Regulating trade may hold key to stopping spread of deadly amphibian disease

The latest on the killer fungal disease that is wiping out amphibian populations the world over points to human trade as the biggest factor in the spread of the disease.

It’s been called “the worst infectious disease ever recorded among vertebrates in terms of the number of species impacted, and it’s propensity to drive them to extinction”. The single-celled chytrid fungus known as Batrachochytrium dendrobatidis (Bd) has decimated amphibian populations around the world. Faced with this killer disease, as well as climate change and habitat loss, hundreds of amphibian species are considered critically endangered or feared extinct.

Unlike other members of the chytrid family, Bd is flesh-eating. It infects the skin of amphibians and releases enzymes that digest their flesh causing fluid imbalance and eventually heart failure. Population decline can be extremely rapid with rare and endemic species most vulnerable to the threat of extinction.

North American Bullfrog. Photo under Creative Commons license.

Although Bd has been around for eons, amphibian population decline only began in the 1980s. It was originally thought to be because of changes to the environment, such as climate change creating ideal conditions for Bd or pollution making amphibians more vulnerable. Yet recent DNA studies have discovered that in the majority of cases where the disease has been deadly the genetic make-up of the fungus has been identical (PNAS, vol 108, p 18732). This suggests that a new, killer strain has emerged.

This has major implications because it means that Bd must be spread through invasive species caused by the human movement of amphibians and not environmental factors. This means that science alone is not enough to solve the problem.

Frogs’ legs. Photo under Creative Commons license.

In addition to amphibians being traded on the exotic pet market, frogs are also commonly traded around the world for leg-meat and research. Making matters worse, some species have been shown to be tolerant of the fungus. This means that healthy frogs may be carriers and spread the disease to other species. Tolerant species which are commonly traded, such as the African clawed frog (Xenopus laevis) for research and the invasive North American bullfrog (Rana catesbeiana) for meat, have been suggested to be one of the principal causes of the spread of the disease.

The trade in amphibians has also been suggested by Matthew Fisher of Imperial College London as one of the reasons for the emergence of the deadly Bd strain because it may have led to the hybridisation of two different strains, creating the lethal strain in the process. Another factor suggested is captive breeding causing artificial selection of strains with higher virulence.

Whatever the cause, one thing is certain. Without adequate regulations governing the trade of amphibians, the disease will keep spreading. These regulations cannot focus on specific species, as CITES does for example, but must cover all amphibian-related trade and all species because every amphibian species may be a potential carrier.

These findings raise serious biosecurity concerns not only about the frog trade, but more broadly. According to Fisher, “we’re seeing a breakdown in global biosecurity that’s having a profound impact on natural environments … we’re seeing it in plant systems, we’re seeing it in animal systems, and we’re seeing it in human systems as well. It’s pretty terrifying.”

Works Cited:

Amphibian Conservation Summit 2005, ‘Amphibian Conservation Action Plan’, <http://www.amphibianark.org/pdf/ACAP_Summit_Declaration.pdf>.

Farrer R. et al. 2011, ‘Multiple emergences of genetically diverse amphibian-infecting chytrids include a globalised hypervirulent recombinant lineage’, Proceeding of the National Academy of Sciences, vol. 108, no. 46, <http://www.pnas.org/content/108/46/18732>.

Holmes B. 2012, ‘Genetic detectives hunt the global amphibian disease’, New Scientist, <http://www.newscientist.com/article/mg21528722.000-genetic-detectives-hunt-the-global-amphibian-killer.html?full=true>.

Marshall M. 2012, ‘Deadly frog disease spreads through tolerant species’, New Scientist, <http://www.newscientist.com/article/dn21583-deadly-frog-disease-spreads-through-tolerant-species.html>.

Sanderson K. 2012, ‘Trade rules must be tightened to halt frog-killing fungus’, Nature News, <http://www.nature.com/news/trade-rules-must-be-tightened-to-halt-frog-killing-fungus-1.10771>.

Will Bibby is currently completing a Masters (MPhil) in Geography and the Environment at the University of Oxford.