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Cases = 75

Crude incidence rate = 1.27/100,000

Histoplasmosis is caused by the dimorphic fungus Histoplasma capsulatum, which grows as a mold in the soil and as yeast in man and animals. Infection results from inhalation of spores that are disseminated into the atmosphere when soil and or organic materials containing the infectious material is disturbed. Infection is common while overt clinical disease is not. Individuals with preexisting, immune-compromising conditions are at higher risk of developing clinical disease after exposure. The number of spores inhaled and host’s susceptibility play roles in the development of clinical disease. The clinical disease may vary from a mild respiratory illness to a chronic disseminated disease or a chronic pulmonary disease resembling tuberculosis. Generally, the disease is mild or unrecognized as histoplasmosis; rarely does it develop to a severe progressive disease.

Indiana is part of the eastern and central United States where exposure and infection is prevalent. In 1999, 75 confirmed cases were reported for a crude incidence rate of 1.27/100,000. Figure 1 shows the county of residence for reported cases for 1999 and Figure 2 shows the county of residence for cases for the past five years. In 1999, Marion County reported 28% of the cases, but only has 14% of the population. Crude incidence rate for Marion County in 1999 was 2.6/100,000, which is approximately double the state rate.

 Histoplasmosis occurs as individual cases or as a local outbreak. Outbreaks have occurred in Indiana when old bird roost sites have been disturbed during the destruction of buildings or movement of dirt at construction sites. Since 1995, an average of 96 cases of histoplasmosis has been reported yearly. The number of cases by year is showed in Figure 3.

Figure 3.

In 1999, the sex-specific incidence rates indicated that males were almost twice as likely to have disease as females. The male incidence rate was 1.67/100,000 and the females incidence rate was .89/100,000. The female:male case ratio was 1:1.9. The age- and sex-specific incidence rates of histoplasmosis in Indiana are presented in Figure 4. Male incidence rates are higher than female rates at all age levels except ages 5-9 and 40-59, and they are 3.5 times higher in the 10-19 age group, 3.6 times higher in the 20-39, and 3.7 times higher in the 60+ age group. The difference in sex-specific rates in the 20-59 year old groups may be in part due to increased risk of occupational exposure as well and the risk of being immunosuppressed due to HIV infections. One female and four male individuals with histoplasmosis were also identified as being HIV positive, and the four males were in the 20-39 year old group. Figure 5 illustrates the 5-year incidence rate and the preponderance for a higher case rate in males than females at all ages. The five-year female:male case ratio is 1:2.4.

Figure 4.

Figure 5.

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