Blastomycosis is a rare fungal infection caused by the fungus Blastomyces dermatitidis, which grows in wood and soil. It is a common infection among dogs, particularly in areas where it is prevalent.
The disease mainly affects the lungs with nearly 50% of patients often showing minimal or no symptoms. The infection may spread to other parts of the body. The skin is most commonly affected and is involved in about 20-40% of cases.
Causes and risk factors
Infection occurs by breathing in the spores that become airborne when contaminated soil or wood is disturbed. Humans and animals such as dogs, rats and cats may become infected. The disease is rarely transmitted from human-to-human or animal-to-human. Very rarely, infection via the skin may occur.
Blastomycosis can be found throughout the world but is most common in south-central and mid-western USA and Canada. The annual incidence in these areas is around 1-2 cases per 100,000 people. Although people of all ages and sex may become infected, people with weakened immune systems such as those with human immune deficiency virus infection (HIV) or organ transplant recipients are at greater risk of severe disease and infection spreading to other parts of the body.
Symptoms may not become apparent until 3-4 weeks after inhalation of the fungus. The mean time for symptoms to manifest is 45 days (range 21-106 days).
Patients usually present with respiratory symptoms that conform to one of the following specific patterns.
|Acute illness resembling bacterial pneumonia||
|Chronic illness resembling tuberculosis or lung cancer||
|Fast, progressive, severe symptoms resembling acute respiratory distress syndrome (ARDS)||
Other symptoms that may occur include bone or joint pain, muscular stiffness and pain, prostatic involvement that may cause pain on urinating, and throat involvement causing hoarseness.
Cutaneous features of blastomycosis
Skin lesions are common on the face, neck and extremities as the infection spreads from the lungs to other parts of the body. One or many lesions may develop.
- Lesions begin as papules, pustules or as subcutaneous nodules.
- Within weeks to months the lesions develop into ulcers and form crusty sores.
- Over a period of months to years lesions grow larger and heal to form raised wart-like scars. Lesions may cover much of the face causing severe disfigurement.
- Irreversible scarring often occurs.
Laboratory and radiological studies are performed to confirm the diagnosis of blastomycosis.
- Sputum microscopy and cultures
- Serologic testing
- Chest x-ray and CT scanning
- Tissue and skin biopsy
Patient history is also important in the diagnosis, particularly in areas where there may be a disease outbreak.
Patients with blastomycosis confined to the lungs and who are not suffering from severe symptoms may not require any treatment. Usually symptoms are self-limiting and the infection clears spontaneously. Treatment should be used if infection in the lung worsens.
When the infection spreads and skin becomes involved, spontaneous resolution does not occur and treatment is necessary.
- Itraconazole given orally is the drug of choice in mild-to-moderate disease involving the lungs, or in disease involving other organs.
- Amphotericin B via intravenous administration is the drug of choice for severe or life-threatening blastomycosis (e.g. ARDS, CNS involvement, immunocompromised patients).
Patients with limited skin lesions and relatively mild lung involvement usually recover completely. Untreated or more severe disease can lead to severe scarring and disfigurement or death.
- Book: Textbook of Dermatology. Ed Rook A, Wilkinson DS, Ebling FJB, Champion RH, Burton JL. Fourth edition. Blackwell Scientific Publications.
On DermNet NZ:
- Blastomycosis – Medscape Reference
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