Blastomycosis is a systemic fungal infection caused by inhaling airborne spores of the fungus Blastomyces dermatitidis or related species. This thermally dimorphic fungus exists as a mold in moist, acidic soil rich with decaying organic matter, such as wood and leaves. When the soil is disturbed, microscopic fungal spores, called conidia, are released into the air. Once inhaled, these spores become the infectious particles that initiate the disease process within the body.
Invasion and Primary Lung Infection
The infectious particles, or conidia, are small enough to reach the lower respiratory tract, specifically the pulmonary alveoli. Upon entering the warm environment of the lungs, the fungus transitions from its mold form to a pathogenic, thick-walled yeast form. This transformation is necessary for the fungus to cause disease. The body’s initial defense involves immune cells like alveolar macrophages and neutrophils, which attempt to destroy the inhaled conidia. However, the yeast form is much larger and its thick cell wall provides resistance against being cleared, allowing the fungus to establish a primary infection within the lung tissue.
The outcome varies significantly, with up to 50% of infected people remaining completely asymptomatic. For those who develop symptoms, the incubation period can range from a few weeks to several months. The resulting illness, acute pulmonary blastomycosis, often mimics a bacterial pneumonia or a severe flu. Symptoms typically include fever, chills, muscle aches, and a nonproductive cough.
In more severe cases, the infection can progress to acute respiratory distress syndrome (ARDS), characterized by rapid onset of severe breathing difficulty and diffuse inflammation. In some cases, the infection can also settle into a chronic form, resembling tuberculosis, with symptoms like low-grade fever, night sweats, weight loss, and a persistent, productive cough.
How the Fungus Spreads Systemically
If the body’s local immune response in the lungs fails to contain the yeast, the fungus can initiate a process called dissemination, spreading beyond the respiratory system. The yeast form actively evades the immune system, including by producing a protein called BAD-1, which helps it bind to macrophages and suppress the host’s inflammatory response. This allows the yeast to survive and multiply, setting the stage for systemic spread.
The primary mechanism for this body-wide spread is hematogenous dissemination, where the yeast cells enter the bloodstream and lymphatic system. Once in the circulatory system, these large, thick-walled yeast cells travel throughout the body, eventually becoming trapped in the capillary beds of distant organs. There, they exit the bloodstream and multiply, establishing new sites of infection.
Systemic spread occurs in 25% to 40% of symptomatic blastomycosis cases, often weeks or months after the initial lung infection. Individuals with compromised cellular immunity, such as those with HIV/AIDS or those receiving immunosuppressive therapy, are at a higher risk. However, blastomycosis is a primary pathogen that frequently causes disseminated disease in people with otherwise healthy immune systems.
Specific Organ Damage and Manifestations
Once the fungus has disseminated, it causes a distinctive inflammatory reaction known as pyogranulomatous inflammation, leading to tissue damage and the formation of abscesses in various organs.
Skin (Cutaneous Blastomycosis)
The skin is the most common site of extrapulmonary disease, occurring in up to 40% to 80% of those with systemic infection. The resulting cutaneous blastomycosis typically presents as painless, purplish-gray skin lesions that can be verrucous, or wart-like, with raised, heaped borders. These lesions can also break down into friable, shallow ulcers that may have small abscesses underneath.
Skeletal System (Osteoarticular Blastomycosis)
The skeletal system is the second most frequent site of spread, involving the bones and joints in up to 25% of extrapulmonary cases. This osteoarticular blastomycosis often manifests as osteomyelitis (infection of the bone itself) or as septic arthritis (infection within a joint space). The vertebrae, pelvis, and long bones are commonly affected, causing chronic pain, local swelling, and the formation of bone-eroding lytic lesions.
Central Nervous System (CNS)
Involvement of the CNS is less common but represents the most severe manifestation of blastomycosis. The fungus can cause meningitis (inflammation of the membranes surrounding the brain and spinal cord) or form abscesses within the brain tissue. CNS blastomycosis is particularly dangerous, with symptoms that include severe headaches, confusion, and focal neurological deficits.
Genitourinary Tract and Abdominal Viscera
The fungus frequently targets the genitourinary tract, especially in men. This spread can lead to prostatitis (infection of the prostate gland) or epididymitis (inflammation of the coiled tube at the back of the testicle). The infection can also affect the abdominal viscera, leading to abscesses in organs like the liver or spleen.