Description

• A fungal infection caused by Histoplasma capsulatum, a dimorphic, soil-dwelling saprophyte, which exists as a mold in the environment and as a yeast in tissues (at 37°C)
• Infection is often caused by inhalation of conidia or mycelial fragments. Initial infection is often asymptomatic.
• Clinical manifestations vary according to extent of exposure, presence of underlying lung disease, and host immune response. Manifestations range from a self-limited flulike syndrome to pneumonia, mediastinal fibrosis, or chronic cavitary disease in those with obstructive lung disease and disseminated histoplasmosis (more frequent in immunocompromised and infants).
• Primary infection: often asymptomatic or flulike (fever, cough, headache, chest pain); can also develop hilar lymphadenopathy calcified pneumonitis, and splenic/hepatic calcifications
• Chronic pulmonary histoplasmosis: cavitary upper lung lesions; usually in white male smokers with history of obstructive lung disease
• Disseminated histoplasmosis infection in immunocompromised patients is a rare opportunistic infection that mimics sepsis syndrome and may progress to multiple organ system failure.
• System(s) affected: gastrointestinal; hematologic/lymphatic/immunologic; pulmonary; skin/exocrine

Geriatric Considerations
Increased incidence of disseminated histoplasmosis in males during 6th and 7th decades

Pediatric Considerations
1/3 of cases of disseminated histoplasmosis occur in infants age <1 year.

Epidemiology

H. capsulatum

• Worldwide distribution: Most endemic region in North America is Central United States; cases also found in the Eastern and Midwestern United States, along the Ohio and Mississippi River valleys and in several states along the U.S. East Coast
• Spores may remain active for up to 10 years.
• Increased exposure among farmers and individuals working with bird or bat droppings
• Exposure in endemic areas is virtually 100%.
• Few patients develop active disease.

Incidence

• ~500,000 new infections in the United States per year
• Occurrence in AIDS patients is 2–5%.
• Disseminated histoplasmosis <0.05% of infections
• >80% of young adults living along the Ohio and Mississippi River valleys have been infected.
• Predominant gender:
  • Acute histoplasmosis: male = female
  • Disseminated: male > female (5 to 10:1)

Prevalence
Disseminated histoplasmosis

• Infants <1 year at higher risk for dissemination
• In adults, increased incidence in age >60 years

Etiology and Pathophysiology

Spore growth enhanced in bird and bat droppings; spores carried by air currents and inhaled. Once inhaled, spore is phagocytized by alveolar macrophages and converts to yeast. The organism survives in the macrophages for weeks and disseminates via the reticuloendothelial system.

Risk Factors

• Spelunking or excavation near bird roosts
• Cleaning chicken coops or performing routine activities in areas with high accumulation of bird or bat droppings
• Demolition or remodeling of old buildings
• Exposure to decayed wood or dead trees
• Immunosuppression
• Exposure prior to developing adequate cell-mediated immunity (typically infants <1 year of age)

General Prevention

• Consider prophylactic therapy in patients with AIDS with CD4 count <150 μcells/mm³.
• Individuals at high risk of exposure to H. capsulatum should wear respirators. Spray demolition work to decrease dust and aerosolized conidia.
• Immunocompromised individuals should avoid spelunking or undertaking renovation projects involving soil or buildings potentially contaminated with H. capsulatum.

Commonly Associated Conditions

Disseminated histoplasmosis is an opportunistic infection in immunocompromised hosts (transplant patients), patients receiving antitumor necrosis factor (α-TNF) therapy, and patients with cellular defects (e.g., HIV).