It’s not easy being green. Frogs, salamanders, and other amphibian species face extinction, due in part to a mysterious parasitic fungus. Now, two studies shed light on its spread and toll.
The global bullfrog trade, which sends millions of frogs each year to the United States, mostly for their meaty legs, may fuel the parasite’s spread and create perfect conditions for a superbug, according to a paper coming out in the next print issue of Molecular Ecology. And a second study, published this week (August 12) in Nature Climate Change, suggests that the increased temperature variability caused by climate change may worsen the parasite's impact.
“For whatever reason, there’s a greater movement of the fungus than in the past,” said Vanderbilt University School of Medicine microbiologist and immunologist Louise-Rollins Smith, who was not involved in either study. “It’s going into areas where it’s never been before, where there are fewer natural defenses against it.”
The population-decimating fungus, Batrachochytrium dendrobatidis (Bd, or chytrid) often enters hosts as they rest their bellies or feet in shallow pools of water or watery soil that contain the fungal spores, Rollins-Smith said. A 2009 study in Science showed that once it gains a foothold, Bd thickens the skin through which amphibians drink and breathe, preventing osmosis of ions like potassium and eventually causing cardiac arrest.
But many factors affecting disease transmission and susceptibility are not clearly understood, making it difficult to save amphibians from the disease. Since 1980, 122 species have gone extinct, and 500 more are currently on the brink.
In the forthcoming Molecular Ecology study, ecologist Lisa Schloegel and her colleagues traced Bd’s spread by sequencing parasite DNA from live bullfrogs purchased at Asian markets in the United States. The team then compared those sequences to strains from bullfrog farms in Brazil, as well as from wild frog populations in South America’s Atlantic Forest.
The researchers found that bullfrogs from Michigan markets harbored a fungal strain that originated in the Atlantic Forest, and with data from a prior study, showed that a Brazilian strain of Bd had made its way to Japan, suggesting that global trade was helping spread the parasite throughout the world. The researchers also detected a hybrid strain that likely emerged through sexual reproduction of the parasite—something that had never been shown before.
The team hypothesized that the deadly global “superbug” is one that emerged through such recombination of two benign strains as a result of global trade, said study co-author Timothy James, a University of Michigan mycologist. “It’s a perfect environment,” he said. “These animals are under stress and have lowered immune systems. You have these animals sourced from different locations put into the same container, and you can have this hybridization event.”
Current methods to save amphibians—through antifungal treatments or environmental cleaning—are too laborious or ineffective for more than a small population of amphibians.
“Once it’s in an ecosystem, we don’t have any reasonable way at this point to mitigate the spread of it,” said James Collins, an ecologist and evolutionary biologist at Arizona State University who was not involved in the research. But limiting live trade may protect naive populations from exposure, he added.
In the second study, disease ecologist Thomas Raffel of Oakland University in Rochester, Michigan, and his colleagues investigated how climate change, which causes larger, more unpredictable temperature swings, affects the fungus. They exposed one group of Cuban tree frogs, which are highly susceptible to the fungus, consistently to a day temperature of 25 degrees Celsius and 15 degrees at night. Another group had the same mean, minimum, and maximum temperatures, but experienced random temperature shifts, sometimes having a warm night with a cool day, for instance, a few hour cold snap, or a uniform temperature for 24 hours. Frogs exposed to the unpredictable temperature regime grew fungus more quickly and died more often, suggesting that more volatile weather caused by global climate change will make amphibians even more susceptible to Bd.
“Parasites are smaller than their hosts and are likely to acclimate more rapidly than their hosts,” Raffel said. This gives the parasites a small window of time where they have the upper hand.
Still, trade and climate change are only one part of the puzzle, Collins said. Toxins, invasive species and habitat loss in particular may also stress the animals, in turn making them more susceptible to the Bd fungus and worsening amphibian decline, he said. “Anything we can do to get that under control is going to be the best way to mitigate the loss of species,” Collins said.
T. Raffel et al., “Disease and thermal acclimation in a more variable and unpredictable climate,” Nature Climate Change, doi:10.1038/nclimate1659, 2012.
L. Shloegel et al., “Novel, panzootic and hybrid genotypes of amphibian chytridiomycosis associated with the bullfrog trade,” Molecular Ecology, doi: 10.1111/j.1365-294X.2012.05710.x, 2012.