Frog-Killing Fungus Paralyzes Vulnerable Amphibian Immune Response

Amphibians are declining across the globe. Especially vulnerable to environmental shifts, this group of animals is suffering quite a bit from climate change and habitat degradation. Now, scientists have found something else that may be impacting amphibians. There's a fungus that's killing them that releases a toxic factor that disables the amphibian immune response.

Amphibians play an important role in ecosystems. They act as efficient predators of insects and provide valuable nutrients to creatures further up the food chain. In addition, some play an important role in nutrient cycling. Yet populations of these species have been declining for more than 40 years. In fact, scientists have discovered that on average, populations of amphibians vanish from habitats at a rate of about 3.7 percent each year. If this rate were to continue, these species would disappear from half of the habitats they currently occupy in about 20 years.

Contributing to this decline is a particular fungus called Batrachochytrium dendrobatidis. This fungus can cause skin infections that eventually kill amphibians. Yet because amphibians have excellent and complex immune systems, researchers have often wondered why they've been so drastically impacted by this fungus.

So what happens exactly? Some frogs produce anti-microbial peptides in the skin that offer a first layer of defense against the fungus. When the fungus gets into the layers of the skin, though, the conventional lymphocyte-mediated immune response should be activated to clear it. In fact, scientists have noted that recognition of the fungus by macrophage and neutrophil cells was not impaired. But during the next stage of the immune response, when lymphocytes should be activated, the fungus exerts a toxic effect.

"Fungal infection causes rapid behavioral changes--frogs become lethargic and start to crawl out of the water--suggesting that even though the fungus stays in the skin, the toxic materials is having effects elsewhere," said Louise Rollins-Smith, one of the researchers, in a news release.

So what exactly does this toxic effect do? It impairs lymphocyte proliferation and induces cell death of lymphocytes from frogs, mice and even humans. In fact, the toxic fungal factor inhibits the growth of cancerous mammalian cell lines, as well. It's also resistant to heat and proteases, suggesting that it's not a protein. Instead, it appears to be a component of the cell wall.

The findings reveal a little bit more about this deadly fungus. This, in turn, could allow scientists to take conservation measures for species in the wild.

The findings are published in the journal Science.