DESCRIPTION

• Histoplasmosis (a.k.a. Ohio Valley disease, spelunker’s lung, cave disease, Darling disease, Appalachian Mountain disease, and reticuloendothelial cytomycosis) is one of the most common endemic mycoses in the United States.
• Dimorphic fungus Histoplasma capsulatum var. capsulatum in the United States (African var. is duboisii, which affects skin, lymph nodes, and bone.)
• Forms include acute, chronic, primary, reactivation, and the affected disease areas (pulmonary, extrapulmonary, disseminated).

EPIDEMIOLOGY

• Endemic in central and eastern states, classically Ohio and Mississippi River valleys; potential range may include other parts of the United States; endemic in large parts of Central and South America and Africa and parts of Europe, Asia, and Australia
• Mold form at <35°C (whereas yeast form in tissue above 37°C) grows best in soil environments: warm, moist, and rich in nitrogen (bird/bat guano); found in caves, under roosting trees, abandoned buildings
• Infection/transmission from inhaled spores from soil; no human or animal-to-human transmission except for rare vertical transmission and organ transplant transmission cases
• Occasional cluster infections usually with soil disruptions (spelunking, building demolition)
• Sporadic cases with some activities: gardening, roofing, installing air-conditioning/heating, cleaning older homes, playing in hollow trees
• Only mammals can be infected (birds do not carry the organism).
• Incubation is typically 3 to 17 days, but reactivation can occur later in immunocompromised patients.
• Annual incidence estimated to be 0 to 4.3 cases per 100,00 population in U.S. states reporting cases: higher rates in the Midwest (6.1 per 100,000); disseminated infection occurrence rate: 1:2,000 cases
• Skin test reactivity as high as 50–80% by the age of 18 years in most highly endemic areas

ETIOLOGY

Infectious mold spores are microconidia of H. capsulatum; warmer temperatures inside the host signal transformation to an oval, budding yeast (i.e., dimorphic forms)

RISK FACTORS

• Males are slightly more at-risk than females based on incidence rates.
• Infections are more common in ages 30 to 60 years, but all ages are affected.
• Disseminated disease risk factors: immature cellular immunity (<2 years of age), large inoculum, acquired immunodeficiency (including tumor necrosis factor [TNF]-α inhibitors), malnutrition

GENERAL PREVENTION

• Avoid areas/dust with likely mold.
• Spray water, oil, or 3% formalin in areas with dust/dirt if work planned.
• Wear cover apparel and face mask that are able to filter particles >1 millimicron (e.g., N95).
• Prophylaxis for certain populations

PATHOPHYSIOLOGY

• Inhaled mold spores germinate in lungs to yeast form; rarely enter via skin or transplanted organs
• Once germinated, some lymphatic and hematologic dissemination even in self-limited disease
• Immune system takes several weeks to respond and involves T lymphocytes, IL-12, IFN-γ, and TNF-α; macrophages kill fungus.