection, 17 (14%) had a disseminated infection, and 12 (10%) had an extra-pulmonary infection.There were four blastomycosis outbreaks identified in 2022. In the first, seven cases were associated with a family cabin on a lake in St. Louis County, which also affected three dogs and one person in 2021. In the second outbreak, three cases were exposed at a dam worksite in Wisconsin. The third outbreak involved three unrelated cases who reported visiting the same beach on a lake in Cass County. Lastly, two outbreak cases were part of a previously identified outbreak associated with a family cabin property in Pine County. This outbreak began in 2019 and now includes a total of six cases. In 2022, 14 cases of blastoycosis occurred in Cass County residents, three times the annual median case count of five cases reported during 2017-2021. There were also six additional cases who live elsewhere but were likely exposed in Cass County. In addition to the outbreak listed above, a cluster of cases occurred around Cass Lake, but many of these cases reported numerous outdoor locations in addition to their residence, so exposure could not be attributed to a specific area. From 1999 to 2022, 1,033 cases of blastomycosis in Minnesota residents were reported. Exposure information was available for 727 cases; of these, 151 (21%), were likely exposed in St. Louis County, 87 (12%) in Itasca County, 82 (11%) in Cass County, 26 (4%) in Hennepin County, and 23 (3%) in Beltrami County. Eighty-one cases (11%) were exposed in Wisconsin. In 2022, the statewide incidence was 2.0 cases per 100,000 population, compared to a 1999-2021 median annual incidence of 0.63 cases per 100,000. A map of average annual incidence of blastomycosis from 1999 to 2022 clearly shows the highly endemic regions, which include the northern and northeastern counties of Minnesota (Figure 1). This is in contrast to the other endemic fungal disease found in Minnesota, histoplasmosis, which has higher incidence levels in southern and western counties. While clinicians should test for both diseases when clinical illness is consistent with a fungal infection, these maps can help assess differential risk in various regions of the state.

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Blastomycosis is an infection caused by a fungus called Blastomyces. In 2021, 82 blastomycosis cases were reported. This continues an increase in cases reported in the past 5 years, with a median of 79 cases/year from 2017 to 2021 compared to a median of 33 cases/year for the prior 18 years. The median age of blastomycosis cases in 2021 was 56.5 years (range, 8 to 89 years), and 51 (62%) were male. In 2021, cases were more likely to be older and female than in previous years. Of the 77 cases for whom race and ethnicity was reported, 63 (82%) cases were white, 6 (8%) were American Indian/Alaska Native, 5 (7%) were Asian, 1 (1%) was Black/African American, and 2 were of another race. One case (1%) was Hispanic. When hospitalization status was known, 54 of 80 (68%) cases were hospitalized, for a median of 8 days (range, 1 to 54 days). Nineteen (23%) cases died, which is a significantly higher case fatality rate (CFR) than the 10% mean CFR during 1999-2020. Blastomycosis was the listed cause of death for 12 cases, 3 cases had other causes of death listed, and the cause of death was unknown for 4 cases. When it was possible to identify the body system infected by Blastomyces, 51 (64%) cases had a pulmonary-only infection, 22 (27%) had a disseminated infection, and 7 (9%) had an extra-pulmonary infection. There were two blastomycosis outbreaks identified in 2021. One case was part of an ongoing outbreak associated with a family cabin property in Pine County. This outbreak began in 2019 and resulted in 4 cases through 2021. A second outbreak-related case was exposed at a dam worksite in Wisconsin; the other case in that outbreak was a Wisconsin resident. From 1999 to 2021, 916 cases of blastomycosis in Minnesota residents were reported. Exposure information is available for 662 cases. The largest number, 138 (21%), were likely exposed in St. Louis County. Eighty (12%) cases were likely exposed in Itasca County, 61 (9%) in Cass County, 24 (4%) in Hennepin County, and 20 (3%) in Beltrami County. Seventysix cases (11%) were exposed in Wisconsin. In 2021, the statewide incidence was 1.4 cases/100,000 population, compared to a 1999-2020 median annual incidence of 0.63 cases/100,000. A map of average annual incidence of blastomycosis from 1999 to 2021 clearly shows the highly endemic regions, which include the northern and northeastern counties of Minnesota (Figure 2). This is in contrast to the other endemic fungal disease found in Minnesota, histoplasmosis, which has higher incidence levels in southern and western counties. While clinicians should test for both diseases when clinical illness is consistent with a fungal infection, these maps may help assess differential risk in various regions of the state.

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In 2020, 84 blastomycosis cases were reported, the highest number since enhanced surveillance began in 1999 but just 5 above last year’s previous high. This continues the increase in cases seen in the past few years. Enhanced surveillance for blastomycosis was affected by response to the COVID-19 pandemic. We were unable to obtain full clinical details or exposure information for all cases. The median age of cases was 50 years (range, 2 to 84 years), and 49 (60%) were male. Of the 74 cases where race and ethnicity were reported, 47 (63%) cases were white, 12 (16%) were Asian, 3 (4%) were black, 5 (7%) were American Indian/Alaska Native, and 6 were of another race. Thirteen cases (18%) were Hispanic. When hospitalization status was known, 52 (72%) were hospitalized for a median of 7 days (range, 1 to 33 days). Six (8%) cases died, and outcome was unknown for 6 cases. Blastomycosis was the cause of death for 5 cases, while the cause of death for the sixth was unknown. When the body system infected with Blastomyces was known, 52 (78%) cases had a pulmonary-only infection, 11 (16%) had a disseminated infection, and 4 (6%) had an extrapulmonary infection. Numerous outbreaks and clusters of blastomycosis among family members occurred in 2020. The first outbreak involved two adults and two young children who reside in Hennepin County and a family member who lives in Wisconsin, who were exposed at the Willow River in Wisconsin. The second outbreak involving a family cabin in Pine County spans 3 years and includes two relatives who were infected in 2019, one woman in 2020, and a fourth 2021 case. The first cluster of 2 cases involved a mother and daughter who usually reside in Hennepin County, but moved to their cabin in Cass County during spring 2020 to reduce their chances of COVID-19 infection. The second cluster involved two sisters who were infected at the family cabin in Hubbard County, the same location where their father was infected in 2014. The third cluster involved a father and son from Faribault County, whose likely exposure was in Cass County. When clusters or outbreaks of blastomycosis occur in Minnesota, the demographics may be different than the sporadic cases. Of the 9 cluster or outbreak-related cases reported in 2020 and the 3 related cases from 2019 and 2021, 8/12 (66%) were persons of color, and 6/12 (50%) were female. In the past 21 years of data, only 20% of all cases occurred in persons of color and only 30% in women. From 1999 to 2020, 834 cases were reported; the annual median number was 34 cases (range, 22 to 84), but the median for the most recent 5 years was 58 cases/year. In 2020, the incidence statewide was 1.5 cases/100,000 population, compared to the 1999- 2020 median annual incidence of 0.63 cases/100,000. Exposure information is available for 617 cases. The largest number, 131 (21%), were likely exposed in St. Louis County. Seventyfive (12%) cases were likely exposed in Itasca County, 56 (9%) in Cass County, 21 (3%) in Hennepin County, and 17 (3%) in Beltrami County. Seventy cases (11%) were exposed in Wisconsin. Average annual incidence of blastomycosis by county clearly shows the highly endemic regions, which include the northern and northeastern counties of Minnesota (Figure 1). This contrasts to the other endemic fungal disease found in Minnesota, histoplasmosis, which has higher incidence levels in southern and western counties. While clinicians should test for both diseases when clinical illness is consistent with a fungal infection, these maps may help assess differential risk in various regions of the state.

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In 2019, 79 blastomycosis cases were reported, the highest number since enhanced surveillance began in 1999, and 21 higher than last year’s previous record. This continues the increase in cases seen in the past few years. The median age of cases was 41.5 years (range, 4 to 76), and 56 (71%) were male. Fifty-six (74%) cases were white, 8 (11%) were Asian/Pacific Islander, 7 (9%) were black, 5 (7%) were American Indian/Alaskan Native, and 3 were of unknown race. Fifty-one (65%) cases were hospitalized for a median of 8 days (range, 1 to 78). Five (6%) cases died, which is a lower fatality rate than the normally observed 9-10%, and improved over 2017-2018 when the case fatality rate was 15%. Blastomycosis was the cause of death for all five. Twenty-two cases (28%) had immunocompromising health conditions or medication use, including 9 (11%) with diabetes, and 1 (1%) on medications for rheumatoid arthritis. Sixty (76%) cases had pulmonary infection, 11 (14%) had disseminated infection, and 8 (10%) had extrapulmonary infection.

One large outbreak occurred in Minnesota residents in 2019; however, the exposure occurred in Wisconsin. Twelve cases all reported tubing, primarily in early August, on a popular Wisconsin river. Eleven cases were part of three different groups. The twelfth case was also part of a group, but no other members developed illness. Half of these outbreak cases were male and half were female, a much different ratio than in sporadic cases. Five (42%) cases were white, 4 (33%) were black, 2 (17%) were Asian/Pacific Islander and 1 (8%) was unknown. The median age was 28 years (range, 21 to 41 years). Eight cases (67%) were hospitalized. From 1999 to 2019, 750 cases were reported; the annual median is 34 cases (range, 22 to 79), but the median for the most recent 5 years is 44 cases/year. In 2019, the incidence statewide was 1.4 cases/100,000 population, compared to the 1999- 2019 median annual incidence of 0.63 cases/100,000. Exposure information is available for 605 cases. The largest number, 131 (22%), were likely exposed in St. Louis County. Seventyfour (12%) cases were likely exposed in Itasca County, 52 (9%) in Cass County, 21 (3%) in Hennepin County, and 17 (3%) in Beltrami County. 

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In 2018, 58 blastomycosis cases were reported, the highest number since enhanced surveillance began in 1999. The 2 previous years (2017, 2016) had the third and fourth highest counts, 44 and 39 cases. The median age was 45.5 years (range, 5 to 90); 45 (78%) were male. Thirty-eight (75%) cases were white, 8 (16%) were black, 2 (4%) were American Indian/Alaska Native, 2 (4%) were Asian/Pacific Islander, 1 (2%) was mixed race, and 7 were unknown race. Thirty-five (60%) cases were hospitalized for a median of 7 days (range, 1 to 197). Eight (14%) cases died, which is a higher fatality rate than the normally observed 9-10%, and the second year in a row with an increased case fatality rate. Blastomycosis was the cause of death for all 8. Twenty-one cases (40%) had immunocompromising health conditions or medications, including 12 (28%) with diabetes, 3 (9%) taking corticosteroids, and 2 (5%) on medications for rheumatoid arthritis. Forty-two (72%) cases had pulmonary infection, 3 (5%) had extrapulmonary infection, and 13 (22%) had disseminated infection.

From 1999 to 2018, 671 blastomycosis cases were reported; the annual median is 33.5 cases (range, 22 to 58). The median annual incidence statewide is 0.63 cases/100,000 population, but was 1.03 cases/100,000 in 2018. Exposure information is available for 559 cases. The largest number, 131 (23%), were likely exposed in St. Louis County. Seventy (13%) cases were likely exposed in Itasca County, 43 (8%) in Cass County, 19 (3%) in Hennepin County, 17 (3%) in Chisago County, and 16 (3%) in Beltrami County.

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• Archive of Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health
  Archive of past summaries (years prior to 2023 are available as PDFs).