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Epidemiology

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Aetiologic Agent

Blastomyces dermatitidis and Blastomyces gilchristii are thermally dimorphic fungi. Both grow as a mould form at 25°C (room temperature), and as a yeast form at 37°C (body temperature). Upon entering the body from the environment, the mould transforms into the yeast-phase as part of the adaptation process to a new environment with an elevated temperature. Unlike other fungi, the dimorphic fungi, including Blastomyces spp. are considered true pathogens and can cause disease in otherwise healthy individuals.

Clinical Presentation

Blastomycosis is a fungal infection that primarily affects the lungs, but can become a systemic infection with extrapulmonary manifestations. Up to 50% of pulmonary cases remain asymptomatic. Pulmonary blastomycosis may be acute or chronic.

Acute pulmonary infection, which often goes undiagnosed, presents as an influenza-like illness with the sudden onset of fever, cough, and a pulmonary infiltrate on chest radiographs. The acute disease often resolves spontaneously after 1–3 weeks. A subset of those with acute infection will go on to severe disease and acute respiratory distress syndrome (ARDS).

Chronic pulmonary infection has a slow onset where initial symptoms of cough and chest pain may be mild or absent. Clinical manifestations may include 2–6 months of weight loss, fever, night sweats, cough with sputum and chest pain, and may be similar to tuberculosis, other fungal infections and cancer. There is a very high mortality rate for patients who develop ARDS with chronic pulmonary infection.

Extrapulmonary disease can occur in patients with blastomycosis, but is more common in patients with chronic pulmonary infection. The most common extrapulmonary site for infection is the skin (cutaneous lesions are often located on the face and distal extremities). Other common sites include bone, the genitourinary system, and the central nervous system, but any system can be affected.

Untreated, chronic and extrapulmonary blastomycosis can eventually progress to death, and a high index of suspicion is required for prompt treatment of all disease to prevent progression.

Modes of Transmission

Inhalation of airborne spores in dust from the mould or saprophytic growth forms. Cases of blastomycosis from direct inoculation into the skin are rare, but can occur.

No person-to-person transmission or zoonotic transmission. Infection in animals, particularly dogs, has been identified, but animals do not appear to directly transmit the disease to humans.

  • Incubation Period
    The incubation period ranges between 21–106 days, with a median of 43 days.

Period of Communicability

No person-to-person transmission, nor zoonotic transmission from infected animals.

Risk Factors/Susceptibility

People who participate in outdoor activities in wooded areas (such as forestry work, hunting, and camping) in endemic areas may be at higher risk of exposure to Blastomyces spp. Susceptibility is general in areas where B. dermatitidis is present in the environment. Immunocompromised individuals have higher morbidity and mortality with blastomycosis infection.

Diagnosis & Laboratory Testing

Although urine antigen and serological testing is available, the sensitivity and specificity are poor, and therefore they are not generally recommended. If a patient has a reactive Blastomyces serology result, it is recommended that appropriate specimens be collected for microscopy and culture.

Treatment & Case Management

Treatment is under the direction of the attending health care provider. Most patients will require treatment. Treatment is indicated for all patients with progressive pulmonary or extrapulmonary diseases as well as those patients who are immunocompromised. Provide cases with information about the infection and how it spreads.

Treat with appropriate drug therapy. Itraconazole is commonly used for less severe infections. Amphotericin B or amphotericin B deoxycholate are used for more severe infections, sometimes with the addition of itraconazole.

Patient Information

Additional Resources

Government of Canada, “For Health Care Professionals: Blastomycosis”.

References

Ministry of Health and Long-Term Care, Infectious Diseases Protocol, May 2018.

 
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