July 9, 2014

Study finds ‘hot’ frogs fight off fungal pathogen

Simple heat treatments may give the frog immune system a boost and help it fight off a deadly fungal pathogen, according to a new study published July 10 in the journal Nature.

Score one for the frogs. Simple heat treatments may give the frog immune system a boost and help it fight off a deadly fungal pathogen, according to a new study published July 10 in the journal Nature.

The findings suggest that it may be possible to “immunize” frogs in captivity, before sending them back into the wild to face the pathogen again, said Louise Rollins-Smith, Ph.D., associate professor of Pathology, Microbiology and Immunology at Vanderbilt.

Rollins-Smith’s group collaborated with Jason Rohr, Ph.D., at the University of South Florida, who led the studies to demonstrate that frogs can acquire resistance to a deadly fungus.

Louise Rollins-Smith, Ph.D., left, Laura Reinert and colleagues are studying using heat to help boost the immune system of frogs to help ward off a deadly fungal pathogen. (photo by Joe Howell)

The populations of amphibian species including frogs have been declining worldwide for more than 40 years. In the late 1990s, researchers discovered that an ancient fungus, Batrachochytrium dendrobatidis, was causing skin infections, and the fungus is now recognized as a leading contributor to global amphibian declines.

To preserve biodiversity, biologists have collected frog species — with the aim of eventually returning them to their natural habitats — but there is no effective method for eradicating the fungus, or protecting the frogs from it in the wild.

In the current studies, the investigators demonstrate that three different species of frogs can acquire resistance to B. dendrobatidis. The researchers exposed frogs to the fungus for a period of time and then heated the frogs (to 30 degrees Celsius) to clear the infection.

“In the warmer temperatures, everything goes faster in the frog, including the immune system,” said Rollins-Smith, “but the chytrid pathogen hates that warm temperature and dies.”

The researchers exposed groups of frogs to B. dendrobatidis and then cleared the infection using heat, for up to four cycles. They found that a greater number of exposures and clearance cycles reduced the fungus remaining on the skin and improved survival of the frogs.

They also found increased numbers of immune system cells in the spleens and demonstrated in culture that these lymphocytes recognized the pathogen and proliferated in response to it.

Additional studies suggested that exposure to dead B. dendrobatidis is effective in generating an immune response and resistance to the pathogen.

“I think what’s key is giving the frog immune system a little bit of an advantage (with the heat treatment) and giving the fungus a disadvantage,” Rollins-Smith said. “There’s always this kind of war going on, and if the fungus can get well-established in the skin, it can protect itself.”

Rollins-Smith and her team reported in Science last year that the fungus produces a toxic factor (or factors) that disables the frog’s immune response.

“If you clear the infection before it gets established, and give the frog’s immune system a bit more time to become activated, maybe the immune system can win in that setting,” she said.

“There’s a hopeful note here, that we may be able to help frogs recognize and fight off this pathogen.”

Vanderbilt contributors to the study included J. Scott Fites, Ph.D., and Laura Reinert. The research was supported by grants from the National Science Foundation, the National Institutes of Health (GM109499), the U.S. Department of Agriculture and the U.S. Environmental Protection Agency.