Amphibian Apocalypse : or farewell to frogs

Amphibians around the world are experiencing unprecedented population losses, and in some cases, total extinction.  While there are multiple causes of declines many are attributed to Batrachochytrium dendribatidis (Bd).  Bd belongd to the division of fungi called Chytridiomyconda (chytrids).

Chytrids are microscopic fungi mostly found in freshwater and wet soil.  Most are parasites of algae and animals and live on organic debris.  However one species,Bd, has been shown to cause disease in frogs and other amphibians.  The chytrid fungi Bd and Batrachochytruim salamanderivotans (Bsal), unwittingly spread around the world by human activity, has damaged amphibian global diversity more than any other recorded disease.  These fungi have driven the decline of at least 501 amphibian species, of these 90 have become extinct, or presumed extinct in the wild.  Another 124 species have declined by more than 90%.

In the early twentieth century human activity such as trade, war and a global trade in amphibians spread the fungus around the world.  As it travelled it evolved, spawning the highly virulent strain responsible for the pandemic.

Bd degrades the keratin in the animal’s permeable skin which is used to breathe, regulate water levels and the nervous system causing  infections which trigger a death spiral ending in cardiac arrest.  Bd spreads very effectively and can target hundreds os species.  It doesn’t kill it’s victims quickly which increase the odds of the fungus spreading.  In addition Bd can spread by touch and water, it’s spores can swim and, in the right conditions, can live outside it’s host months, maybe even years.

Scientists first started noticing frog die-offs in the late twentieth century but they didn’t realise that they were a global phenomenon until the 1990s.  Karen Lips of the University of Maryland watched the fungus spread over her research site in Panama which lost more than 2/5of it’s amphibian species between 2004 and 2008.  At a site in Australia Ben Scheeb, of the Australian National University, says that he had to watch his step when out at night for fear of stepping on tree frogs.  Now they are nearly impossible to find.  In 2009 Matt Fisher of Imperial College London witnessed an outbreak of Bd amongst midwife toads in the Pyrenees which resulted in them dying by the hundreds on  the shores of the mountain lakes.  He said “early in the outbreak midwife toads would be calling at full volume, a beautiful chorus, since the disease the world has become very quiet”.

More than a decade ago Reid Harris watched a mother salamander as she marched in a figure of eight pattern through her clutch of eggs rubbing her skin against them.  Harris wondered if she might be showing him the solution to the scourge killing thousands of amphibians.  Now at his lab at James Madison University, Harrisonburg he is investigating how to use this episode to design effective anti-Bd probiotics.  Probiotics are beneficial bacteria given to individuals to improve their health.  As all animals host symbiotic microbes, these experiments can be applicable to other systems, including humans.

Researchers have identified thousands of anti-fungal microbes that fight fungi in the lab and hopefully this could lead to real solutions in the wild.  Work is being done in Cairns, Australia, to determine whether acquired immune response in healthy tree frogs with chytrid results in protection against re-infection.  Meanwhile Stephanie Shaw is working at Auckland zoo. In New Zealand, investigating Bd in native frogs.  She plans to re-infect a small group of animals that have recovered from Bd in the wild, to see if they have acquired immunity.

The damage already done to wildlife cannot be reversed, and eliminating Bd from the environment is impossible because topical fungicides cannot be applied on a global scale.  A 2018 study confirmed that all major strains of Bd are present in pet shop animals.  Researchers world-wide are urging governments to curb the trade in wild amphibians, protect habitats and support captive breeding programmes.

Wendy Palen of Simon Fraser University in British Columbia has described Chytridiomyconda as  “The most destructive Pathogen ever described by science.”

Footnote 

For lovely pictures and other stuff visit the non-profit website “Save the Frogs”  www.savethefrogs.com