B. dermatitidis is asexual form of Ajellomyces dermatitidis. (These can be differentiated on basis of presence or absence of "A Antigen", B. dermatitidis being positive for it.) It is a dimorphic fungus.
Blastomyces dermatitidis from a lesion in dog.
Inhaled conidia of B. dermatitidis are phagocytosed by neutrophils and macrophages in alveoli. Some of these escape phagocytosis and transform into yeast phase rapidly. Having thick walls, these are resistant to phagocytosis and express glycoprotein BAD-1, which is a virulence factor as well as an epitope. In lung tissue, they multiply and may disseminate through blood and lymphatics to other organs, including the skin, bone, genitourinary tract, and brain. The incubation period is 30 to 100 days, although infection can be asymptomatic.
Signs and symptoms
Blastomycosis can present in one of the following ways:
a flu-like illness with fever, chills, arthralgia (joint pain), myalgia (muscle pain), headache, and a nonproductive cough which resolves within days.
an acute illness resembling bacterial pneumonia, with symptoms of high fever, chills, a productive cough, and pleuritic chest pain.
a chronic illness that mimics tuberculosis or lung cancer, with symptoms of low-grade fever, a productive cough, night sweats, and weight loss.
a fast, progressive, and severe disease that manifests as ARDS, with fever, shortness of breath, tachypnea, hypoxemia, and diffuse pulmonary infiltrates.
skin lesions, usually asymptomatic, can be verrucous (wart-like) or ulcerated with small pustules at the margins.
bone lytic lesions can cause bone or joint pain.
prostatitis may be asymptomatic or may cause pain on urinating.
Once suspected, the diagnosis of blastomycosis can usually be confirmed by demonstration of the characteristic broad based budding organisms in sputum or tissues by KOH prep, cytology, or histology. Tissue biopsy of skin or other organs may be required in order to diagnose extra-pulmonary disease. Blastomycosis is histologically associated with granulomatous nodules. Commercially available urine antigen testing appears to be quite sensitive in suggesting the diagnosis in cases where the organism is not readily detected. While culture of the organism remains the definitive diagnostic standard, its slow growing nature can lead to delays in treatment of up to several weeks.
However, sometimes blood and sputum cultures may not detect blastomycosis; lung biopsy is another option, and results will be shown promptly.
Itraconazole given orally is the treatment of choice for most forms of the disease. Ketoconazole may also be used. Cure rates are high, and the treatment over a period of months is usually well tolerated. Amphotericin B is considerably more toxic, and is usually reserved for immunocompromised patients who are critically ill and those with central nervous system disease. Patients who cannot tolerate deoxycholate formulation of Amphotericin B can be given lipid formulations. Fluconazole has excellent CNS penetration and is useful where there is CNS involvement after initial treatment with Amphotericin B.
Mortality rate in treated cases
0-2% in treated cases among immunocompetent patients
In the United States, blastomycosis is endemic in the Mississippi river and Ohio river basins and around the Great Lakes. The annual incidence is less than 1 case per 100,000 people in Mississippi, Louisiana, Kentucky, and Arkansas. The cases are greater in northern states such as Wisconsin, where from 1986 to 1995 there were 1.4 cases per 100,000 people. It also frequently affects hunting dogs in northern Wisconsin and the upper Mississippi and Wisconsin Rivers.
In Canada, most cases of blastomycosis occur in Northwestern Ontario, particularly around the Kenora and areas like Killarney (Georgian Bay) Area. The moist, acidic soil in the surrounding woodland harbors the fungus.
Blastomycosis is distributed internationally; cases are sometimes reported from Africa.
Blastomycosis was first described by Thomas Casper Gilchrist in 1894 and sometimes goes by the eponym Gilchrist's disease. It is also sometimes referred to as Chicago Disease.
Budding yeasts in cytoplasm of giant cells at arrows. Broad-based budding and double countoured cell wall seen in the giant cell in the center is characteristic of Blastomyces dermatiditis.
Granuloma with early suppuration. Fungal organisms difficult to recognize at this low magnification.
Large yeast-like fungi seen within giant cells at arrows.
Nodular skin lesions of blastomycosis, one of which is a bullous lesion on top of a nodule.
^Alvarez G, Burns B, Desjardins M, Salahudeen S, AlRashidi F, Cameron D (2006). "Blastomycosis in a young African man presenting with a pleural effusion". Can Respir J13 (8): 441–4. PMC2683332. PMID17149463.