Amphibians on the brink

On May evenings, the cacophony of spring peepers is less deafening than before. Frogs are in trouble — a lethal fungal disease called chytridiomycosis is threatening little backyard frogs and their counterparts across the globe.

Frogs play an important role in every ecosystem by eating insects and serving as prey for fish, birds, snakes and mammals. Frogs also help scientists study ecosystems, because they are an indicator species. In other words, healthy frogs usually mean a healthy ecosystem.

But many frog species are not healthy. In the last 30 years, chytrid has caused the extinction or catastrophic population decline of over 200 species of frogs. Read on to learn more about this lethal fungus.

What is chytrid, and where does it come from?

Short for chytridiomycosis, chytrid is an infectious disease that affects amphibians, especially frogs and salamanders. Caused by the fungus Batrachochytrium dendrobatidis (Bd), chytrid is found on every continent except Antarctica, and has been linked to massive amphibian declines in Australia, Africa, the Americas and Europe. Currently, the fungus affects more than 350 species of amphibians.

Scientists are not sure where chytrid originated, but there are two basic hypotheses. One is that the fungus originated in a single location and then spread to new host species in other regions. According to this theory, chytrid was likely spread by African clawed frogs, which were shipped around the world in the early 1940s for use in pregnancy testing research. The second hypothesis is that the fungus has existed on multiple continents for a long time. In this case, either the fungus has recently become more lethal, or amphibians have somehow become more susceptible.

Recent research by Rhys Farrer, from Imperial College London, suggests that both hypotheses are correct. His team sequenced the genomes of Bd samples from Europe, North America, South America, Australia, and Africa. Most of the Bd samples were genetically identical, representing a virulent strain of the fungus. By examining mutations in the fungal genome, Farrer could tell that the strain formed within the past 100 years. Farrer thinks that this exceptionally deadly strain formed when modern amphibian trade brought two weaker forms of Bd into contact. The new strain then spread to multiple continents via trade.

What happens to infected frogs? 

Chytrid infects an amphibian’s delicate skin, especially on its belly, pelvis, and feet. Frogs have keratin layers within their skin, the same substance that makes up human hair and fingernails. Chytrid causes these keratin layers to thicken. Many frogs absorb water and oxygen through their skin, and the thicker keratin makes it difficult for them to get the water and oxygen they need, causing death.

Mysteriously, chytrid affects some species more than others. Chytrid causes complete mortality in some species, killing every infected animal. Other species, like the American bullfrog, are able to fight the infection. Many bullfrogs survive, still infected, and spread the fungus to different locations.

How is it spreading, and what is being done to stop it?

Scientists are not sure how chytrid is continuing to spread into new areas. Possible methods of transmission include: wind-borne spores, transmission via water, infected frogs carrying the fungus to a new environment, and human shipments of amphibians. Frogs and salamanders are often transported for use as food, pets, bait and subjects of scientific research. Climate change is causing some amphibian species to expand their range, potentially bringing chytrid fungus into new environments. Recent research predicts that bullfrogs will spread farther into South America as the planet warms, exposing more species to chytrid.

Vance Vredenburg, a biologist at San Francisco State University, is experimenting with using bacteria to fight the fungus. He gave Sierra Nevada yellow-legged frogs baths containing the bacteria Janthinobacterium lividu, which produces a substance that is toxic to the fungus. J. lividium is naturally present on the skin of some amphibian species, and the bacteria kept growing on the frogs after the bath. Vredenburg’s experiment showed that frogs exposed to the bacteria still become infected with chytrid, but with a very low level of infection. Individually bathing wild frogs is not a practical solution to the chytrid epidemic, but Vredenburg’s research could eventually lead to a feasible preventative treatment.

To learn more about chytridiomycosis, visit Amphibiaweb.org. Vredenburg is a key author of the section on chytrid, which offers detailed information on the fungus and a comprehensive review of current research.