Amphibian decline: Frogs fight back!
In the worldwide extinction crisis, the most depressing stories concern amphibians — four-legged animals like frogs and salamanders that undergo metamorphosis to reach their adult form. The International Union for the Conservation of Nature says about 30 percent of the 6,285 amphibian species are threatened with extinction.
Climate change — especially warming and drying in tropical highlands — gets some blame for the crisis, but a fungus called “chytrid” gets more. Chytrid attacks the skin, preventing amphibians from absorbing water and controlling electrolyte levels, which eventually stops the heart.
Last year, scientists found that it suppresses the amphibian immune system, helping account for its lethality.
The disaster is so quick and so global that biologists are capturing frogs and sending them to zoos for breeding, hoping to reintroduce them to the wild if and when a solution to the chytrid epidemic is found.
Light at the end of the tunnel
Now we hear a mote of good news concerning the chytrid fungus Batrachochytrium dendrobatidis (bd). Since it was identified in 1998, bd has seemed unstoppable, although not uniformly fatal. “There is pretty clear evidence in certain locations for certain frog species, that exposure to the fungus can cause extirpation or extinction, but not in others,” says Jason Rohr, an associate professor of biology at the University of South Florida.
Extirpation is the eradication of a local population. If that is the only population, extirpation equals extinction.
Rohr, first author of a new study on acquired immunity to bd in frogs, says, “The fungus is implicated in a couple of hundred amphibian extinctions over four decades, but we have also come to realize that in many cases, it does not cause extirpation.”
To check if some frogs could be mounting a defense against chytrids, Rohr and colleagues performed lab tests to see if frogs could acquire resistance, defined as a reduction in “infection loads upon subsequent exposure to a pathogen or antigen caused by changes in behavior, innate immunity or adaptive immunity within the lifetime of an individual.”
To expose frogs to bd without killing them, Rohr’s group sprayed them with zoospores, the infectious phase of the fungus, then placed the animals in a warm (30°C) enclosure for 11 days, long enough to kill the fungus, rendering the animals healthy — but still exposed to the deadly chytrid fungus.
Staying out of trouble
Aware that some animals can detect and avoid certain pathogens, Rohr’s group divided a container in two, placed fungal spores in one half, and saw that previous exposure to bd affected where the frogs chose to be. While unexposed (“naïve”) frogs randomly chose either side, frogs exposed once to chytrid spent more than 60 percent of their time in the bd-free section. After a second exposure, they spent 70 percent of their time in the healthy zone.
“We knew that amphibians avoided candida, another fungus,” says Rohr, “and we were interested in evaluating whether there was any innate or learned avoidance. We saw no evidence of innate avoidance, but after one exposure it was clear that there was a capacity to learn to avoid chytrid.”
Although it seems surprising that amphibians can learn, “there is a lot of evidence that they can, and it’s probably critical for survival, as it is for birds that cache seeds,” Rohr says. He speculates that the frogs may detect bd through skin irritation: The fungus makes enzymes that dissolve keratin, a skin protein in amphibs.
Immune system fights back!
Three lines of evidence showed that the frogs were mounting an immune response to bd:
After three cycles of exposure (followed by heat-treatment to clear the infection), the number of fungal spores surviving on the frogs’ skin was reduced by 75 percent compared to naïve frogs
The number of lymphocytes (white blood cells that carry “immune memory”), rose after each exposure to the pathogen
Survival improved from 20 percent for naïve frogs to 60 percent after the fourth exposure. “If we added in behavioral avoidance, that could increase survival even further,” Rohr says.
It’s not clear how long the immunity lasts, Rohr admits. “If it’s short, it may not be very effective at the population level. But we showed, despite the immunosuppression from bd, that the immune reaction reduced the number of chytrid spores on frogs and improved survival.”
The new study suggests that inducing acquired resistance in captive-bred animals could protect them while being reintroduced into the wild. Because dead fungal spores also evoked immunity, it’s conceivable that vulnerable amphibians could be dusted with dead spores as a vaccine against the deadly chytrid fungus.
– David J. Tenenbaum
Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer
- Amphibians acquire resistance to live and dead fungus overcoming fungal immunosuppression, Taegan A. McMahon et al, Nature, July 11, 2014. ↩
- Amphibian Ark ↩
- Is the Frog-Killing Chytrid Fungus Fueled by Climate Fluctuations? ↩
- Asian frogs becoming extinct before they can be identified, biologists warn ↩
- Study: Salamanders in the Appalachians are smaller ↩