Etiology and Epidemiology

H. capsulatum is a thermal dimorphic fungus. At 25°C, it grows on Sabouraud agar as a fluffy-white mycelium, which bears microconidia and also characteristic tuberculate macroconidia. The organism is free-living in nature in this form. At 37°C, it grows as a small (2- to 4-/i,m diameter) yeast. The organism is found in this form in infected tissue.

A minor disturbance of fungus-laden soil may scatter spores into the air. The microconidia are inhaled, causing infection. Within the lung the organism converts to the yeast phase, which is not infectious. Person-to-person transmission does not occur. So-called epidemics of histoplasmosis are more accurately point-source outbreaks.

Within a highly endemic area, the organism is widely but not uniformly distributed. Microfoci with high concentrations of organisms are found by chicken coops and starling roosts, and in caves inhabited by bats. The nitrogen-rich excrement of birds and bats provides a favorable growth environment for the fungus. Exposure of small groups of people to high concentrations of organisms at such sites may result in outbreaks of symptomatic infection. These are fairly easy to identify because a severe respiratory illness occurs simultaneously in a group of people who were together for a particular activity 14 days earlier. Extremely large outbreaks of longer duration have occurred during excavation for road or building construction. A good example is the community-wide outbreak of histoplasmosis that occurred during the building of a swimming pool and tennis court complex in Indianapolis, Indiana, infecting perhaps over 100,000 people (Wheat et al. 1981).

Most cases of histoplasmosis, however, are sporadic and result from casual exposure to environmental spores. Patients with sporadic illness probably inhale fewer spores and are more likely to be asymptomatic or minimally symptomatic. The vast majority of these infections are never recognized and are known to exist only as a result of skin test surveys.

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