Blastomycosis

Blastomycosis refers to disease caused by fungi of the Blastomyces genus. These are dimorphic fungi which grow in mould form at 25 to 28 degrees Celsius, and in yeast form at 37 degrees Celsius. Blastomyces dermatitidis and Blastomyces gilchristii are the most common species associated with blastomycosis. Within the last decade, several other species of the Blastomyces genus have been identified. While some have only been identified internationally, B. helicus has been found in areas bordering the Rocky Mountains of western North America. Although there are genetic differences between the species, there are no clinically relevant differences, and differentiation may not be attempted in clinical laboratories. This summary will refer to the etiologic agent of blastomycosis as Blastomyces species. 

Infection with Blastomyces species occurs when the organism’s conidia (fungal spores) are inhaled into the host’s lungs. Upon entering the body from the environment, the mould form of the agent moves into the yeast phase due to the change in temperature. From here, both pulmonary and extrapulmonary manifestations can occur. There is no person-to-person transmission.

Blastomyces is a moist soil and decaying organic matter associated fungus. Risk areas for exposure to B. dermatiditis range from Saskatchewan to Quebec, particularly bordering the Great Lakes, the St. Lawrence Seaway, and Canadian Shield lakes; in the United States in the Ohio and Mississippi River valleys and eastwards

The diagnosis of blastomycosis is often protracted because this fungal infection is uncommon and its symptoms non-specific. Ontario added blastomycosis to its reportable disease list mid-2018. In Ontario in 2023 the average incidence rate was 0.8/100,000 (126 cases), though in Northwestern Ontario, a hyperendemic area for blastomycosis, between 2020 and 2024 the annual incidence ranged from 16.8 to 49.3/100,000. In Ottawa in 2024, 6 reported cases of blastomycosis gave an incidence of 0.5/100,000. For detailed information on blastomycosis counts and rates in Ottawa, please see Ottawa Public Health’s “Diseases of Public Health Significance Dashboard” and select “blastomycosis” from the disease of interest drop-down menu.

Infection with Blastomyces species is asymptomatic in up to 50 per cent of cases. Incubation periods between exposure and clinical disease are typically 2 weeks to 3 months (median 43 days). Rarely, an incubation period may last many months to years due to reactivation or latent infections. When associated with clinical disease, there can be a wide range of clinical presentations. Blastomycosis can be difficult to diagnose as symptoms can be easy to mistake for other more common illnesses. Non-specific symptoms can include fever, malaise, fatigue, and weight loss.  

Pneumonia is the most common presentation of blastomycosis, occurring in up to 90% of symptomatic cases. Symptoms can be similar to an acute bacterial pneumonia (fevers, chills, cough productive of sputum, hemoptysis) and can progress to acute respiratory distress syndrome in up to 15% of hospitalized patients. More frequently, patients develop a chronic pneumonia syndrome (weight loss, night sweats, fever, productive cough, hemoptysis over a period of 2 to 6 months). 

Disseminated disease is common and can occur in 25% to 40% of all symptomatic patients. Although it is possible to develop blastomycosis through direct inoculation (e.g., from trauma), the vast majority of extrapulmonary manifestations are due to disseminated disease following a pulmonary infection. Disseminated disease most commonly affects the skin, bone, genitourinary system, and central nervous system, although infection can occur in any organ system.  

Skin manifestations are the most common extrapulmonary finding, occurring in 40% to 80% of cases with disseminated disease. Skin manifestations can take on two major forms: verrucous and ulcerative lesions. The verrucous lesions have an elevated heterogeneous border and the lesions appear crusted overtop of an abscess in the underlying subcutaneous tissue. The ulcerative lesions are usually open with draining exudate. 

Bone infection is most commonly reported in the vertebrae, pelvis, sacrum, skull, ribs, and long bones, but theoretically any bone can be involved. The most common forms of genitourinary blastomycosis are prostatitis and epididymo-orchitis. Meningitis or cerebral abscess are the most common neurologic manifestations of blastomycosis, with abscess formation having a predilection for the cerebellum. 

Please see “Lab Testing” subheading in Physician Resources section for link to Public Health Ontario’s lab testing webpages.

Culture remains the gold-standard for diagnosing blastomycosis. Direct detection of appropriately-sized, broad-based budding yeast with microscopy is a presumptive positive and remains an important part of the diagnostic algorithm. B. dermatitidis is most commonly identified via microscopy from sputum, tissue, or wound exudate specimens. The organism can also be visible on histology using special fungal stains.

Serology can also be sent for Blastomyces species; however, while relatively specific for detecting antibodies to Blastomyces, due to the lesser sensitivity of the test, serology is difficult to rely on for diagnosis – a negative result does not rule out the possibility of current infection.

All patients with suspected blastomycosis infection, either pulmonary or extra-pulmonary, should have a chest x-ray done to assess for lung involvement 

If you have a suspected or confirmed case of blastomycosis, it should be reported as soon as the next business day by fax or telephone: Monday to Friday from 8:30 am to 4:30 pm: Call 613-580-2424, extension 24224 or fax 613-580-9640. See also: Reporting a communicable disease.

In the immunocompetent host, acute pulmonary blastomycosis can be a mild, self-limited illness that does not require treatment. Referral to an infectious diseases specialist should be considered in all cases of confirmed or suspected blastomycosis as treatment may still be offered in these circumstances in order to prevent development of disseminated infection. All immunocompromised patients, those with moderate to severe symptoms, or those with disseminated (extrapulmonary) disease should be offered antifungal therapy.

The choice of antifungal therapy is dependent on the location and severity of disease and tailored based on independent host factors. Intravenous amphotericin B is the initial treatment of choice for those with moderate to severe pulmonary disease, disseminated blastomycosis, or disease involving the central nervous system. It is also the treatment of choice for immunocompromised patients with blastomycosis. Once clinical improvement has been noted with intravenous amphotericin B, oral step-down to itraconazole for the remainder of therapy is reasonable. The duration of suggested therapy ranges from 6 to 12 months depending on the site of disease.

For those who present with mild acute pulmonary disease, initiating therapy with oral itraconazole is reasonable; again, consultation with an infectious diseases specialist is recommended.

Preventive measures are not yet fully understood. It is possible, particularly for immunocompromised persons, that risk of infection may be reduced by avoiding activities that cause disruption of the soil.

There is currently no approved vaccine against blastomycosis. 

Reporting to public health is important, as the incidence of blastomycosis in Ontario may be increasing. If endemic areas are able to be identified by reporting of disease, strategies to improve awareness among residents as well as healthcare providers of these areas can be put into place.

General Information on Blastomycosis 

Blastomycosis, Health Canada

International Resources on Blastomycosis

Blastomycosis, U.S. Centers for Disease Control and Prevention (CDC)   

Lab Testing

Go to  Diseases and Conditions Index, Public Health Ontario and click on “B” to find “Blastomycosis”, and follow this link to then select “Laboratory Testing” under which will appear the current links for applicable test information.

General Information on Blastomycosis

Infectious Diseases Protocol - Appendix 1: Case Definitions and Disease Specific Information Disease: Blastomycosis, Ontario Ministry of Health

Blastomycosis in Ontario, Public Health Ontario

Blastomycosis, Health Canada

Pathogen Safety Data Sheet – Blastomyces dermatitidis, Health Canada

International Resources on Blastomycosis

Blastomycosis, U.S. Centers for Disease Control and Prevention (CDC)

Guidelines on management of blastomycosis, Infectious Diseases Society of America (IDSA)