The Ecological Impact of Fungal Infections on Wildlife Populations

Despite the increasing interest in the impact of infectious diseases on natural populations1xEcology of Infectious Diseases in Natural Populations. Grenfell, B.T. and Dobson, A.P. CrossrefSee all References, 2xHost-Parasite Evolution: General Principles and Avian Models. Clayton, D.H. and Moore, J. See all References, we have little information on the ecological impact of fungal disease agents on wildlife populations. Because of their often facultative nature of infection, fungi are seldom even categorized as (micro)parasites. Besides having ecological and pathological properties observed in many directly transmitted parasites (which are most likely to have detectable effects on the host3xScott, M.E. and Dobson, A. Parasitol. Today. 1989; 5: 176–183Abstract | Full Text PDF | PubMed | Scopus (82)See all References3), fungal pathogens possess intriguing biological characteristics, many of which differ from those of the more typical microparasite groups (viruses, bacteria, protozoa). The analyses of the influence of these characteristics on host individuals and populations could provide new insights to the studies of wildlife diseases.Although most parasitic fungi are considered to be relatively harmless to their natural hosts, fungal disease agents include organisms that can be disseminated, and are pathogenic to their wild hosts4xInfectious Diseases of Wild Mammals. Davis, J.W., Karstad, L.H., and Trainer, D.O. See all References4. More importantly, it is reasonable to assume that fungal infections also affect their host through accumulation of energy costs that are subtle and likely to have a negative effect on the survival of the host (eg. through reduced ability to escape from predators) and to the reproduction of the host, in like manner to the subclinical infections of other microparasites5xFeore, S. et al. Proc. R. Soc. London Ser. B. 1997; 264: 1457–1461Crossref | PubMed | Scopus (66)See all References5. The fact that many opportunistic fungal pathogens are important secondary infections (as clearly demonstrated by fungal infections in AIDS patients) emphasizes their importance in studies of interactions between various disease agents; it also points to the difficulty of documenting the impact of fungal parasites. Furthermore, the fact that many fungi are transmitted by airborne particles that remain localized in the lungs diminishes the probability of finding fungal microorganisms in routine parasitological (blood, fecal) surveys.Fungi comprise a diverse group of organisms. Many fungi can live either as hyphal saprobes in the soil (or vegetation) or as systemic pathogens in the host tissue. Fungi are abundant, often ubiquitous, and the few baseline surveys done show that fungi are common in many wildlife species (eg. see Ref. 6xHubalek, Z., Rosickky, B., and Otcenasek, M. Folia Parasitologica. 1980; 27: 269–279PubMedSee all ReferencesRef. 6). Many dimorphic fungi, however, are typically endemic in a certain geographical area (eg. Cryptococcus neoformans, Histoplasma capsulatum). Since meteorological factors affect viability7xEllis, D.H. and Pfeiffer, T.J. J. Clin. Microbiol. 1990; 28: 1642–1644PubMedSee all References7 and transmission of the environmental forms of fungi (as well as the pathogenicity of fungi8xSystemic Mycoses. Mackinnon, J.E. : 164–178See all References8), the impact of fungi can be expected to vary according to the particular host–parasite relationship and to the geographical area in which they occur.The close association with the soil has important consequences for the transmission of fungal parasites. The habitat choice and behaviour of the host animals determines how intense is the contact between the host and the fungi in the soil. Burrowing animals and those resting in caves (or buildings) are very likely to be exposed to fungal infections9xDenton, J.F. and Di Salvo, A.F. Sabouraubia. 1968; 6: 213–217Crossref | PubMed | Scopus (4)See all References9. Some fungi show close association to certain plants7xEllis, D.H. and Pfeiffer, T.J. J. Clin. Microbiol. 1990; 28: 1642–1644PubMedSee all References7, indicating that not only invading or introduced animals, but also such plants might enchance the transmission and distribution area of fungal parasites.The practise of dust-bathing (common in many birds and mammals) appears to have antiparasitic functions, but it might also increase the transmission of fungal parasite spores from the soil to the lungs of the animals. The high body temperature of birds, on the other hand, might reduce the ability of fungi typical for mammals to infect avian lung tissue.The existence of environmental forms in many fungi affects the transmission of fungal parasites also at the population level. If animals congregate in good habitat patches, transmission of fungal parasites can be efficient regardless of the overall host density because of the environmental form of the fungus capable of initiating the infection. Because fungal infections might spread as airborne spores, many randomly occurring large-scale events (storms, earthquakes) might initiate an epidemic. In such a case, any one fungal species might have a great influence on the genetic composition of the host species through differences in survival between host populations. As a result, the fungal diversity (as parasite diversity in general) found in any particular host population could be transient, and very different from other similar host populations. The fragmentation of habitats that results from human activities is likely to increase further the variation in parasite diversity between populations of any one species.The few population dynamic studies of wild animals involving fungal organisms10xLaakkonen, J. et al. Can. J. Zool. 1995; 73: 961–966CrossrefSee all References, 11xLaakkonen, J. et al. Parasitology. 1999; 118: 1–5Crossref | PubMed | Scopus (21)See all References indicate that, as in other microparasites, the abundance of fungi in host populations varies remarkably, both temporally and spatially. While some species seem to be relatively common all the time, other fungal species are found only occasionally, and are found not to be related to the population density of the host. Unfortunately, we cannot fully estimate the effect of environmental forms on the transmission of the fungi in the above studies since the life cycle of the most abundant fungi found in those studies is not fully understood.