Blastomycosis

Pathogen and infectious cycle

Blastomyces dermatitidis is a dimorphic fungus: it exists as a filamentous mold in the environment (at room temperature) and transforms into a broad-based budding yeast in human tissues at 37°C. This transformation is essential to its virulence. The related species Blastomyces gilchristii, which was identified more recently, is responsible for some outbreaks in Ontario and possibly Quebec. Contamination is exclusively environmental: no human-to-human or animal-to-human transmission has been documented, although dogs are also susceptible to contracting the disease and may indicate common exposure to an environmental source.

Risk factors and vulnerable populations

Clinical manifestations

Blastomycosis presents with three main clinical patterns depending on the site of infection and the host’s immune status. Approximately 50% of infected individuals remain asymptomatic.

Diagnosis

The diagnosis of blastomycosis is often delayed due to its relative rarity and the similarity of its manifestations to other pulmonary or skin conditions. A history of exposure in a natural environment within an endemic area should systematically guide clinical suspicion.

• Direct Mycological Examination: Demonstration of budding yeasts with characteristic broad bases on sputum, bronchoalveolar lavage (BAL), skin, or bone biopsy smears (calcofluor white or KOH staining)
• Fungal culture: reference method, but slow (2 to 4 weeks); performed on Sabouraud's medium using sputum, BAL, urine, or tissue biopsies
• Urinary antigen of Blastomyces (MiraVista Diagnostics): rapid enzyme-linked immunosorbent assay (ELISA), 90% sensitivity in disseminated forms; possible cross-reactions with Histoplasma and Coccidioides
• Serology (anti-Blastomyces antibodies): limited sensitivity (30–60%), rarely used in routine practice
• Chest X-ray and CT scan of the lungs: infiltrates, consolidations, masses, or cavitations; no specific radiological features
• Tissue biopsy with histological examination: granulomas with giant cells and characteristic yeasts (most specific method)
• Lumbar puncture and CSF analysis if neurological involvement is suspected
• Extension scan: thoraco-abdominopelvic CT scan, bone scan or MRI depending on suspected sites of spread

Antifungal treatment

Treatment depends on the severity of the disease, the site of involvement, and the patient's immune status. The Infectious Diseases Society of America (IDSA) guidelines guide management.

Therapeutic follow-up and healing criteria

• Serum itraconazole level two weeks after starting treatment to confirm adequate therapeutic concentration (target: random level > 1 µg/mL)
• Close clinical surveillance at weeks 2, 4, 8, then monthly until the end of treatment
• Urinary antigen testing Blastomyces to track the decrease under treatment
• Control imaging (chest X-ray or CT scan) at 3 and 6 months to document pulmonary response.
• Liver function monitoring every 1 to 3 months while on azoles (risk of hepatotoxicity)
• In case of relapse or therapeutic failure: reassessment of adherence, serum levels, and referral to infectious diseases

Prevention and protection measures

There is no vaccine for blastomycosis. Prevention relies on reducing exposure to fungal spores during high-risk activities, especially for immunocompromised individuals.

• Wear an N95 respirator mask when performing work that involves disturbing wet forest soil in an endemic area
• Avoid returning piles of leaves, decaying wood, or old organic materials without respiratory protection.
• Immunocompromised individuals should discuss high-risk activities with their doctor and consider limiting them in endemic forest areas.
• Report any persistent respiratory symptoms occurring in the weeks following outdoor activity in an area known for blastomycosis to a doctor.

Consult at Clinique Omicron

In cases of antibiotic-resistant atypical pneumonia, unexplained skin lesions, or prolonged fever after returning from activities in forest or riverside environments in Quebec, Clinique Omicron physicians may initiate targeted mycological investigation, including the prescription of urinary antigen for Blastomyces and coordination of appropriate microbiological sampling. Prompt referral to infectious disease specialists is facilitated in cases of suspected disseminated or severe forms. Book an appointment online or by phone at one of Clinique Omicron's service points on the South Shore and elsewhere in Quebec.

The content of this page is provided for informational purposes only and is not intended to replace the advice of a qualified healthcare professional. Consult a physician for any symptoms, questions or decisions you may have regarding your health.