Histoplasmosis is caused by the dimorphic fungus Histoplasma capsulatum, which exists in the mold (mycelial) phase mainly in soil. Infection results from inhalation of the spores (conidia) that are disseminated into the atmosphere when infected material is disturbed. Individuals with preexisting, immune-compromising conditions are at higher risk of developing clinical disease following exposure. Generally, factors such as the number of spores inhaled and host susceptibility play decisive roles in the development of clinical disease. Patients usually have primary lesions in the lungs. Definitive diagnosis requires certain laboratory tests with compatible clinical characteristics.

Indiana lies in the "histo belt" area of the Midwest. The incidence rate of histoplasmosis was 1.71/100,000 persons in 1997. Figure 1 shows the areas of reported cases.

Figure 1.

The disease can occur as isolated cases or in local outbreaks. Outbreaks often happen when a dust containing spores is produced such as during destruction of an old bird roost site, removal of an old chicken house, or excavation into contaminated soil. Since August 1988 when histoplasmosis became reportable in Indiana, surveillance data indicate that the number of cases has decreased gradually (Figure 2).

Sex-specific incidence rates indicate males have about twice the risk of getting the disease than females. The incidence rate for males was 2.3/100,000 vs. only 1.15/100,000 for females. The age-specific incidence rates of histoplasmosis in Indiana in 1996 are illustrated in Figure 3. Individuals in the 20-59 year old age group appear to be at higher risk of developing clinical disease.

Control measures for histoplasmosis are unsatisfactory at present, but it is possible to reduce the dissemination of spores into the atmosphere and to minimize exposure to dust in a contaminated environment by wearing appropriate personal protective equipment and gently misting potential infectious dust sources with water before beginning work.