Chytrid Fungus, Batrachochytrium dendrobatidis
The Situation: Upwards of 40% of amphibian species are in decline worldwide, owing to several factors such as habitat loss, environmental degradation, pollutants, and disease. Recently the fungal pathogen Batrachochytrium dendrobatidis has emerged as a major threat to amphibians. Amphibians infected with B. dendrobatidis develop chytridiomycosis, which eventually causes death in susceptible species. The first documented outbreaks of chytrid fungus occurred in the late 1990s simultaneously in Australia and Central America. Since then the pathogen has been detected in over 100 amphibian species and has been associated with severe population declines or extinctions in several regions throughout the world. A great deal is still unknown about the biology of this pathogen, therefore it remains an active area of research for disease ecologists and conservation biologists.
Damage: B. dendrobatidis is an external pathogen that attaches to keratinized portions of amphibians, including the mouthparts of tadpoles and the skin of adults. The fungus reproduces via sporangia, and may be spread by movement of flagellated zoospores, direct contact between hosts, or between host stages. Growth of the fungus leads to degradation of the keratin layer, which eventually causes sloughing of skin, lethargy, weight loss, and potentially death. The physiological mechanism for chytrid-induced mortality is not known, but it appears to stem from disruption of skin function - such as fluid transport or gas exchange.
The chytrid fungus is known to infect over 100 species, but susceptibility to disease is highly life stage and species specific. For example, in mountain yellow legged frog (Rana muscosa) tadpoles suffer generally mild sublethal effects, with most mortality occurring at metamorphosis when there is a rapid production of newly keratinized skin tissue. Conversely, several other amphibian species appear to be relatively tolerant to B. dendrobatidis - including some widespread exotic or invasive species, such as the marine toad (Bufo marinus), American bullfrog (Rana catesbeiana), and African clawed frog (Xenopus laevis)).
At the population level, chytrid fungus outbreaks have been associated with local and possible species extinctions in Australia, Central America, and the United States. For example, in 2004 chytrid fungus prevalence in parts of Panama increased from 0 to nearly 60% over approximately 4 months, with concomitant declines in amphibian density and diversity of over 80% and 60%, respectively. B. dendrobatidis is thought to thrive in cool, moist habitats. This has been used to argue that cooling trends observed in parts of Central America are driving chytrid-induced amphibian extinctions in these regions.
Distribution: One explanation for the recent emergence of chytridiomycosis in amphibians, the "novel pathogen hypothesis", is that B. dendrobatidis existed historically as a locally distributed pathogen that only recently was spread to new regions. Alternatively, the "endemic pathogen hypothesis" posits that the chytrid fungus was historically widespread but that recent environmental change (e.g., climate change, pollutants, habitat degradation) altered its pathogenicity. The relative importance of these two mechanisms is currently a source of debate. Low genetic diversity among geographically distant B. dendrobatidis strains is consistent with the first hypothesis, but synchronicity of chytrid fungus outbreaks in disparate, intact habitats supports the latter hypothesis.
The first described outbreaks of chytrid fungus occurred in 1998 in both Australia and Central America. Since then B. dendrobatidisinfections have been documented throughout the Americas, including Mexico and the U.S., Europe, and most recently in Southeast Asia.
The oldest known chytrid fungus infections are from museum specimens of African clawed frogs (Xenopus laevis) collected in 1938. These specimens have been used to argue for an African origin for B. dendrobatidis. It is believed that the chytrid was then spread to other continents in the 1960s and 70s through commercial trade of these African frogs.
Research: The link between chytridiomycosis and amphibian decline is an active area of research worldwide. The genome of B. dendrobatidis has been sequenced, which should prove useful for identifying the origin, mechanisms of virulence, and potential control methods for this pathogen. University of California researchers have been studying this pathogen for several years, especially the impacts of chytrid fungus on populations of the mountain yellow legged frog (Rana muscosa) in the Sierra Nevada Mountains in California. This once abundant alpine frog has undergone severe declines in recent years, with numerous local die-offs. Research is being conducted on the spatial epidemiology of disease in R. muscosa, to understand why some local populations persist whereas others go extinct. Projects include identifying the modes of pathogen spread, impacts of outbreaks on alpine food webs, and the population genetic consequences of outbreaks for frogs. With regard to frog population and disease management, experiments include evaluating the efficacy of anti-fungal treatments and the feasibility of reintroducing frogs into previous outbreak areas.