Research Highlight: Curing Disease with a Nice Hot Bath

Some like it hot, some don’t. A new study led by a graduate student researcher at Scripps Institution of Oceanography at UC San Diego has provided evidence of a way of fighting an infectious fungus by turning up the heat.

Amphibians are the most endangered group of animals on the planet and a prime reason for their alarming population declines and species extinctions is Batrachochytrium dendrobatidis (Bd), which causes a potentially lethal disease called chytridiomycosis. The spread of Bd is known to decimate entire amphibian communities and ecosystems, leading to the pathogen’s reputation as one of the most deadly invasive species on the planet.

Matthew Forrest of Scripps Oceanography stumbled into this research during a class field trip led by Scripps Professor Paul Dayton to the Lower San Pedro River in Arizona, where Forrest met Martin Schlaepfer of the State University of New York, who was surveying populations of local amphibians.

During a handful of field expeditions stretching over the next several years, Forrest and Schlaepfer studied several amphibian species—mostly lowland leopard frogs—at 12 sites in Arizona.

Forrest and Schlaepfer analyzed animals captured in a variety of “microhabitats,” including areas in warm geothermal hot springs, and collected hundreds of skin samples for laboratory analysis by gently swabbing the undersides of the frogs. They evaluated the prevalence of the chytrid fungus across different temperature settings and found that Bd, which is known to thrive under cooler conditions, didn’t fare well in hot environments.

“We provide clear evidence that the prevalence of Bd decreases significantly when amphibians have access to higher water temperatures,” said Forrest, a Ph.D. student in the Scripps Center for Marine Biodiversity and Conservation (CMBC). “Our results have important conservation implications because they highlight how water temperature can modulate the presence of the amphibian chytrid fungus in the wild, and indicate that geothermal ecosystems may provide some protection from this potentially fatal pathogen.”

The findings are described in the Dec. 21 issue of the online journal PLoS ONE.

While research on frogs in warm environments isn’t a traditional track of study at an oceanographic and earth science institution, Forrest’s investigation continues Scripps’ legacy of “out of the box” research. Forrest came to Scripps with a strong background in ecological studies in marine geothermal systems, working at a shallow-water hydrothermal vent in Baja California for his master’s degree. While CMBC research primarily covers marine issues, the center’s core mission is to understand, protect, manage, and restore biodiversity around the planet. Forrest’s Ph.D. work at Scripps centers on desert springs and Scripps Professor Richard Norris, his academic advisor, encouraged him to study the physical and geochemical factors that make thermal springs the last refuge for many species of amphibians, fish, and other organisms.

As frogs and other amphibian species decline, Forrest believes hot spring watersheds may become exceptionally valuable for conservation efforts because their elevated water temperatures can provide refuge from the deadly fungus. Forrest says Bd appears to have been recently introduced to new locations such as Arizona, although there is genetic and historical evidence that it has been present for a long time in Africa, Japan, and eastern North America. Although Bd’s origin is not known, it is clear that global trade in amphibians for food, laboratory research, and pets is responsible for the fungus’ presence in new locations.

“Emphasizing protection and restoration efforts, as well as native species translocations into suitable geothermal ecosystems could help recover some threatened and endangered species,” said Forrest.

Forrest added that experimentally increasing temperatures at locations that do not feature hot springs could offer infected animals a way to clear themselves of the pathogen in the wild, as opposed to current treatments performed in captivity with antifungal drugs or heat. Possible links between the disease spread and climate change are unclear, he said.

“This paper shows that there is some cause for hope despite the devastating loss of amphibians from chytrid infections,” said Norris, a paleobiologist. “It certainly looks like some of the last holdouts for certain native frog populations are in hot springs, so these environments achieve added value for conservation. Clearly we are not the only ones to like (and need) a hot bath.”

Supporters of the research include The Nature Conservancy, Jef Jaeger of UNLV, NSF’s Integrative Graduate Education and Research Training program at Scripps, and the Halliday Fund.

– Mario C. Aguilera

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