Very few of the common indoor moulds are likely to infect healthy individuals.
However, professionals involved in indoor air quality including remediation and demolition workers are constantly at risk of fungal infections by opportunistic or true pathogenic fungi.
The two most important pathogenic fungi that would be of concern are Histoplasma capsulatum and Cryptococcus neoformans. Indoor air quality professionals may also be at risk of infection by opportunistic fungal pathogens such as Aspergillus fumigatus.
Histoplasma capsulatum is a dimorphic fungus (i.e., has both yeast and mycelial growth phases) found throughout the world in substrates rich in nitrates such as bird droppings, chicken manure and bat guano. It is also common in soils enriched by bird droppings. It causes a disease called histoplasmosis. Humans are infected by inhalation of spores in air or carried in fine dust.
It is estimated that 95% of infections may produce mild-to-no symptoms, depending on the infected person’s resistance to the fungus. However, chronic infection may also occur resulting in pneumonia, blindness, and even death.
Histoplasma capsulatum usually thrives only at bird roosts that have existed for 3 years or more. Spore formation is poor under the acidic conditions of fresh droppings and therefore an active bird roost may only produce a few spores. When the droppings have dried, massive amounts of spores can be released, especially if the droppings are disturbed. Once established, H. capsulatum will survive in the soil under abandoned roosts for many years.
Cryptococcus neoformans causes a disease called cryptococcosis. The fungus may be present in pigeon or bat droppings. Infection of humans occurs through inhalation of the yeast-like vegetative spores of the fungus. These spores measure 1-3 µm in diameter and are readily airborne. The spores may also be present in organic dust contaminated with pigeon or bat droppings. Up to 50-million colony forming units (CFU) of Cryptococcus neoformans per gram of pigeon droppings have been recorded.
Many people get exposed to the fungus but do not develop disease symptoms. Of those who develop the disease, symptoms of lung infection may or may not be present and the infection may spread to the central nervous system, resulting in cryptococcal meningitis. Cryptococcal meningitis is difficult to diagnose and fatal if not properly and promptly treated.
A number of Aspergillus species are opportunistic pathogens. However, Aspergillus fumigatus, a thermophilic fungus found almost everywhere, causes 90% of infections. It is found in air, soil and decaying organic debris. It is particularly abundant in compost facilities and is the most important cause of all forms of invasive and non-invasive aspergillosis.
In indoor environments, Aspergillus fumigatus has been isolated from kitchens, bathrooms, basements, and flower pot soil. It thrives on damp materials with water activity values ranging from 0.90 to 0.95.
Infection by A. fumigatus occurs primarily through inhalation of airborne spores. Due to their small size (2 to 3 µm in diameter), spores of A. fumigatus are inhaled deep into the lung alveoli. Dissemination of spores into the air depends on disturbances of the environment and strong air currents. Again due to their small size the spores tend to remain airborne both indoors and outdoors.
Therefore, all humans inhale at least several hundred spores of A. fumigatus spores per day. However, in healthy individuals the spores are normally eliminated by the immune mechanisms, and only occasionally do aspergilloma ("fungus ball") and allergic bronchopulmonary aspergillosis infections occur.
Fungal infections are rare, particularly in healthy individuals. However, professionals involved in mould investigation, mould abatement and demolition or renovation buildings are at risk of infections. When they occur, fungal infections are generally difficult to treat, especially if not diagnosed in time. Proper protection is extremely important when working in environments likely to be contaminated with pathogenic fungi.
Karen H. Bartlett, Laura MacDougall, Sunny Mak, Colleen Duncan, Sarah Kidd, and Murray Fyfe (2004). Cryptococcus gattii, a tropical pathogen emerging in a temperate climate zone. Presented at the 16th Biometeorology and Aerobiology. The American Meteorological Society.
Hoog, G.S. de, J. Guarro, J. Gené, M. J. Figueras (2000). Atlas Of Clinical Fungi, 2nd. Edition, 2000, Centraalbureau von Schimmelculture / Universitat Rovira i Vigili. ISBN 90-70351-43-9.
Gravesen S, Nielsen PA, Iversen R, Nielsen KF. Microfungal contamination of damp buildings – examples of risk constructions and risk materials. Environmental Health Perspectives 1999;107:505-8.
Latge, J. P. (1999). Aspergillus fumigatus and aspergillosis. Clin. Microbiol. Rev. 12, 310-350.
Summerbell RC, Staib F, Dales R, Nolard N, Kane J, Zwanenburg H, Burnett R, Krajden S, Fung D, Leong D. Ecology of fungi in human dwellings. Journal of Medical and Veterinary Mycology 1992;Suppl.1:279-85.
U. S. Army Environmental Hygiene Agency (1992). Managing health hazards associated with bird and bat excrement. USAEHA Technical Guide NO. 142.
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