Blastomycosis

Basics

Description

  • Systemic infection caused by the dimorphic soil fungus Blastomyces dermatitidis
  • Dimorphism is characterized by a mold phase (mycelial form) that grows at room temperature and a yeast form that grows at body temperature.
  • Incubation period estimated at 30 to 45 days

Epidemiology

  • Similar to other dimorphic fungi, B. dermatitidis is a soil saprophyte (mycelial form).
  • Congenital infections occur rarely.
  • Infection is endemic in the Upper Midwest and Southern United States, particularly in the wooded Mississippi and Ohio River valleys and the Great Lakes. The highest incidence in the United States is in Wisconsin, Mississippi, and Tennessee followed by Minnesota, Illinois, North Dakota, Alabama, and Louisiana.
  • Substantial disease occurs in the Canadian provinces of Manitoba and northwestern Ontario. Other reported areas of infection include Africa, India, and South America.
  • Children account for 3–11% of all cases of blastomycosis.
  • Blastomycosis is a very uncommon diagnosis even in endemic regions.

Risk Factors

  • Blastomycosis is more common in males. This is thought to be due to occupational or recreational activities that increase risk of exposure.
  • Underlying immunodeficiency is rarely observed among children with blastomycosis.

General Prevention

  • No special precautions for hospitalized patients are indicated.
  • The natural reservoir is undetermined.

Pathophysiology

  • Inhalation of the fungus into the lung is followed by an inflammatory response with neutrophils and macrophages.
  • Blastomycosis most commonly presents as subacute pulmonary disease, but the clinical spectrum of the disease extends from asymptomatic to disseminated disease that can involve lung, skin, bone, and central nervous system (CNS).
  • As many as 50% of infections are asymptomatic.

Etiology

  • Infection is almost always caused by inhalation of spores from B. dermatitidis.
  • Rarely, blastomycosis has occurred through accidental inoculation, dog bites, conjugal transmission, and intrauterine transmission.
  • Point-source outbreaks have occurred with occupational and recreational activities that occur in areas with moist soil and decaying vegetation, such as along streams and rivers.
  • Natural infection occurs in humans and dogs.

Commonly Associated Conditions

  • Pulmonary blastomycosis
    • Most common form of infection by Blastomyces in children
    • Can be acute, subacute, or chronic
    • Illness severity can vary greatly, from asymptomatic to upper respiratory tract infection, bronchitis, pleuritis, pneumonia, or severe respiratory distress.
  • Cutaneous blastomycosis
    • Skin manifestations are variable and include nodules, verrucous lesions, subcutaneous abscesses, or ulcerations.
    • Cutaneous disease usually occurs after pulmonary infection with dissemination to the skin, rarely by direction inoculation.
  • Bone blastomycosis
    • Bone disease usually occurs after pulmonary infection with dissemination to the bone resulting in osteomyelitis and bone destruction.
  • Disseminated blastomycosis
    • Recent series suggest that this occurs in <1/2 of children.
    • Usually begins as pulmonary infection, with subsequent spread to involve skin, bone, genitourinary tract, and/or CNS
    • Can disseminate to virtually any organ
    • The classic triad of lung, bone, and skin disease occurs in ≤15% of children.

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