In the late 20th century, in the Australian state of Queensland, something extraordinary began to happen among the region’s wildlife. Little by little, in upland sites and tropical forests, frog populations began to disappear.

Although extensive monitoring of these population crashes by biologists, the cause remained elusive for many years. That was until 1993, when a scientific mission to Big Tableland, near Cooktown, identified yet more dead and dying frogs strewn along the edges of tropical creeks.

One of the amphibians observed was the sharp-snouted day frog - a species endemic to the region. A group of these frogs were collected and taken to James Cook University for further study and analysis. Just three months later, the wild population of this species disappeared entirely. Although this was yet another tragedy for amphibian conservation, analysis of the collected individuals eventually identified the cause of their ailments - a tiny fungus found growing on the skin.

Humans have been a critical player in the spread of the fungus, mainly via the global amphibian trade.

An Amphibian Pandemic

The fungus in question is known scientifically as Batrachochytrium dendrobatidis (Bd) – often referred to as chytrid fungus. Since its discovery just before the turn of the century, it has spread like wildfire through amphibian populations across the globe and has been implicated in the decline of more than 500 species, of which 100 have been classified as extinct. As a result, it is currently considered the most significant disease threat to global animal diversity.

Bd exists in the environment as a waterborne pathogen, thought to spread by dispersing zoospores. Once these spores reach an amphibian host, they enter via the skin and cause havoc with the body, disrupting ion channels and causing rapid skin keratinisation. The skin is a crucial organ for amphibians, as it helps them absorb oxygen from the water and lead the semi-aquatic lifestyles they are known for. Any disruption to the skin can, therefore, be catastrophic.

Bd has already been identified in more than 50 countries and is a severe problem in the Americas. For example, upland regions in Central and South America saw more than 40% of their amphibian populations collapse in the early 21st century. This followed previous extinctions such as that of the golden toad - a species that has since become something of a poster child for the global amphibian plight. Golden toads were once common to one area of pristine cloud forest in central Costa Rica before the population crashed in the 1980s. The last ever individual was seen in 1989, with the absence of pollution and human interference in the region making chytrid fungus the prime suspect.

Humans to Blame?

There is some debate over whether Bd has always occurred in the areas where it now causes problems or whether it has been introduced through external means. Worryingly, there is evidence to suggest that humans have been a critical player in the spread of the fungus, mainly via the global amphibian trade.

For example, the African clawed frog has been - and continues to be - used extensively in scientific research, especially in early forms of pregnancy testing. This particular species is a rare example of an amphibian that carries chytrid fungus without any adverse effects. This, therefore, makes it an essential vector in the fungi’s transmission. The use of African clawed frogs in laboratories worldwide may have helped spread the fungal disease far more rapidly than it would have done under normal circumstances.

More recently, trading of another Bd carrier - the American bullfrog - may have had a similar effect. American bullfrogs are widely used both as dissection specimens and for food, but their invasive nature has made them a conservationist’s nightmare. Introduced populations are now established widely across parts of Europe, Asia and South America, and it is feared that the species may be spreading chytrid fungus to native frogs. Besides, many amphibian species are transported worldwide for use in the pet trade, with the United States alone importing more than 700,000 amphibians every year. Each one has the potential to spread Bd, especially if they escape into the environment. 

As amphibian populations continue to decline and disappear, the potential for knock-on effects within ecosystems is severe. And the recent discovery of a second chytrid fungus, Batrachochytrium salamandrivorans, affecting newts and salamanders, only adds to the challenges facing this group of animals. With no clear solution available and a lack of international efforts to reduce the trade-in amphibians, the future for many species hangs in the balance.

References

Kilpatrick, A.M., Briggs, C.J. and Daszak, P., 2010. The ecology and impact of chytridiomycosis: an emerging disease of amphibians. Trends in ecology & evolution, 25(2), pp.109-118.

Skerratt, L.F., Berger, L., Speare, R., Cashins, S., McDonald, K.R., Phillott, A.D., Hines, H.B. and Kenyon, N., 2007. Spread of chytridiomycosis has caused the rapid global decline and extinction of frogs. EcoHealth, 4(2), pp.125-134.

Further readings

https://www.theguardian.com/environment/2019/apr/20/battle-to-save-frogs-from-global-killer-disease-amphibians-pathogens

https://www.discoverwildlife.com/animal-facts/amphibians/facts-about-chytridiomycosis-disease/