Fatal Frog Fungus
The potentially lethal fungal disease chytridiomycosis has been associated with declining amphibian populations around the globe. This rapidly emerging disease, and the chytrid fungus that causes it, have forced scientists to scramble to learn more. There are still plenty of mysteries about the origin and spread of the fungus. With today's episode we will shed some light on what we know and what we can expect for the future of amphibians. Join us as we interview USGS ecologists Mike Adams and Tara Chestnut, as well as USGS hydrologist Chauncey Anderson.
[Intro Music: Scott Pemberton Trio, Little Bobby Cobalt]
[Segment #1: Introduction to Chytrid Fungus]
[Steven Sobieszczyk] Hello and welcome. This is the USGS Oregon Science Podcast for Tuesday, February 23, 2010. I'm Steven Sobieszczyk.
For today's episode we're examining a disease that has been plaguing frogs and other amphibians around the globe. Throughout the world scientists have witnessed a rapid decline of amphibian populations which is thought to be caused by the fungal disease chytridiomycosis. The chytrid fungus has been linked to large die-offs within amphibian populations and is prevalent on at least four continents. Scientists with the USGS in Oregon have been studying its effects here in the Pacific Northwest. In today's episode we'll have two interviews. First, our own Damon Rundberg will sit down with USGS amphibian ecologist Mike Adams and discuss what the chytrid fungus is, how it affects amphibian populations, and what impact it has had in Oregon. Later in the show, we'll play a segment of an interview I recorded earlier with USGS hydrologist Chauncey Anderson and ecologist Tara Chestnut. They'll bring us up to speed on how we monitor the spread of the chytrid fungus and what the future holds for amphibians.
But first, Damon and Mike will give us some background on the chytrid fungus and what effect it's having on amphibians in Oregon. Please stick around.
[Midtro Music #1: Scott Pemberton Trio, Little Bobby Cobalt]
[Segment #2: Interview with Mike Adams, USGS Ecologist]
[Damon Runberg] Welcome back to the USGS Oregon Science Podcast. I am Damon Runberg. For this segment we will be speaking with U.S. Geological Survey amphibian ecologist Mike Adams about his work on the chytrid fungus and its effect on amphibian populations.
Welcome Mike, thanks for joining us today.
[Mike Adams] You bet, thanks.
[Damon Runberg] Can you start by telling our listeners what the chytrid fungus is?
[Mike Adams] Well, it's a fungus, like the name says. It's pathogenic to amphibians; it was discovered, I think, in the late nineties or so. It's something that affects skin and keratinized tissues on amphibians, and it seems to be a problem.
[Damon Runberg] Do you have any idea where the fungus comes from?
[Mike Adams] Oh, there is a hypothesis that it originated in Africa, but we are not sure.
[Damon Runberg] Does the fungus infect all species, or are certain species more likely to obtain it?
[Mike Adams] The fungus specifically affects amphibians. I think that we have found it in every species that we have looked at. It seems more prevalent in some species than in other species, but we didn't find it in tailed frogs. I guess that's the one species where we have not found it, and I don't think I know of anyone finding it, but I don't think there have been that many samples.
[Damon Runberg] When looking at infected frogs, can you see any visual signs of infection?
[Mike Adams] You can see it in tadpoles. Sometimes it shows up just as deformities in the tooth rows in their mouth parts. You have to look pretty closely to see it. If it is really severe, there will be inflammation in the tissue. The tadpoles survive with it, and they will go ahead and transform. In the adults and juveniles, they seem sick and lethargic, and it can kill them.
[Damon Runberg] Do these infected frogs and tadpoles spread the fungus among one another or is there another source of infection?
[Mike Adams] We really don't know how it spreads. It could probably spread through populations by direct contact. But, we know that the spores can show up in water samples, which suggests that it can spread within a water body. Beyond that, we don't know how it spreads. It seems to spread pretty well, but we don't know how it's doing that.
[Damon Runberg] Is there a link between the chytrid fungus and deaths among frogs and toads?
[Mike Adams] Yeah, there are definitely pretty good observations of the fungus arriving in naive populations that haven't experienced it before and of quite massive die-offs following shortly thereafter. However, there are also lots of observations of populations having the fungus, and we don't see any apparent problem. That doesn't mean there is not a problem, that there wasn't a problem in the past, or that there might not be a problem in the future. It seems that it is quite widespread to the extent that we can tell by just showing up every now and then, which is not a really great extent. We are frequently not seeing problems; in fact in my studies, we are never seeing problems.
[Damon Runberg] Have you or other scientists found a difference between amphibian species regarding their response to the fungus?
[Mike Adams] Only that we find differences in the prevalence of the fungus within populations and among populations. There are certain species, like both species of spotted frog, where we almost always find it. In one species of spotted frog, the Oregon spotted frog, I think every single population in Oregon has the fungus present, and it seems fairly prevalent within the population, meaning that many individuals within the population have it. For other species, it's not nearly so common, but how that's affecting them, we don't know. In fact, we don't know if finding that it's prevalent within a certain species means that it is a problem for that species or if maybe that's a good thing. It might mean that they persist perfectly well with the fungus, or maybe that they experienced mortality in the past and now they are more resistant to the fungus. We don't know how to interpret that information. We just know that in some species it is very common and widespread, and in other species it is not.
[Damon Runberg] I understand that you found frog and toad populations were more likely to acquire the fungus if there was a high level of human activity in the region. What is it about this human disturbance that may lead to an amphibian species being more vulnerable?
[Mike Adams] We don't know, and we don't know if there is a connection. We just found a correlation. There happened to be greater prevalence, and it was the only pattern that we found. There was greater prevalence of the fungus within populations, meaning a higher proportion of the populations were infected in areas with this higher human footprint index. Why that is we really don't know.
[Damon Runberg] How high of a human footprint are you talking about? Would a large footprint be within a city or more like a popular hiking trail?
[Mike Adams] It's a ten-point scale, and I think the highest we got on the scale was a seven. It certainly was not within a city; typically, I think it was heavily agricultural.
[Damon Runberg] Since it's difficult to tell just by looking at a frog if it's infected, could you describe the process of determining if a frog has the fungus?
[Mike Adams] Yes, we catch the frog. We use a little rayon swab, which looks like a Q-tip, that we rub around on the skin. There are particular places that we like to look for, between toes, on mouth parts, and on some hard, keratinized tissue. We rub the swab around, and then we follow a few steps for preserving the sample. As far as our role goes, that is about it. We ship the swab off to a lab where they use a method called polymerase chain reaction, to specifically amplify genetic material from this species of fungus. This particular species, by amplifying it, they will be able to determine if it's there. That will return either a positive or negative result of the frog being infected or even a quantitative result of how much of the fungus is present.
[Damon Runberg] So, it seems that up until this point in Oregon the fungus has been less harmful compared to other regions. Should we in the Pacific Northwest be worried about future amphibian losses due to the spread of the chytrid fungus?
[Mike Adams] Yes, we are pretty concerned about it. We just don't quite know what to think about it, but we see that it is causing some severe problems elsewhere, and we don't know what the difference is. We don't know if perhaps this is going to sit here until we get climatic conditions, such as weather in a particular year that is more conducive to a severe infection, which could cause die-offs. We don't know if maybe that has happened in the past and just wasn't really observed here. But, we are definitely concerned about it.
[Damon Runberg] Thank you very much for your time today, Mike.
[Mike Adams] You bet, thanks.
[Damon Runberg] Now that we have discussed the chytrid fungus and its effect on amphibians, we will find out how USGS scientists study the spread of the fungus through our lakes and rivers. Make sure to stick around.
[Midtro Music #2: Scott Pemberton Trio, Little Bobby Cobalt]
[Segment #3: Interview with Chauncey Anderson, USGS Hydrologist, and Tara Chestnut, USGS Ecologist]
[Steven Sobieszczyk] We are back, hope you enjoyed that interview with Mike Adams. Now it's time for our final segment in our all amphibian episode. We are joined by two USGS scientists, hydrologist Chauncey Anderson and ecologist Tara Chestnut. Both Chauncey and Tara work out of USGS Oregon Water Science Center here in Portland, Oregon. Tara is also a PhD candidate at Portland State University. They are here to share with us what the future holds for the monitoring efforts and the spread of chytrid fungus in our lakes and rivers.
[Steven Sobieszczyk] Chauncey, I figure I'll start with you, before we get into a discussion of what chytrid fungus detection methods you guys use, I was wondering…to what extent and for how long has the USGS been involved in amphibian research?
[Chauncey Anderson] So, the USGS has an amphibian research and monitoring initiative (ARMI) which was started around 2000 and has been working for the last 10 years, or so, on a variety of issues related to amphibians across the country. The program involves herpetologists, hydrologists, climate change people, geographers and various other ecologists and disease pathologists (quite a wide range of disciplines within the USGS). So the USGS has, since around 2003-2004, recognized the threat of chytrid fungus for various amphibian populations. And in 2004 and 2005 was when my group started doing work trying to detect chytrid in water, as an attempt to be able to provide a complimentary tool to those who were analyzing for it on amphibian skin.
[Steven Sobieszczyk] Tara, your turn. Can you give a little background on the chytrid fungus and what makes the amphibian strain so unique?
[Tara Chestnut] Chytrids are a very ancient fungus. And there are about 1200 species that are described. Most of them are plant decomposers. They decompose diatoms and algae, and about 10 percent of the fungal flora inside of ruminoids, so cows and bison, are chytrids. They're well distributed across the environment. What's unique about the amphibian chytrid is that it is the only chytrid fungus that affects a vertebrate species.
[Steven Sobieszczyk] So what is the field testing method you have been working on? What are you testing for? And how will scientists actually use what they learn from this in the future?
[Chauncey Anderson] The method that we developed uses a genetic test for the DNA of chytrid fungus to be able to detect whether the fungus is in a water body that we sample. Similar tests can be used to test whether the DNA is on a frog and seeing if the frogs are infected. But in the absence of affected frogs, if it is in a pond, chances are good we can detect it if we sample it at the right time of year and if we sample enough water. The next question is where are we going with that? And there's actually a lot of different ways that can be used. One possible use for that is for reintroductions of threatened or endangered species in places where they have previously been extirpated, or otherwise are no longer present. So, there have been several instances where people have tested ponds that are likely candidates for reintroduction and then, depending on the results of the water, then that's one thing that would go into the consideration of reintroducing a species there. Another question that is being looked at is what is the relationship between presence in water and presence on an amphibian? Which came first? There's a sort of chicken and egg question. And part of the work we're doing is trying to be able to use the water test to give us an idea of if there is a way to predict whether species there will become infected, what the exposure routes are, and also get a sense for the timing of when infection may occur.
[Steven Sobieszczyk] Are the results from this test just "presence/absence"? Or can you actually determine the amount of fungus present in the water?
[Chauncey Anderson] Well, so, the first response is we can do this quantitatively. It can either be done quantitative or presence/not detected-type of approach. Particularly, in the water, our method is a quantitative method. What we don't know yet is what's the exposure mechanism? Is there a dose/response-type interaction going on? Where there needs to be a certain threshold. For instance, density of chytrid spores in the water for frogs to become infected. A lot of may have to do with where do they spend their various life history stages. Are they in the mud? Are they in the plants? And how are they exposed to chytrid? We're really not there yet and being able to answer those questions. But that's the direction.
[Steven Sobieszczyk] How does this relate to what you do in the lab?
[Tara Chestnut] What I do in the lab is a two step process. The first is after we extract the DNA, I ask the presence/absence question. So, is chytrid DNA present in my sample? And if the answer to that is yes, then I go to the second step and try to measure how much chytrid is there. It saves us time and it is also more cost-effective to approach it that way. Because the tests are fairly expensive to run and if we can determine whether it's there or not first, then we don't have to take it to the second step.
[Steven Sobieszczyk] We've spent most of the episode talking about how devastating the chytrid fungus is. Have there been any attempts to develop a vaccine or cure? What about natural immunity? Are certain species less susceptible to the disease then others?
[Chauncey Anderson] There has been some work looking at various treatments for individual animals or maybe several individuals at one time. A couple different types of solutions that animals have been bathed in and it appears to help them cure themselves or helps kill the amphibian chytrid from them. How to treat a larger…or even an individual pond, much less an entire ecosystem, has a very different question. There's all kind of considerations that go in. We don't know if it can be done effectively and if the amphibians that are treated once will be susceptible to it again.
[Steven Sobieszczyk] Lastly, what's the take home message for our listeners? Is this disease something that will wipe out frogs everywhere? What should we know about chytrid fungus?
[Chauncey Anderson] It's a potentially important disease in many parts of the world, including here, but it also doesn't necessarily mean the end for every species that is out there. Because some species are less susceptible than others and some species may have adjusted to it in their populations already.
[Tara Chestnut] I think the take home message there is that we're most concerned about species that have never been exposed to it before. And because…until very recently we didn't have reliable methods to detect it in the environment. We didn't know how to detect amphibian chytrid unless we had infected animals [Chauncey Anderson: and a die-off in process]. And potentially a die-off. What our work allows researchers to do, not just here at the USGS, but in other places is be proactive about monitoring not just the amphibians but also there environment. So that we can have more tools that allow us to predict areas that may be candidates for new exposure. So that places where we haven't found chytrid before, we can test for it in the water and development management plans that will, perhaps, allow for the conservation of species that we believe haven't been exposed to chytrid before.
[Steven Sobieszczyk] Well, that is all the questions I have. Thanks guys for joining us. [affirmative response] That is it for this month's podcast. For links to topics discussed, check out the show transcripts on our Website at: or.usgs.gov/podcasts. If you have any questions, comments, or complaints about the USGS Oregon Science Podcast, please email us at firstname.lastname@example.org. Thank you for listening. To hear more about other research the USGS is doing around the country, check out the USGS social media Website at: usgs.gov/socialmedia. There you can listen to other USGS podcasts, as well as find links to USGS on twitter and YouTube.
Until next time. I'm Steven Sobieszczyk.
This podcast is a product of the U.S. Geological Survey, Department of the Interior.
[Outro Music: Scott Pemberton Trio, Little Bobby Cobalt]
An OPB Oregon Field Guide episode in 2008: