Recurrent exposure to Histoplasma capsulatum in modern air-conditioned buildings

Background: Between 1989 and 1996, an epidemic of histoplasmosis occurred on a medical school campus. There had been numerous construction projects on the campus that involved previously wooded land and were adjacent to a large bird sanctuary. Methods: We investigated the epidemic with active surveillance to detect cases, a histoplasmin skin-test survey, inspection of the air-filtration systems of the involved buildings, and cultures of soil samples. The investigation also included a simulation of entry into air-intakes of the buildings from spore sources by means of a wind-tunnel analysis of a model of the campus that used inert gas. After control procedures were instituted, sentinel population groups had follow-up with yearly serological tests. Results: From 1989 through 1996, there were 29 cases of histoplasmosis that occurred among school employees. All cases with a defined onset began during periods of ongoing campus construction. Positivity rates for histoplasmin skin testing were higher among on-campus personnel (47%) than among off-campus employee control subjects (28%) (P < .001); the rates were highest in employees who worked on the upper floors of 2 research buildings. The air-handling units on the roofs of these buildings were not designed to exclude Histoplasma spores. The wind-tunnel experiment indicated that spores aerosolized in the bird sanctuary were not taken into campus buildings. Conclusions: The major sources of employee exposure to H. capsulatum spores were the construction sites. Low-level, recurrent exposures occurred over several years inside modern research buildings. This phenomenon, which has not been previously described, may play a role in the epidemiology of spore-transmitted diseases in urban settings.