Blastomycosis, also known as Gilchrist's disease, is a fungal infection, typically of the lungs, which can spread to brain, stomach, intestine and skin, where it appears as crusting purplish warty plaques with a roundish bumpy edge and central depression.

[8][14] Blastomycosis also affects a broad range of non-human mammals, and dogs in particular are an order of magnitude more likely to contract the disease than humans.

Because the symptoms are variable and nonspecific, blastomycosis is often not even considered in differential diagnosis until antibacterial treatment has failed, unless there are known risk factors or skin lesions.

[8] If symptoms are present they may range from mild pneumonia resembling a pneumococcal infection to acute respiratory distress syndrome (ARDS).

[8] Common symptoms include fever, chills, headache, coughing, difficulty breathing, chest pain, and malaise.

[21] In disseminated blastomycosis, the large Blastomyces yeast cells translocate from the lungs and are trapped in capillary beds elsewhere in the body, where they cause lesions.

[20] The signature image of blastomycosis in textbooks is the indolent, verrucous or ulcerated dermal lesion seen in disseminated disease.

In western North America, many cases of blastomycosis are caused by Blastomyces helicus, which most commonly attacks immunodeficient people and domestic animals.

[23] In lung tissue, the cells multiply and may also disseminate through blood and lymphatics to other organs, including the skin, bone, genitourinary tract, and brain.

[26] Once suspected, the diagnosis of blastomycosis can usually be confirmed by demonstration of the characteristic broad based budding organisms in sputum or tissues by KOH prep, cytology, or histology.

Commercially available urine antigen testing appears to be quite sensitive in suggesting the diagnosis in cases where the organism is not readily detected.

[28] While culture of the Blastomyces organism remains the definitive diagnostic standard, its slow growing nature can lead to a delay of up to four weeks.

[43] An incidence of 277 per 100,000 was roughly calculated based on 9 cases seen in a Wisconsin aboriginal reservation during a time in which extensive excavation was done for new housing construction.

Human blastomycosis is primarily associated with forested areas and open watersheds;[20][46][47][48] It primarily affects otherwise healthy, vigorous people, mostly middle-aged,[49] who acquire the disease while working or undertaking recreational activities in sites conventionally considered clean, healthy and in many cases beautiful.

[20][40] Repeatedly associated activities include hunting, especially raccoon hunting,[50] where accompanying dogs also tend to be affected, as well as working with wood or plant material in forested or riparian areas,[20][51] involvement in forestry in highly endemic areas,[52] excavation,[43] fishing[49][53] and possibly gardening and trapping.

[34][43] There is also a developing profile of urban and other domestic blastomycosis cases, beginning with an outbreak tentatively attributed to construction dust in Westmont, Illinois.

[53] Based on a similar finding in a Louisiana study, it has been suggested that place of residence might be the most important single factor in blastomycosis epidemiology in north central Wisconsin.

[20][34][37] Ethnic group or race is frequently investigated in epidemiological studies of blastomycosis, but is potentially confounded by differences in residence and in quality and accessibility of medical care, factors that have not been stringently controlled for to date.

In the United States, some studies show a disproportionately high incidence and/or mortality rate for blastomycosis among Black people.

The pathogenic group of onygenalean fungi that give rise to conditions including blastomycosis and histoplasmosis emerged approximately 150 million years ago.

[71] At the Koster Site in Illinois, evidence pointing to possible blastomycosis infections among Late Woodland Native Americans has been identified.

At that site, Dr. Jane Buikstra found evidence for what may have been an epidemic of a serious spinal disease in adolescents and young adults.

There are two modern diseases that produce lesions in the bone similar to the ones Dr. Buikstra found in these prehistoric specimens: spinal TB and blastomycosis.

[73] In the early 1950s, blastomycosis was first determined to be a primarily respiratory disease, with most skin lesions caused by systemic spread from an initial lung infection.

[73] Before 1950, the fatality rate for disseminated blastomycosis was 92%, and treatment options were limited to iodide compounds, radiation therapy, and surgery.

Since that time, genomic analysis has identified multiple other Blastomyces species causing blastomycosis, including B. gilchristii (2013), B. helicus (reassigned from the genus Emmonsia in 2017), B. percursus (2017), and B. emzantsi (2020).

[71] The largest-ever blastomycosis outbreak in United States history occurred at an Escanaba, Michigan, paper mill in 2023.

As of April 2023[update], one person had died and almost a hundred more had fallen ill.[76][77] Blastomycosis affects a broad range of mammals.

[15] Cases of blastomycosis have also been reported in captive lions and tigers, in a wild North American black bear, and in marine mammals such as the Atlantic bottlenose dolphin.

[15] This may be due to a shortened incubation period, caused by the dog inhaling larger quantities of Blastomyces spores than the human.