A Global Amphibian Crisis
Of all the threats facing amphibians today, one stands out as particularly devastating: chytridiomycosis, a disease caused by the aquatic fungus Batrachochytrium dendrobatidis (commonly abbreviated as Bd). Since its discovery in the late 1990s, this pathogen has been linked to the decline or extinction of more amphibian species than any other single cause in recorded history — making it one of the most destructive infectious diseases ever documented among vertebrates.
How Does Chytrid Kill Frogs?
The chytrid fungus infects the keratin-rich outer layers of a frog's skin. In adult frogs, this disrupts the skin's vital function of regulating water and electrolyte absorption. Infected frogs essentially experience a form of heart failure driven by electrolyte imbalance, losing the ability to maintain proper salt and water levels in their bodies. Symptoms include:
- Lethargy and unusual daytime exposure
- Reddening or peeling of the skin
- Loss of righting reflex (inability to flip back when turned over)
- Abnormal posture and reduced responsiveness
- Rapid death in highly susceptible species
How Does the Disease Spread?
Bd spreads through contact with contaminated water, soil, and other infected animals. The global wildlife trade has played a significant role in dispersing the pathogen across continents. The American Bullfrog and African Clawed Frog — both widely traded and highly resistant to Bd — have acted as carriers, introducing the fungus to naive amphibian populations that had no evolutionary immunity.
Which Species Are Most at Risk?
Susceptibility varies enormously between species. Some, like the American Bullfrog, can carry the fungus with little apparent harm. Others — particularly island endemics and high-altitude species — have suffered catastrophic losses. Several Central American frog species, including the iconic Golden Toad (Bufo periglenes), disappeared entirely in the late 20th century, with Bd implicated as a major driver.
Conservation Responses
The conservation community has responded to chytridiomycosis with a range of strategies:
- Captive assurance colonies: Zoos and amphibian conservation centres house threatened species in sterile, disease-free environments to preserve genetic diversity
- Probiotic research: Scientists are exploring beneficial skin bacteria that may help wild frogs resist Bd infection
- Biosecurity protocols: Field researchers now follow strict decontamination procedures to avoid carrying the fungus between sites
- Trade regulation: Calls to improve screening and regulation of the global amphibian trade to reduce spread
- Habitat protection: Maintaining healthy, intact habitats where frogs can develop natural resistance and recover populations
A Second Threat: Bsal
A closely related fungus, Batrachochytrium salamandrivorans (Bsal), poses a similar threat specifically to salamanders and newts. First identified in Europe, it has already caused dramatic declines in some European salamander populations and is considered a serious emerging threat should it reach North America or Asia.
What You Can Do
Individual actions matter in the fight against chytrid disease. Never release captive amphibians into the wild. Always clean boots, equipment, and containers when moving between water bodies. Support organisations working on amphibian conservation and disease research. Awareness is the first step toward meaningful action.