Blastomycosis

Blastomycosis is an invasive fungal infection caused by the spores of the dimorphic fungi belonging to the genus Blastomyces, most commonly Blastomyces dermatitidis and occasionally Blastomyces gilchristii, Blastomyces helicus or Blastomyces percursus.

Epidemiology and distribution

Blastomyces dermatitidis is endemic in the soils of the Ohio and Mississippi River Valleys, Great Lakes region, and the southeastern United States. Early efforts to isolate Blastomyces from soil samples relied on culture or recovery from intravenous injection of soil samples into mice, which was a laborious and often unsuccessful process. Thanks to PCR technology, Blastomyces have been successfully identified in the expected ecologic niche (moist, acidic soil, particularly in wooded areas near rivers or other water sources) and tissues of animals exposed to these environments.

As a result, Blastomyces are frequently recorded in people who work outside, fish or hunt, or have recently been exposed to regions where soil and plants have been disturbed, for example, for excavation or construction. Moreover, in the endemic regions, the risk of blastomycosis is particularly high in dogs, especially hunting and sporting dogs. This high risk is likely due to their close proximity to the soil, together with sniffing and digging behaviour, reinforcing the soil-spore link. (1, 2)

Risk factors

Apart from living in the endemic areas and carrying out the activities mentioned above, the other main risk factor for invasive infection is a weakened immune system. However, in contrast to other invasive fungal diseases that predominantly affect immunocompromised hosts, blastomycosis is rather prevalent in immunocompetent hosts. (3)

Symptoms

Blastomycosis most commonly presents as a pulmonary infection; however, the clinical manifestations of blastomycosis can be quite diverse, ranging from acute to subclinical and chronic pulmonary infection and systemic pyogranulomatous disease. The initial infection occurs through the lungs via inhalation of aerosolized conidia released by the fungus and found in the disrupted soil. In the lungs, these conidia convert to thick-walled budding yeast, leading to symptoms of a pulmonary infection.

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In approximately 25 to 30% of patients, extrapulmonary disease occurs after hematogenous dissemination from the lungs, and the skin is the most common site of extrapulmonary disease. Although extremely rare, primary cutaneous blastomycosis can also occur if the fungus inoculates the skin after a cut or another traumatic event. Other sites of extrapulmonary disease are bones and the genito-urinary system. (1, 2)

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Diagnosis and treatment

Blastomycosis diagnosis can be challenging. A definitive diagnosis requires direct visualization of Blastomyces dermatitides. Sputum specimens stained with 10% potassium hydroxide or another fungal stain have an approximate 80% yield. The most sensitive method for the detection and diagnosis of blastomycosis is culture. However, growth can often be detected only after up to 30 days, especially if there are few organisms in the specimen. In addition to culture, chest x-rays or CT scans can be used to screen for pulmonary involvement, though findings often lack specificity. In particular, blastomycosis can often resemble lung cancer on radiographical analyses. Therefore, better and more specific diagnostic tests for blastomycosis represent an unmet clinical need. (4)

With regards to treatment, mild to moderate infection is treated with the antifungal medication itraconazole, while amphotericin B is usually prescribed for severe blastomycosis in the lungs or in case of extrapulmonary involvement. (5)?