Blastomycosis is a rare, sometimes serious fungal infection caused by an organism living in the environment. It is not contagious and cannot be spread through casual contact between people or animals, though extremely rare transmission has been reported (e.g., via dog bite or sexual contact). Understanding the source is important for minimizing the risk of exposure, especially for individuals living in or traveling to endemic regions. The disease primarily affects the lungs after spores are inhaled but may progress to involve other organ systems.
The Fungal Source and Habitat
The cause of blastomycosis is the fungus from the genus Blastomyces, specifically Blastomyces dermatitidis and the recently identified Blastomyces gilchristii. This organism is thermally dimorphic, meaning it exists in two different forms depending on the temperature: a mold form in the environment and a yeast form once inside a host. The fungus thrives in moist, acidic soil that is rich in decaying organic material, such as wood and leaves.
It is commonly found near waterways, including the Great Lakes region, and the Mississippi, Ohio, and Missouri River valleys in the United States. The fungus is also endemic to parts of Canada, particularly provinces bordering the Great Lakes and the St. Lawrence Seaway. While difficult to isolate directly from its natural habitat, its presence is strongly associated with wet, undisturbed soil that has a high nitrogen and organic content. This environmental pattern defines the geographical range where human and animal infections are most likely to occur.
Mechanism of Transmission
Infection occurs almost exclusively through the inhalation of airborne fungal spores, known as conidia, from the environment. These microscopic spores are released into the air when the soil or organic matter where the fungus lives is disturbed. Activities that involve disrupting the ground, such as construction, excavation, farming, hunting, or collecting firewood, can aerosolize the infectious particles.
Once inhaled, the conidia travel deep into the lungs, which is the primary site of infection. The host’s body temperature triggers the fungus to transform from its environmental mold form into a large, thick-walled yeast form. This yeast form is better equipped to evade the host’s immune defenses, allowing the infection to take hold and potentially spread. While the respiratory tract is the usual point of entry, rare infections have been reported through direct traumatic inoculation into the skin.
Recognizing the Signs of Infection
The incubation period for blastomycosis can range from approximately three weeks to three months after exposure. About half of those infected are asymptomatic or experience an illness so mild it goes undiagnosed. When symptoms manifest, the initial presentation, known as pulmonary blastomycosis, often resembles a common bacterial pneumonia or a severe flu-like illness.
Common signs of pulmonary blastomycosis include fever, chills, cough, muscle aches, night sweats, and chest pain. The cough may be dry or productive, sometimes yielding bloody or purulent sputum. In a minority of cases, the infection can progress to a severe condition called acute respiratory distress syndrome (ARDS), where the lungs fill with fluid, making breathing extremely difficult.
If the infection is not contained in the lungs, it may disseminate, or spread, through the bloodstream to other parts of the body. This disseminated blastomycosis most commonly affects the skin, where it can cause lesions that appear as papules, pustules, or wart-like plaques that may ulcerate. The second most common site of spread is the bones and joints, leading to osteomyelitis and painful swelling. The fungus can also reach the central nervous system, causing meningitis or abscesses, or affect the genitourinary system, especially in men.
Diagnosis and Treatment Protocols
Diagnosis requires a high degree of clinical suspicion, especially in non-endemic areas, because symptoms mimic other respiratory illnesses. Definitive diagnosis relies on laboratory identification of the fungus. This involves microscopic visualization of the characteristic large, budding yeast forms in samples like sputum or tissue biopsies.
Fungal culture remains the most sensitive method, though growth can take days to weeks. Rapid diagnostic tools include antigen detection tests, which measure fungal cell wall components in the urine or blood, and chest X-rays, which show signs of pulmonary involvement. Treatment protocols depend on disease severity, but all symptomatic infections are medically managed to prevent dissemination and recurrence.
For mild to moderate pulmonary disease, the first-line treatment is oral itraconazole, taken for six to twelve months. Severe, life-threatening infections, or those involving the central nervous system, require initial intravenous therapy with Amphotericin B. Once clinical improvement is shown, treatment transitions to oral itraconazole for a prolonged course. Prevention focuses on avoiding activities that disturb soil in known endemic areas.