Date Published: September 13, 2018
Publisher: Public Library of Science
Author(s): George S. Deepe, Donald C Sheppard.
This fungus is considered to be dimorphic; it grows both as a mold and a yeast. The phase depends on temperature. At 25 °C, H. capsulatum exists as a mold and generates two types of conidia or spores-macroconidia and microconidia. These forms are asexual ovoid structures produced at the tips of hyphae. The portal of entry is the lung, and microconidia are considered to be the form that reaches the alveoli and terminal bronchioles of the lung because of their small size (2–5 μm). Exposure of the mold phase to 37 °C induces an orderly change in gene expression, driving conversion of spores into yeast cells that are typically 2–4 μm in diameter . It is this morphotype that causes nearly all the pathology associated with histoplasmosis . Since yeast cells are not transmissible from human to human, they do not promulgate transmission of the fungus in outbreaks.
The habitat of H. capsulatum is soil laden with bird excreta or bat guano. Contact with the fungus usually requires disturbing the soil, thus aerosolizing spores. Droppings from several avian species have been implicated in supporting the growth of the fungus; these include starlings, blackbirds, pigeons, and, less commonly, oilbirds (found in South America) and grackles . It was not until the 1940s that H. capsulatum was recovered from the soil, thus establishing incontrovertibly that infection is acquired from the environment. The fungus was isolated from soil containing chicken excreta surrounding a chicken coop . The organism is usually found within 20 cm of the ground surface. It thrives in acidic soil rich in nitrogen. This compound is more freely accessible in decomposing rather than fresh excreta. In heavily infested soils, the number of H. capsulatum particles has been estimated to reach 105 per gram of soil [10, 11]. Of note, conidia (or spores) have been identified in soil, and this finding documents the presence of this morphotype in nature .
Numerous reports of epidemics or outbreaks of histoplasmosis in the US and Canada have been described over the years, dating back to the late 1930s [14–16]. An outbreak is defined as involving at least two cases. In 1963, 31 cases of histoplasmosis were identified in Montreal along the St. Lawrence river valley. Although no specific source was detected, the authors noted that the city had been in a construction boom during this period. Perhaps the most ironic episode of multiple infections transpired on Earth Day in 1970. A large number of junior high school students and faculty in Delaware, Ohio were infected after clearing a school courtyard that had been a bird roost .
An often neglected but important source of Histoplasma outbreaks is a cave . Bats have been associated with the development of histoplasmosis for many years, and seasoned spelunkers are aware of the risk. Bats harbor the fungus, and it has been isolated from their feces and other tissues. However, the caves do not necessarily have to be in the traditionally endemic regions since cave-associated histoplasmosis has been reported from Florida, South Africa, Tanzania, Cyprus, Australia, and Zimbabwe.
Not all the exposures to H. capsulatum require individuals to be in proximity to areas that are being excavated or where soil is disrupted. One outbreak occurred in a hotel in Acapulco, Mexico in which cleaning of air ducts and the use of stairwells was associated with the spread of spores that caused illness in 21 students who were vacationing over spring break . Another outbreak was attributed to the dissemination of spores via the air handling system at the University of Texas Southwestern Medical School . The likely source was a bird sanctuary near campus. The transmission was noted to be strictly indoors, and cases were more frequent in the upper floors. The air handling system was not constructed to remove small particles such as H. capsulatum spores. Other outbreaks related to air handling systems include one in which room air conditioners spread spores that had been swept off a roof of the building.
Infection with H. capsulatum is not a reportable disease, and therefore, tracking the number of cases is a difficult task. Usually, histoplasmosis comes to our attention when there are outbreaks, whether they involve a few or a few thousand individuals. Once considered a rural disease of the Midwest and Southeast, the pattern of outbreaks has shifted to involve urban areas. Concomitantly, recognition of the disease as one of the Americas is simply no longer true. The geographic extent of the disease has broadened considerably, with an increasing number of reports from Asia. Occupational workers must be provided with the appropriate knowledge regarding the risks when they are tasked with environmental remediation for a site that potentially contains H. capsulatum spores. Unfortunately, no safe soil disinfectant is available that will kill spores; hence, the best protection for workers is the proper protective equipment and wetting of the soil to minimize aerosols.