From the CDC:
H. capsulatum grows in soils throughout the world. In the United States, the fungus is endemic and the proportion of people infected by H. capsulatum is higher in central and eastern states, especially along the valleys of the Ohio, Mississippi, and St. Lawrence rivers, and the Rio Grande. The fungus seems to grow best in soils having a high nitrogen content, especially those enriched with bird manure or bat droppings. The organism can be carried on the wings, feet, and beaks of birds and infect soil under roosting sites or manure accumulations inside or outside buildings. Active and inactive roosts of blackbirds (e.g., starlings, grackles, red-winged blackbirds, and cowbirds) have been found heavily contaminated by H. capsulatum. Therefore, the soil in a stand of trees where blackbirds have roosted for 3 or more years should be suspected of being contaminated by the fungus. Habitats of pigeons and bats, and poultry houses with dirt floors have also been found contaminated by H. capsulatum.
On the other hand, fresh bird droppings on surfaces such as sidewalks and windowsills have not been shown to present a health risk for histoplasmosis because birds themselves do not appear to be infected by H. capsulatum. Rather, bird manure is primarily a nutrient source for the growth of H. capsulatum already present in soil. Unlike birds, bats can become infected with H. capsulatum and consequently can excrete the organism in their droppings.
To learn whether soil or droppings are contaminated with H. capsulatum spores, samples must be collected and cultured. The culturing process involves inoculating mice with small portions of a sample, sacrificing the mice after 4 weeks, and streaking agar plates with portions of each mouse's liver and spleen. Then for four more weeks, the plates are watched for the growth of H. capsulatum. Enough samples must be collected so that small but highly contaminated areas are not overlooked. On several occasions, H. capsulatum has not been recovered from any of the samples collected from material believed responsible for causing illness in people diagnosed from the results of clinical tests as having histoplasmosis.
Until a less expensive and more rapid method is available, testing field samples for H. capsulatum will be impractical in most situations. Consequently, when thorough testing is not done, the safest approach is to assume that the soil in regions where H. capsulatum is endemic and any accumulations of bat droppings or bird manure are contaminated with H. capsulatum and to take appropriate exposure precautions.
