Non-transmissible Fungal Infections
Non-transmissible fungal infections are those for which the host is a dead end, and no further onward transmission occurs; this category includes the largest grouping of pathogenic fungi as it includes all those that are opportunistic infections and infect immunocompromised hosts.
Non-transmissible infections are also referred to as accidental parasites or accidental pathogens to describe free-living organisms that can multiply in a suitable host, but do not derive obvious evolutionary benefits from the association.Reservoir hosts are biological hosts that a pathogenic fungus can complete its life cycle in, but are not the primary focus of concern. For example, small mammals are known to be the reservoirs for human mycoses, such as bamboo rats as reservoir hosts for human Talaromyces marneffei infection (Cao et al. 2011; Henk et al. 2012) and kangaroo rats as reservoirs of infection for Coccidioides sp. and P. dipodomyis (Pappagianis 1988; Henk and Fisher 2011). Environmental reservoirs are substrates that support the life cycle of a pathogenic fungus outside of susceptible hosts and can act as the source of inocula for susceptible animal hosts. For example, composting sites are an environmental reservoir for very high burdens of Aspergillus fumigatus conidia which can lead to human disease if exposure occurs.
Sapronoses is a recently introduced term for the class of pathogens that grow and complete their life cycle as free-living stages in the environment; however, it can use within-host growth as a replication strategy. Sapronoses, also known as environmentally growing opportunists, comprise a large number of serious fungal infections that affect humans and other animals, for instance, species of Aspergillus, Cryptococcus, Coccidioides and Histoplasma are all examples of fungal sapronoses. Sapronoses are generally considered as non-transmissible fungal infections; however, this may not be strictly accurate as there is some evidence to suggest that parasitism of small mammals by Coccidioides and T. maιrneffei can result in ongoing transmission following death of the host (Fisher et al.
2001; Henk et al. 2012). The lack of host-to-host transmission causes the epidemiology of sapronoses to manifest dynamics that differ substantially from those that characterise transmissible fungal pathogens. This is because factors that are extrinsic to the host, such as ecological interactions with other free-living organisms leading to resource competition, predation and parasitism, alongside environmental variation in temperature and water availability, will all impact on transmission to susceptible hosts.‘Fungi are the only group of organisms that have been convincingly shown to cause extinction'. This remark by Arturo Casadevall (Olsen et al. 2011) is sadly true and relates to an unfortunately rather common combination of traits that characterise fungal infections. While traditional epidemiological theory assumes trade-offs between virulence and transmission owing to the optimisation of the numbers of secondary infections that are caused by a primary infection (R0 > 1), free-living fungi are largely freed from this constraint owing to their ability to replicate outside of the host. When sapronotic life-styles are combined with high virulence, long- lived environmental stages and generalist host ranges, then high lethality can occur leading to local extirpations or complete host extinctions (Fisher et al. 2012). Such population processes have been observed to have caused the widespread extinction of at least 200 species of amphibian by B. dendrobatidis (Fisher et al. 2009; Alroy 2015) (Chap. 14).