A Froggy Apocalypse
What do you do when a fungus is taking over your community?
Bodies lay scattered. An eerie silence enveloping the jungle. The sour smell of death in the air… dun dun dun!
Nope, it’s not the plot of the fourth Indiana Jones movie where psycho frogs start chomping on explorers. Although…maybe there’s potential in that idea? (hit me up Lucasfilm and we can discuss)
No, let’s get serious. This is the day-to-day reality for the increasing number of frogs who have become victims to the horrible chytrid panzootic (animal pandemic).
Chytridiomycosis, or chytrid (KY-trid) for short, is the worst disease to impact biodiversity globally. It’s killing even more animals than rats and house cats are (and we know how many presents Fluffy likes leave for us).
Chytrid is caused by the fungi Batrachochytrium dendrobatidis (Bd) in frogs and Batrachochytrium salamandrivorans (Bsal) in salamanders, chytrid has caused the decline of over 500 species. Of those, 18% are extinct in the wild, and another 25% are experiencing a reduction of over 90%.
It can spread to any amphibian; frog, newt, salamander, or caecilian (and also some crayfish!), but in this article I’ll be focusing on frogs, the most affected species.
What happens to the amphibians?
Imagine a little microscopic fungus crawling over your skin and burrowing deep into your body. Isn’t that such a warm, fuzzy feeling?
Well, that’s basically what chytrid does. It invades the outer keratin layer of the skin and then creeps inwards to damage the internal organs (mainly the nervous system).
It’s an excruciating process but also microscopic, so it’s invisible to the naked eye.
Skin is one of the most important organs for amphibians. It needs to be kept perfectly healthy since it’s what allows them to maintain homeostasis, a constant internal environment. Its job is to regulate things like temperature and mineral levels. In addition, the skin helps amphibians to hydrate and breathe. Many species don’t even need lungs!
When chytrid damages the skin, frogs lose electrolytes and end up with really low levels of sodium and potassium. These minerals are vital for maintaining fluid balance in the body, making sure organs are running properly, and preventing stroke. (Maybe some Gatorade could help?)
As a result, after weeks or even months of intense suffering, Kermit the frog will eventually die of heart failure (if he isn’t already so weak that he dies of starvation or becomes somebody else’s dinner.)💀
How to tell if a frog has chytrid
Ok, so I guess I spoiled the (un)happy ending, but there were tons of signs that the frog was going to die!!
There are over 7,000 amphibian species and each one responds in a different way to the fungus. This means that there’s a whole lot of different symptoms one can have.
These can include:
- Lethargy (drowsiness)
- Hyperkeratosis (extra layers of thick, rough skin)
- Abnormal posture (withdrawn position, won’t get back up if knocked over)
- Cringing at touch
- Red underbody
- Opaque skin smearing off cuticles
- Slowing of heart and breathing
In addition to these symptoms, even though infected frogs spend practically all their time in the water, a closer look at their dorsal (back) skin shows it’s still dry and taught. Chytrid can cause a calcium deficiency in frogs too which leads to edema (fluid retention) and bloating. It’s like the day after Thanksgiving everyday until you die. 👀
Once there’s been damage to the nervous system which can emerge in the form of toe curling, paralysis, or a tucked in head (literally what it sounds like: a permanent frog double chin), it’s too late to save the frog.
Mountain yellow frogs take ~2–3 weeks to die but other species can take days or even months.
How it took a world tour
Welp, that was depressing. I’m sorry to say that this section isn’t going to be much hoppier. But stay with me! I’ll even do some storytime!
Chytrid spores all have a tiny little tail called a flagellum which they use to swim around in water with. Even without a host, the spores can survive for a few weeks in water, depending on the temperature.
Chytrid is pretty hardy stuff and anything that’s wet or moves water could become a transmitter of the disease. It can also go from one disease carrying animal to another.
A select few species are even naturally immune, meaning they can become super-spreaders. Recognize some of that language? Some frogs can also be asymptomatic carriers, just like with COVID-19, and move on to infect more vulnerable populations.
Old hypothesis: South Africa
There are a few hypotheses about where Bd, the fungus that causes chytrid, originated from.
The older one, my personal favorite due to its wackiness (you’ll see), suggests that the disease might have come from South Africa in the 1930’s. During that time, African Clawed Frogs were being imported by the boatful to hospitals around the US.
Why? Well because of pregnancy tests. Duh.
British scientist Lancelot Hogben had a thing for frogs. In 1927 he moved to South Africa to “spend more time” with them. He even named his house after the African Clawed Frog (xenopus), his favorite species.
One night he got really high (jk, but maybe 🤷♀️) and injected a frog with some ox pituitary gland extract he happened to have on hand (juices from a bustling hormone hub at the base of the brain).
The hormones stimulated the frog’s ovaries to laying eggs shortly after her little treatment.
Knowing that ox extract chemically resembled human chorionic gonadotropin (HCG), present in pregnant women, Lancelot Hogben wondered if xenopus could be used as a living fertility test.
He tested it and was right. Since then, every time you take a pregnancy test, there’s a little frog sitting inside of the stick waiting to be peed on. The end.
Haha no, we have better technology now, but at that time frogs were the latest and greatest way to find out if you were pregnant!
A fine improvement at least over rabbit and mice impregnation. Because frogs make eggs, there’s be no need to slaughter and dissect them to see the results.
Hogben’s success sparked the US’ love for African Clawed Frogs as they started shipping tens-of-thousands in every year.
Although the frogs weren’t tested for it at the time, many were carrying Bd. Xenopus is immune to the effects of chytrid so people had no reason to suspect anything might be wrong with the frogs.
After the development of new technology, hospitals and clinics became left with tons of frogs that they just let loose into the wild. Toaday, African Clawed Frogs are an invasive species and will eat literally anything they can fit in their mouth without a frogment to spare. (Seriously! Check out the videos at the end of the article!)
They spend the majority of their time splashing around in streams and ponds and unknowingly transmitting the disease.
Latest hypothesis: Korea
As much as I like the story behind the South Africa hypothesis, this one is probably the real origin of Bd.
With whole genome sequencing, scientists could trace lineages of Bd back to its origins. One lineage, BdASIA-1, has genetic hallmarks of an ancestral population that started the whole panzootic. It’s from the Korean peninsula and emerged in the early 20th century.
This time works with when global expansion of commercial amphibian trade began to boom. Chytrid is still actively being transmitted around the world from East Asia.
The problem with trade
As you can probably tell, humans are a big reason why this chytrid thing is such an issue. Our craving for all things exotic and foreign ensure that we continue importing diseased species into the country without thinking twice.
Yes, I know it’s not all bad and of course we’re never going to stop trading wildlife with other countries. Just in 2019 $4.3 billion of legal wildlife were imported to the US. That’s approximately 200 million live animals and thousands more of illegal shipments. Instead of completely blocking out trade, we have to regulate it better.
I don’t think the problem is necessarily the shipments themselves, it’s how they get handled and checked for diseases (if they even do).
The US has no laws requiring disease surveillance for wildlife entering the country. (Zero!) There’s also no federal agency that screens wildlife coming in for contagious disease and illness. The only species that get checked are those that “present a significant public health concern,” a list too short and incomplete.
We don’t even know how many diseases are being imported since there’s no one checking or keeping track! The lack of coordination has led to a false sense of security that someone else is checking the animals, when no one is. This results in thousands of infected animals entering unchecked every year.
One example of such a species is the American Bullfrog. The US imports nearly 2.5 Million of them a year; more than any other live amphibian species! That’s a lot of French food! These animals introduce an insane number of new infections as they often run away before making it onto Michelin star plates.
American Bullfrogs destined for a life as Little Suzy’s new best friend also have the added risk of being released into the wild due to little kids’ short attention spans.
Other ways of spreading
Besides traveling through trade, there are a ton of other ways chytrid can spread. These include:
- Contaminated human objects like shoes, hands, and fishing equipment. Exchanging pond items or walking through multiple puddles are invitations for Bd hitchhikers.
- Contaminated natural objects such as bird feathers, soil, plants, and rain runoff.
- Other animals, mostly frogs, that move around looking for food and drink. Even some non-amphibian species too though. Field collections in Louisiana and Colorado found that up to 29% of live crayfish were harboring the fungus.
- Human interference like taking wild frogs and breeding them as pets or relocating them to other habitats without disease testing first.
Now I know you might feel a bit like this right now:
But the battle isn’t over!! There are many different strategies scientists are trying to mitigate the impacts of chytrid in the wild. Here I’ll be talking about the pros and cons of 9 of them.
Translocation and repatriation
This is the most popular method right now because it’s (falsely) perceived to be easily implementable, cost-effective, and transferable. People get the feeling that ‘something is being done’ which makes them want to keep doing it, even if it isn’t the best solution.
Translocation and repatriation (aka reintroduction) means removing animals from where they currently live and bringing them to a location where their species natively inhabited.
Oftentimes scientists and conservationists will breed rescued frogs in captivity and then release them into land the species once populated, but now doesn’t due to chytrid.
Attempts have been made in Europe, North America, Africa, and the Caribbean to repatriate amphibians affected by chytrid but none of them have been successful is long-term amphibian re-establishment.
Without a solid understanding of how Bd interacts with its host, the translocation/repatriation strategy won’t work long-term.
For one, there’s nothing stopping chytrid from reemerging to strike again. In the majority of trials, the frogs were safe at first in their new home, but then the disease re-emerged to wreck havoc. This strategy underestimates chytrid’s ability to spread, all the while not offering protection for if it does come back.
What’s more, we’re not sure what the cause behind the failure to re-establish was. I mean yes, it was chytrid coming back, but why? How did it get there?
Deadly chytridiomycosis might be a secondary consequence of another process going on meaning that conservation efforts on the fungus would be focusing on the wrong things. The fact that we don’t even know what is causing our failures is evidence against the idea that this technique is simple and transferable.
For all we know, it could be Indiana Jones infecting the frogs 😲 @lucasfilm.
This demonstrates how little science there actually is to back up the effectiveness of amphibian repatriation to mitigate chytrid and violates the important requirement to mitigate the threat before reintroduction.
The Royal Society states, “Given our incomplete understanding of Bd dynamics and the potential for development of resistance to Bd in wild populations, the use of translocations/reintroductions as a research tool is perhaps more appropriate than as a mitigation strategy against Bd.”
Bioaugmentation means adding helpful bacteria to the frogs’ skin to help them fight chytrid.
There are a couple different reasons why this is a good choice for amphibians. First of all, the amphibian skin bacterial community changes with age being more resistant to Bd earlier in life. This means that choosing targets for field intervention could be age-specific to the more vulnerable demographics.
On top of that, a subset of bacteria with the ability to inhibit Bd growth in vitro (in a lab) has already been isolated from living amphibian skin all around the world.
Scientists are hopeful that by augmenting the skin of frogs and other amphibians with more inhibitory bacteria they’ll be able to help them coexist with Bd.
There’s even been a field trial done to test the idea! It was limited, but successful. The trial was published alongside a strategy for how to isolate and apply probiotics to species’ skin to augment their microbiomes.
The strategy even proposes the possibility that probiotics could be applied to the environment to benefit the entire community.
Buuut, as with any new solution, there’s also a downside. We need to consider the potential risks that probiotics could pose to the ecosystem and public health as a whole. We’re also not really sure how practical probiotic development is and how effective the treatment will actually be.
For example, just because something works in a lab, doesn’t mean that it will work on an actual animal in unruly, unpredictable nature.
As an experimental way to try and control Campylobacter — the number one zoonotic disease in humans in the EU since 2005 — scientists also gave poultry probiotics. 🐔This trial taught us a couple things about what to expect.
One, the process won’t be as straightforward as it seems on paper. There are bound to be some hurdles, for example some inhibitory bacteria may be ineffectual to preexisting infections.
Two, we’re going to need a better understanding of the bacterial and amphibian community. (ikr, surprise surprise) More specifically, this means a deep knowledge of how bacterial communities assemble, stabilize, and permeate in a constantly evolving group of amphibians whose bodies do all sorts of weird things like skin secretions. 👀
The probiotic treatment needs to be effective on all Bd genotypes (strains) which might be difficult since scientists already know that just because a bacterium can inhibit one isolate of Bd, that doesn’t mean it’ll work on all types around the world.
It also needs to be suitable for mass production, including having a long shelf-life.
One final factor is that the vast amount of research required to understand all these aspects deeply will probably make bioaugmentation less cost-effective, implementable, and transferable than using chemical treatments (which I’ll talk about later.)
Furthermore, if requirements for animal experiments are many and not well-justified, it might be ethically questionable. BUT, if it was pulled off, it could be a much better alternative to chemicals and the less controversial choice.
Using Antifungals alone
Applying antifungals to frogs in the wild to reduce the load of fungus on their skin allows their immune system to catch up and fight Bd fungi naturally. This is an effective short-term solution that alters infection dynamics and can ease disease.
The problem is though, that once the treatment stops the benefits are over if the frog isn’t toadally recovered. A study discovered that the repeated use of antifungals on naturally infected frogs had no benefits after the treatment was stopped. It worked as a way to extend the length of survival but only as a short-term mitigation measure.
Application is also painstakingly done by hand which means that people like Jonathan Kolby have to camp out in the jungle for months disinfecting frogs. It’s too much work for this to result in any long-term conservation success.
Combining Antifungals with Chemical Disinfection
While antifungals alone won’t work, antifungal treatments combined with chemical treatment of the environment might. In an isolated, simply structured ecosystem with only one amphibian host species, this method eliminated Bd and clearance persisted for many years.
These findings suggest that this approach could be a great strategy for the places that fit the category above. We’ve been using pesticides to grow food for decades and have evidence that it results in significantly greater crop yields. But when farmers use pesticides they also need to reapply them regularly. In order for fungicides to be effective against chytrid in the long run, they can’t rely on a one time application.
Of course, like the solutions I discussed earlier, there’s also potential risk we need to factor in: impacts on biodiversity, ecosystem function, human health, the potential for chytrid to develop resistance to the fungicide treatments and grow harder to bring under control, etc. I mean, that’s why some people buy organic produce! 🍓
Little investigation has been done on if any existing fungicides are effective against chytrid or to try and develop a new chemical cocktail to specifically attack it.
Still, scientists have found evidence that some chemicals can reduce chytrid infection rates without affecting the development or survival of amphibians.
By giving frogs a sort of vaccine in captivity, scientists hope they’ll be able to prevent them from becoming ill and dying when they’re released back into the wild.
An experiment by Marie Toothman and Cherie Briggs infected frogs with Bd and treated them with antifungal before they died. This decreased their likelihood for infection and death and increased their survival rate for if they did become infected.
These results open up the possibility of priming amphibians with the infection before their release into the wild. So froggin’ awesome!!!
The limitations of the experiment are that it was only performed on frogs post metamorphosis (after becoming a frog) so it might not work the same way on pre-metamorphic frogs (tadpoles). This is important because they’re at the highest risk of chytrid mortality during transformation.
Frogs are highly sensitive to the survival of recently metamorphized juveniles (like young adults) so controlling the numbers of amphibian larvae that live is vital.
Finally, fungal vaccines are pretty dang hard to make. Many manufacturers only have experience with viral and bacterial vaccines and almost all human fungal vaccines are still in preclinical development. There are very few veterinary fungal vaccines.
So for now at least, vaccines probably aren’t how we’ll solve chytrid. With that in mind, the research we’ve done on it will aid us in learning more about amphibian immunity and host-pathogen interactions which we can apply to other solutions.
The idea behind this strategy is that animals are able to coexist with some really nasty pathogens if their genes are more resistant. Dr. Lee Berger, PhD says, “If we can get resources, we should be able to just take what’s been done in agriculture and provide that for the frogs to get them to be able to persist in the presence of the fungus again.”
As we learn more about chytrid, we’re finding that some species are using natural selection to develop immunity to the disease. This has resulted in the discussion of two new potential strategies.
The first one just relies on the immunogenic variations in frog communities to naturally select and develop stronger defense systems over time. The frogs will need to be able to persist in the face of chytrid on their own (with the possibility of humans using translocation and repatriation during the process to ensure they don’t die out.)
The second strategy involves humans breeding frogs selectively for more tolerant or resistant genotypes. This makes the assumption that it’s even possible to breed for better chytrid protection.
For these two strategies, the same pros and cons apply as the ones for translocation/repatriation with the addition that we need to understand resistance and tolerance in captive populations before any release would be ethical.
With selective breeding, we only know of one example where this process successfully created a clear difference in immunity in an amphibian: xenopus. (Yup! Lancelot’s favorite!) This took decades of research to gather enough data for it.
There have been advances in technology since that was done in 2014, but we’re still a long way off from being able to use selective breeding to overcome chytrid.
And even if we were able to produce genotypes that are resistant to chytrid, who knows if they’ll work in a highly volatile natural setting? The environment changes all the time and depending on the temperature, frogs’ immune responses can be strengthened or weakened.
Right now, we’re not sure whether selective breeding will be helpful in controlling this panzootic but it’s clear we need more info first.
What sets this method apart from the others is that it relies completely on changing the environment that the animals live in, not the frogs themselves.
It’s like environmental disinfection which we talked about earlier, but broader. It includes things that are chemical (like altering water salinity), physical (like the temperature), or biotic (for example encouraging the growth of organisms that like to eat Bd zoospores). 😋
Different organisms like different living conditions, just like people. When environmental factors aren’t ideal for Bd, it gets angry and starts dying. Even species that are usually highly susceptible show reduced numbers of Bd hitchhikers when it’s grumpy!
Manipulating the environment will probably be the easiest solution to implement because most conservation groups already do a lot of habitat management.
So is this one the perfect solution?! No, of course not. Manipulating the environment will provide pockets of resistance but won’t work for species that are spread out over a large area or have extremely complex ecosystems.
Like translocation/reintroduction, we need to fully understand the species we’ll be affecting and like environmental disinfection, we’ll have to weigh the benefits against possible risk to biodiversity.
Furthermore, while changing the environment might decrease the amount of chytrid in some life stages, it could increase it in others. An example of this is that while pond draining would allow for the survival of more tadpoles, that also means that there’ll be more juveniles to infect and spread the disease around when they’re older.
In the Caribbean, conservation groups are utilizing the abundance of sunlight to their advantage. 🌞🌴 Chytrid prefers cooler, wetter climates with optimal temperatures being 15–23 C (59–73 F) but able to survive in 4–25 C weather. Anything above 25 C is really hard for chytrid to grow in.
The Mountain Chicken Recovery Program (a type of frog, not bird lol) is experimenting with using canopy removal, solar heated ponds, and basking sites as a way to increase the temperature of the environment and kill Bd.
Lower levels of the forest are usually cool and damp, perfect for the fungus. By removing portions of the tree canopy, sunlight is able to reach areas it wasn’t able to before and with it increase the temperature. These sunny spots provide a safe haven for frogs to prevent and treat chytrid. They also allow for the implementation of other solar heating methods.
Of course, we can’t froget the fact that it is deforestation and whether the frogs or the trees are more important will be fiercely debated.
With the new sunlight that’s able to reach all the way down to the ground, the conservation group can also install solar heated ponds. Cool water is what chytrid loves to replicate in so by increasing the temperature they’re able to kill the fungus before it can reproduce.
Frogs also spend much more of their time in the water once they’re infected because it allows them to cope with the increased pressure on their body. Keeping the water cool enough for the frogs but warm enough so that chytrid stays away can provide a safe space for infected frogs to bathe.
Around these ponds, the Mountain Chicken Recovery Program has also added basking sites. These rocks placed in direct sunlight can reach over 42 C! When frogs lie on them they’ll be able to heat shock their underbellies (which is where chytrid is usually the worst).
Amphibians are cold-blooded and will sometimes try to raise their body temperature if they’re ill to help their immune system fight off whatever they’re faced with. You sly old frog! Some chytrid-infected amphibians have been observed doing this too so it’s likely that frogs will want to bask on these hot rocks.
One thing that scientists and conservationists need to be cautious of though is raising the temperature too much. Frogs have similar preferences to Bd and if it gets too hot or dry for them they’ll move up North bringing the fungus with them. This will further spread it to species who’ve never been in contact with it before.
Improving regulations and resources
It took ~20 years of chytrid spreading around for it to even be diagnosed. There is such a lack of resources for wildlife disease, who knows how many other panzootics are flying under the radar.
The US Fish and Wildlife Service has suspended imports of 201 species known to carry Bsal (salamander chytrid), which is great, but native species are still in danger of getting it from other sources. The US, like most countries, lacks a formal mandate to inspect wildlife imports for pathogens or disease so Bsal can and will still find its way in. Switzerland has similarly banned the import of all salamanders but that doesn’t mean they’re out of the woods yet.
These are great first steps for Bsal, but every single species reduction (except one) has been the result of Bd, not Bsal.
The US continues to import millions of American Bullfrogs a year yet at the same time they’re spending millions of dollars to try and combat the effects frogs are having on native species. It’s like they’re mopping up the floor without first turning the faucet off.
The World Organization of Animal Health has recommended methods for countries to adopt to control trade-driven spread of Bd but only few took action. We need to ensure that countries take the threat of the disease seriously and understand the devastating consequences if they don’t act now.
Looking to the future
There are many possible ways to make sure chytridiomycosis gets beaten. All of them need to be thoroughly researched and thought through before being implemented.
Researchers need to come from different backgrounds and actively engage with conservation managers. They must be dynamic and willing to alter their research programs based on the results of collaborative decision making and forward thinking.
We need as many people as we can working to develop a legal and ethical solution (or combination of a few).
We must remember that no matter how cheap, effective, reliable, transferable, or simple a solution is, if it’s illegal, it won’t happen.
One ethical issue we’ll be faced with is which species to conserve and which ones to cull or let go extinct. No matter how difficult these questions are to address though, any decision will be more ethical than not acting considering humans are the main reason for the severity of this whole thing.
Whatever the solution(s) ends up being, we need it ASAP. We have to be able to rapidly and critically assessed it so we can get it out there before more species go extinct!
What can you do?
Well, besides becoming a world class super amazing rock star scientist who comes up with the perfect solution to fix everything, there are a few other (more simple) ways you can help out amphibians.
- Drain your frog pond in winter (chytrid season)
- Clean your boots and gear with bleach before and after walking in ponds/puddles
- Don’t use amphibians as fishing bait (also illegal in many places)
- Don’t release Little Suzy’s new best frog friend into the wild if he ribbets too much!!!
- Share what you’ve learned with friends, family, and on social media to get the word out about chytrid
TL;DR (not judging you if you skipped straight here)
- Chytrid is a disease caused by the fungi Bd and Bsal
- It affects all amphibians but is especially devastating to frog species
- More than 500 species on 6 continents are declining in numbers due to it
- The fungus originated in Korea but spread through a mixture of human intervention (mostly trade) and natural means
- Translocation and repatriation is a common technique to save species from chytrid but isn’t as straight-forward as it might seem
- Bioaugmentation of species by treating them with Bd resistant probiotics could be a good solution but there is still a lot we don’t know about it
- Antifungals are not feasible as a solution on their own, but paired with chemical disinfection of the environment it could clear habitats of Bd and maintain it for years to come
- The chemicals would need to be reapplied regularly and there are also some potential risks to using them that must first be researched
- Fungal vaccines are hard to make but there’s some promising data for the efficacy of them on post-metamorphic frogs
- Immunity can come either naturally or with the help of selective breeding. We don’t have much experience doing that on amphibians so aren’t yet sure if this will be possible
- Some conservation groups are already manipulating the environment to make conditions less favorable to Bd growth. Raising the temperature could kill Bd but it might also cause a frog migration North which would further spread the disease
- Trade regulations are ultimately what are going to decide whether the spread of chytrid is slowed. We can try and treat animals that are already infected, but if we don’t stop the fungus from coming into their habitats in the first place we’ll always be a step behind the disease.
🐸🍴 Here are the froggy food videos I promised (heads up, they’re kind of disturbing. Apparently frogs aren’t very good at keeping food in their mouth):
📚🤓 If you’re a nerd like me and thought this was super interesting, I’d encourage you to read this paper. It’s the best research paper I’ve read on chytrid (and trust me, I’ve read A LOT!)
🛑 And before you go, I’m Klara — a 14 y/o student studying all things science and tech at The Knowledge Society 🧠 Everyday, I make it a goal to learn something new and share it with others around me. If you want to grow your knowledge along with me, be sure to follow me here on Medium, connect on LinkedIn, or get in touch with me at email@example.com! Also subscribe to my newsletter to get monthly updates on what I’ve been up to and cool resources I’ve found! ✌