Upping the threat level: can we mitigate combined threats to amphibian biodiversity?

by ZSL on

By Bryony Allen

Chytridiomycosis or chytrid was recently cited to be responsible for the greatest loss of biodiversity ever seen due to a wildlife disease. Combining this with the threat of another wildlife disease that can simultaneously co-exist in the same host – means it really is time to raise the alarm. Leading experts in the field of amphibian disease ecology will be meeting this week at ZSL’s amphibian symposium in order to figure out a way forward for mitigating the potential impacts these two diseases are set to cause.

Photograph of a toad sat on a mossy rock

You may or may not have heard about the mass amphibian decline. It is shockingly large for something so under-reported, and much worse than we anticipated. Just last month a paper published by Ben Scheele et al, including ZSL’s Trenton Garner and many of our symposium speakers, explicitly called out the amphibian disease chytrid as one of the worst wildlife diseases ever recorded for its impact on biodiversity. The fungal pathogen behind the disease is indiscriminate in the species it infects; it is a broad host generalist. While some species show signs of resistance, the sheer number of species that have been documented with chytrid infections and suffering population declines is staggering. Over 500 species are declining, 25% of them have experienced a 90% reduction. 

But chytrid is not the only disease affecting amphibians globally. Another highly virulent, emerging infectious disease is out there. Ranavirus, a viral pathogen, also has the ability to infect a diverse range of amphibian species, with over 100 species documented so far. Just like chytrid fungi, the ranaviruses have been deemed a major conservation threat to amphibians and linked to severe mortality events across the globe.

Close up photograph of a green and brown frog

Over the last 30 years, the amphibian research community have accumulated a huge body of knowledge on both diseases. Researchers have tackled the devastating declines from many angles; they have nailed down the phylogenetic lineage of both pathogens, explored the temperature tolerances of the pathogens and host species, and have attempted to understand how the pathogens are transmitted from an infected animal to a susceptible one.

The overwhelming nature of these diseases and the associated declines in amphibian populations means there has been a tendency to focus in on one-pathogen and one-host. This approach was fundamental in building a detailed understanding of the host-pathogen dynamics from the bottom up.  However, we as a research community are increasingly aware that this is not enough. We need to look beyond these devastating diseases in isolation. Co-infection is rife in wildlife populations. Recent field studies show that chytrid fungus and Ranavirus pathogens can co-infect a wide range of amphibian species. The threat co-infection poses is immediate and we urgently need strategies in place to study and mitigate it. 

The ZSL Science symposium, on the 24-25th of April, presents a golden opportunity for experts on both diseases to share their work - from fresh field findings, to new methodologies and conservation initiatives. The 25 speakers represent a stimulating array of specialisms and all have the goal of bringing about conservation practices to save amphibians around the world. With workshops throughout the event, we hope to identify the key threats and develop mitigation strategies. And where better to host it than at ZSL’s Institute of Zoology based at ZSL London Zoo, which is a scientific hub for ground-breaking conservation and wildlife health research. 

Extremely close-up photograph of a Sterling’s toothed toad

As a second year PhD student, I am privileged to work alongside many of these researchers. I work with Prof. Andy Fenton up at the University of Liverpool, Prof. Trent Garner down here at the Institute, and Ben Tapley, Curator of Reptiles and Amphibians. As supervisors, they combine theoretical co-infection knowledge with amphibian research and conservation specialism.  

Whilst I started my PhD research thinking about the transmission of chytrid between species with differing susceptibilities to the disease, it quickly became clear that transmission would likely change not just with the host species response to infection but with the infection scenario. In response to the growing number of field studies that are detecting amphibians with both pathogens, we decided to explore this multi-pathogen, multi-host disease system. If we can understand the role an individual, infected with either or both pathogens, is playing in transmitting the disease to others, we can start to figure out how these diseases are being maintained and spread on such a large scale.  

So, are we ready to tackle both? Join us on the 24-25th of April to hear the latest research and find out whether collectively amphibian-disease experts can find a path for mitigating two terrible diseases. If you can’t make it to the event, you can follow the conversation on Twitter using the #ZSLtalks hashtag.

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