Table 1 The ecology of Batrachochytrium dendrobatidis (Bd) opportunism indicates disease risk factors critical for focused management.

Pathogenicity

A review of the pathogenesis of Bd leads to the conclusion that the fungus is well adapted to the skin of amphibian hosts [228]. However, Bd can also be detected in the water column [218, 229, 230], and on moist substrates [20, 231]. Although saprophytic growth is not strongly indicated, Bd forms biofilms in culture that could, hypothetically, improve survival under harsh or variable environmental conditions. Environmental longevity may entail life-history trade-offs that occur in response to culture conditions of temperature and nutrient availability [160, 161], and specific host adaptations are not unlikely. Bd appears to exhibit chemotaxis toward favorable substrates [232], or away from unfavorable substrates (B. Lam & R.N. Harris, unpublished). Virulence of Bd appears to vary with the strain in laboratory experiments [49, 68, 69], although the determinants of differential virulence are not well understood [228].

Susceptibility

Susceptibility to amphibian chytrid occurs on a spectrum. Bd is an opportunistic pathogen that can be present transiently, cause sublethal host damage, or cause uncontrolled infections leading to death. Developmental stages of amphibians are not equally affected by exposure to Bd, and disease outbreaks of some species are associated with metamorphosis [206, 233, 234]. Figure 2 illustrates several outcomes of exposure to Bd. The contribution of immunopathology to host damage has not been characterized, however, at least in the cases of Bufo bufo and Bufo boreas tadpoles, immunopathology or physiological trade-offs can result from exposure to Bd even without infection [34, 235] (Figure 3). Amphibian susceptibility is extremely sensitive to environmental context.

Environment

Outbreaks of chytridiomycosis are often the result of environmental forcing [36]. For example, although Bd can be widespread across a landscape, mid- to high-elevation populations are often more severely affected by disease than low-elevation populations of the same species. Seasonal disease dynamics are another manifestation of environmental context-dependency, and high infection prevalence is often associated with cool seasons (reviewed in [207]). Climatic variability is also associated with epizootic disease dynamics [67]. A growing number of studies demonstrates that exposure to Bd does not always cause infection, and many species and regions appear to be unaffected by disease [64, 207, 236] (Figure 1). This begs the crucial question for disease management: What factors lead to host protection?