The source of the amphibian chytrid fungi

As some of you may know, I have spent much of my career focusing on the devastating emerging infectious disease of amphibians known as amphibian chytridiomycosis. One of the most important papers published on this topic in many years came out earlier this year in the journal Science. A group of researchers in London, including staff from the London Zoo, compared the DNA sequences from the entire genome of over 200 chytrid fungus samples from around the world. Their data are robust, and their conclusions are important and sobering.

In short, there are two known chytrids that are known to be pathogenic to amphibians (known so far, anyway). We call these by the nicknames Bd and Bsal, and they evolved in southern Asia. A paper in the 1990s had proposed that the origin was in southern Africa, and that the chytrid was spread globally by a trade in African clawed frogs (Xenopus laevis) for research and human pregnancy tests in the last century. While we now know that the origin part of that story is incorrect, the trade aspect remains very relevant.

The new paper clarified what we know about the four distinct genetic strains of Bd. One of those strains is the hyper-virulent form we call the Global Panzootic Lineage, or GPL – the scary one that has obliterated so many amphibian populations, including entire species. Their work also uncovered a fifth genetic strain that was previously unknown. This research group’s improved data on the genetic sequences that make up these lineages have advanced our knowledge of the hybrids that exist – hybrids created when multiple strains came together genetically in space and time. The hyper-virulence of the GPL is a direct result of such a hybridization. Based on the identification of the five genetic strains of Bd, we now know that all five have been detected in global shipments of amphibians (for food, pets, research, etc.). This means that the global trade is a melting pot of chytrid strains, and this very bad situation is a perfect storm for creating additional hybrids.

If the original chytrids were from southern Asia, how did other parts of the world become infected? The answer is simple: unregulated movement of amphibians among continents. We know that no amphibian swam from Korea to Australia or the Americas, but we do know that millions of amphibians are moved between continents every year.

Sadly, our conservation efforts for the last 20 years have shown us that we cannot control chytrid epidemics in the wild. Today, the fight against amphibian losses is an issue of policies and their enforcement or compliance. Moving amphibians around the globe with no disease screening caused this problem, and can continue to make it worse. One good example is the case of the recently-discovered chytrid Bsal that is primarily associated with declines of salamanders. After Bsal was discovered in 2013, biologists with the U.S. Geological Survey formed the multinational Bsal Response Working Group, and I was invited to join. Applying the hard lessons we had learned from Bd, we came up with templates for responding to the discovery of Bsal in commercial trade, and possibly even in the wild. We were also successful in getting emergency legislation approved through the U.S. Fish & Wildlife Service to get policies in place to stop imports of selected species of salamanders into the U.S. We are still working with lobbyists and the U.S. Fish & Wildlife Service to update and increase the effectiveness of this legislation.

The good news is at Bsal has not yet been found in North America. The U.S., specifically here in our Southeast region, is the world’s hotspot for salamander diversity. The introduction of Bsal could erase much of that natural legacy. Not surprisingly, the research paper I discuss here, with a very well-prepared overview by my close colleague, Dr. Karen Lips, emphasizes the necessity of regulating global trade in amphibians. Such clear and strong statements, especially published in a journal like Science, really bolster our case when we are trying to develop regulatory policies.

I’ll end with one more very relevant example. Before I came to Zoo Atlanta, I had a position at Utah State University in the Biology Department. One day, one of my graduate students came into the lab with a small frog he had caught in nearby Logan Canyon. The frog was a fire-bellied toad (Bombina orientalis), and it had been found resting under a rock and completely healthy in the Utah mountain habitat. This species is exported from Korea and China in large numbers for the pet trade. Until the 1980s, Logan Canyon had an abundant population of native western toads (Anaxyrus boreas). They were gone by the time I arrived in 1997, never to be seen again. While it’s unlikely that the specific toad my student found in the canyon caused the disappearance of the native toads, we know how it came to be there. The Asian toad was imported from southern Asia and sold in a pet store, and a well-meaning person released it into the canyon. This is how Utah lost all but one or two of its populations of an abundant native toad. All of the Rocky Mountain states have the same story, and now we have a clear image of how it started.

Publications:

The original paper: O’hanlon, S.J., Rieux, A., Farrer, R.A., Rosa, G.M., Waldman, B., Bataille, A., Kosch, T.A., Murray, K.A., Brankovics, B., Fumagalli, M. and Martin, M.D., 2018. Recent Asian origin of chytrid fungi causing global amphibian declines. Science, 360(6389), pp.621-627.

A wonderful commentary and overview: Lips, K., 2018. The hidden biodiversity of amphibian pathogens. Science, 360(6389), pp.604-605.

Nice coverage in The New York Times!
Joe Mendelson, PhD
Director of Research