Chytrid: The Killer Fungus

The disappearance of entire populations of amphibians around the world sounds like the script for a science fiction movie. But the frightening fact is that a fungus, Batrachochytrium dendrobatidis, or Bd for short, has swept across every continent where amphibians are found since it was identified in 1999. Also known as frog chytrid (KIH-trid), it was thought to be the only chytrid fungus, among 1,000 species, that infects vertebrates, mainly frogs and toads, until the identification in 2013 of B. salamandrivorans, literally “salamander-eating fungus.” Scientists suspect there may be more as yet unidentified Batrachochytrium species contributing to the die-offs of amphibians globally.

Infection goes skin-deep to kill

Bd settles in the cells of an amphibian’s outer skin layers, making it extremely infectious through contact or exposure in water. While amphibians’ mucous coatings contain powerful antimicrobial compounds, in many cases they’re not enough to fight off the virulent fungus.

High concentrations of Bd lead to chytridiomycosis, a disease that thickens and hardens amphibians’ usually permeable skin. When a frog or toad can no longer absorb water and electrolytes like sodium and potassium through its skin, the resulting chemical imbalance stops its heart. Lungless salamanders that breathe solely through their skin suffocate. Chytrid cannot penetrate amphibian eggs, but it can be present on tadpoles’ mouth parts, leaving the animals open to infection after metamorphosis.

Fast-moving Bd has ravaged amphibian populations and even wiped out entire species, sometimes in a matter of months. It is spreading rapidly throughout the western United States and in Central and South America, imperiling already endangered species. Even infected amphibians that don’t become sick can spread disease from one location to another.

But it was the international trade in amphibians, for food, lab animals, pets, or display animals, that spread Bd around the globe. The IUCN has called it “the worst infectious disease ever recorded among vertebrates in terms of the number of species impacted and its propensity to drive them to extinction.” Because sensitive amphibians are important indicators of the overall health of their ecosystems, the large-scale devastation caused by Bd could foreshadow similar catastrophic problems for other organisms, including humans.

There is hope: Bd can be diagnosed and treated with antifungal medications. This has been successful in projects that rescue animals from wild populations decimated by Bd. It has also been used on aquarium and zoo animals that arrived with the infection. Treating amphibians in their natural habitat, however, remains a challenge. A conservation group is collecting symbiotic bacteria from Bd-resistant species for application with infected wild populations to try to reduce mortalities.

Make a change to help

Avoid buying amphibians from other countries because imported animals can spread Bdfrom one region to another. And never release pet amphibians, which could be carrying chytrid, into the wild.