Wednesday, June 12, 2013

Amphibian decline: Chytrid fungus

(Atelopus varius) Costa Rican Variable Harlequin Toad 

When you think about why a species goes extinct, what comes to mind? Habitat destruction, pollution, overharvesting, climate change, invasive species, and infectious disease are all factors that contribute to a species decline. What we are seeing with the amphibians is not the result of just one cause but rather all of them acting together and the result is catastrophic. Amphibians, which includes, frogs, toads, salamanders, and caecilians are facing a mass extinction in our lifetime if we do not act soon. In this post I will touch on the various factors leading to the extinction of amphibians but then focus on one of the main causes for amphibian decline; infectious disease in the form of the chytrid fungus.  Many species of amphibians are endemic (meaning that they are only in one particular area) which makes amphibian species very vulnerable to changes in their environment. Habitat destruction in areas such as the rain forests is a major player in causing many of these endemic species to go extinct.  As amphibians inhabit both water and terrestrial habitats, many types of pollution can have a huge effect on amphibian populations.  One aspect of pollution that dominates in amphibian loss is the use of pesticides like atrazine. In recent studies, atrazine has shown to affect the reproductive organs and health of the animals. These amphibians are changing sex or even growing multiple reproductive organs when exposed to even low levels of atrazine (Raloff). Climate change is changing the seasons, temperature, moisture, and many other factors to which amphibians are very vulnerable.  Overharvesting is also an issue for amphibians in the form of a food source (frog legs), medical, or educational purposes and until recently, impacts from overharvesting have not been addressed.

Dead frogs killed by the amphibian chytrid fungus.
Photograph by Joel Sartore, National Geographic
Amphibians have this super permeable skin where water, oxygen, and other nutrients can travel from the environment into the animal. This is how these animals can live deep in soils for long periods of time.  For example the Spade Foot Toad that lives in the high desert in Southeastern Oregon will bury itself deep into the ground and only come out when it rains. They are able to survive for years without coming back up to the surface.  The Chytrid fungus (Batrachochytrium dendrobatidis) causes a thickening of the skin which leads to oxygen and electrolyte deficiencies. There are currently two current ideas regarding the toxicity of the fungus. The first being that it secretes toxic enzymes and the second being that the loss of oxygen and electrolytes lead to poor osmoregulation.  Without an appropriate osmoregulation, the heart will go into cardiac arrest (Lee). The chytrid fungus does not only infect amphibians, recent research has shown that animals such as the crayfish can be carriers of this disease and the chytrid fungus can remain in the environment for long periods of time. This means that even if we transplant amphibians back into an environment that has lost them; we may still see amphibian death due to the fungus (McMahon).

Now that we know what the chytrid fungus is and what it has done to the amphibian populations the next questions are: Where did it come from? How does it spread across the globe? Several avenues of the spread of this disease have been postulated but none have been confirmed as the one way this disease has spread so quickly. The original thought was that African Clawed Frogs were the original carrier of this disease and it was spread to other populations due to human uses. These frogs were used until the 1970s as pregnancy tests. Doctors would inject a female human’s urine into the frog and if the frog laid eggs then the female human was pregnant (Lee). These frogs have also been widely sold in pet stores across the globe. The thought is that these frogs have gotten loose into the wild and spread the chytrid fungus to other species of amphibians in various locations around the world.  African Clawed frogs in the wild have tested positive for the chytrid fungus which verifies that there is a possibility that this was one path taken by the chytrid fungus. Another path of the spread of this disease is believed to be the American Bullfrog. Although the American Bullfrog is a native species to North America, it has become an invasive species in areas such as California and is spreading the chytrid fungus to other amphibians. The American Bullfrog is widely used as frog legs and since the fungus is not harmful to humans and is not visible to the naked eye, human consumption of this delicacy has not slowed down (Upton). The American Bullfrog’s movements can be correlated with the path of the chytrid fungus (Lee).  The chytrid fungus has also been found hitching a ride on the bottom of bird’s feet which could attribute to the widespread reaches of the disease. Whatever the path is that delivers the chytrid fungus, we know that it is deadly and it has been around for about 40,000 years (Lee). Some amphibians seem to simply be carriers and are immune to the actual disease which may contribute to why this disease has been around for so many years but is just now making a global appearance.   Several groups of scientists are working on tracking the fungus so that we can be one step ahead of the infection.  One group has found that 42% of the world’s amphibian population is currently affected by the fungus and this is spread over 52 countries around the globe as the map below shows (Olson).  
Olson et al.

So why should we care about saving amphibians from this deathly fungus? Amphibians provide quite a few ecological services. Without frogs we would see an enormous increase in the amount of insects and possibly an increase in infectious diseases carried by insects. Like any other group of animals in the wild, frogs and other amphibians play a role in food webs and the loss of amphibians could be a possible destruction of the animals that consume amphibians as their food source.  Many antimicrobial agents and various types of medications have been developed using amphibians and more are still left to be discovered. Amphibians have also played an important role in education and research. Amphibian embryos are transparent and have been widely used in the field of developmental biology.

What are some things we should do to help? Generally we think that just one person cannot make that big of a difference but we hear all the time that one person can make all the difference in the world. Here are some easy guidelines to live by that will impact amphibians in a positive way. Just doing your part such as conserving resources, reducing your carbon footprint, and reducing your effect on global warming will greatly help our frogs and other amphibians. If we stop purchasing wild-caught frogs we do not run the risk of setting them free and possibly infecting others in the wild. Refraining from using pesticides or eating frog legs will also help our frogs.  There are many ways to help save these important animals and if we work together we may just be able to.

Save the frogs!

And finally here are a couple of links to learn more about this subject.


Chatfield M, Moler P, Richards-Zawacki C, Sturtevant J. “The Amphibian Chytrid Fungus, Batrachochytrium dendrobatidis, in Fully Aquatic Salamanders from Southeastern North America”. Plos ONE [serial online]. September 2012;7(9):1-5.

Lee , Jane. "African Clawed Frog Spreads Deadly Amphibian Fungus." National Geographic . 15 05 2013: n. page. Web. 29 May. 2013. <>.

McMahon, T.,  Brannelly, Laura A., Chatfield, Matthew W. H., et al. Chytrid fungus Batrachochytrium dendrobatidis has nonamphibian hosts and releases chemicals that cause pathology in the absence of infection.PNAS 2013 110 (1) 210-215

Olson D, Aanensen D, Fisher M, et al. “Mapping the Global Emergence of Batrachochytrium dendrobatidis, the Amphibian Chytrid Fungus”. Plos ONE [serial online]. February 2013;8(2):1-13. 

Raloff J. Herbicide makes frogs Mr. Moms. Science News [serial online]. March 27, 2010;177(7):9.

Upton, John. "Despite Deadly Fungus, Frog Imports Continue." New York Times 07 04 2012, n. pag. Web. 29 May. 2013. <>.

No comments:

Post a Comment