The ecology of the Zygomycetes and its impact on environmental exposure

Abstract

Zygomycetes are unique among filamentous fungi in their great ability to infect a broader, more heterogeneous population of human hosts than other opportunistic moulds. Various members of the Zygomycetes have been implicated in zygomycosis, although those belonging to the family Mucoraceae are isolated more frequently than those of any other family. The environmental microbiology literature provides limited insights into how common zygomycetes are in the environment, and provides a few clues about which ecological niches these fungi are found in. Mucorales are thermotolerant moulds that are supposedly ubiquitous in nature and widely found on organic substrates, including bread, decaying fruits, vegetable matter, crop debris, soil between growing seasons, compost piles, and animal excreta. The scientific and medical literature does not support this generalization. Sporangiospores released by mucorales range from 3 to 11 microm in diameter, are easily aerosolized, and are readily dispersed throughout the environment. This is the major mode of transmission. However, there are very few data concerning the levels of zygomycete sporangiospores in outdoor and indoor air, especially in geographical areas where zygomycosis is particularly prevalent. Airborne fungal spores are almost ubiquitous and can be found on all human surfaces in contact with air, especially on the upper and lower airway mucosa. Inhalation of sporangiospores must be a daily occurrence. Surprisingly, members of the Mucorales are very rarely found in nasal mucus, suggesting that spores in the mucus of airway mucosa are cleared by mucociliary transport or that there is a low level of airborne contamination. Zygomycetes are found occasionally in water-damaged buildings, as demonstrated by air-sampling, and analysis of settled dust by quantitative PCR. Moreover, inhalation of sporangiospores in dust has been linked to outbreaks of rhinocerebral or pulmonary zygomycosis due to excavation, construction, or contaminated air-conditioning filters. Whereas most zygomycete infections are community-acquired, nosocomial acquisition due to percutaneous routes of exposure is very important. Sporadic cases, and pseudo-outbreaks, have been linked to contaminated bandages and adhesive dressings, needles, and tongue depressors used to construct splints for intravenous and arterial cannulation sites in preterm infants. Insect bites or stings have been implicated in disease transmission in cases of cutaneous and subcutaneous zygomycosis, e.g. diseases caused by the Entomophthorales. Traumatic implantation of spores in dirt or in contaminated water, e.g. as occurred during the Asian tsunami, has led to infection in multiple patients.