UP IN the mountains of northern Majorca, a group of toads live in a series of rocky, rain-fed pools. They aren’t much to look at, with their spotted, greyish-brown skin and pale underbellies. But they are, in their own way, very special amphibians.
If you had visited the rocky pools a few years ago, you would have found the toads in an awful state: those not dead would have been lethargic, some with red, peeling skin. They were infected with a deadly fungus that has been careening through the world’s amphibians, and has already wiped out dozens of species of frogs, a group of animals that includes toads. The Majorca midwife toads are the only species of frogs in the wild in which the disease has been eradicated.
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Now there is a feeling among conservationists that we have to protect our remaining frogs before they croak their last. “It’s such a race against time, because as this fungus spreads further, it’s affecting more and more species,” says conservationist . “It’s not being controlled at all and we’re running out of clean habitat.”
Failing to put the brakes on this situation would be disastrous, and not just because frogs are beautiful animals that act as linchpins for their ecosystems. We have also recently learned first-hand how terrible it can be when animal diseases spill over into other species. Luckily, we are making progress. We know where this “frog pandemic” came from and – although it will be difficult – we are in the early stages of a global fightback.
We first realised frogs were in trouble in 1993, when several species started dying off at once in Queensland, Australia. Researchers quickly realised the frogs were all being killed by the same mystery disease, which we now call chytridiomycosis. It wasn’t until a few years later that the culprit was identified. In 1998, biologist , or Bd, and showed that it caused the disease.
“501
Number of amphibian species infected with the fungal disease chytridiomycosis”
“90
Number of amphibian species that have been driven to extinction by this disease”
“1
Number of wild frog populations in which the disease has been eradicated Source: ”
Once we knew about chytridiomycosis, we began finding more of it. Combing through old tissue samples in labs around the world revealed that the disease had been killing frogs here and there since at least the 1960s. We also began to see it pop up in more places around the planet and in other amphibians. It has now spread to every continent except Antarctica, making it a panzootic, the animal equivalent of a pandemic.
The offending fungus is part of a much broader group of aquatic fungi called chytrids. Most of them just feed on pollen or algae. The ones that affect frogs are a rare exception. Amphibians have thin, water-permeable skin, which they breathe through. The Bd fungus gets into the skin and begins to attack. We don’t fully understand why, but not every infected animal develops chytridiomycosis. Those that do get it experience problems with their nerves and with regulating their water and oxygen levels. “With amphibians, you have the full spectrum,” says Kolby, who runs the . “Some species nearly can’t become infected, they’re so resistant. Others absorb it like a sponge and die in five days.”
It is difficult to determine exactly how the disease causes death because infected frogs are often in remote places.”, there’s heart arrhythmias. So, essentially a fungal infection of the skin causes a heart attack,” says at the Zoological Society of London. After parasitising an amphibian, the Bd fungus goes through its reproductive cycle, releasing spores into the water, where they can latch on to other animals.
A closely related fungus called Batrachochytrium salamandrivorans, or Bsal, which also causes chytridiomycosis, mostly in salamanders and newts, was discovered in 2013. The most comprehensive study we have, published in 2019, found that Bd and Bsal have affected more than 500 amphibian species, 90 of which have already gone extinct. Some biologists say this makes them the most destructive pathogens the world has ever seen.
Can we fight back? We have plenty of creams that are effective at stopping fungal infections, so one early idea was to use these. In 2008, researchers from the National Museum of Natural Sciences in Madrid, the Zoological Society of London and Imperial College London decided to try this out in Majorca. They began capturing midwife toads and their tadpoles and treating them with the antifungal drug itraconazole. The researchers then drained the pools to get rid of any spores and allowed them to naturally refill with rainwater. But when they returned the following year, the toads were all infected again.
It turned out the Bd fungus had a trick up its sleeve. “The neat but scary thing about chytrid is that it’s an alternate saprobe, ” says Kolby. In other words, it can switch from being a parasite to getting its nutrition from decaying organic matter. This meant traces of chytrid could hide out in rocky crevices and feed on detritus while the pools were empty. When the toads came back, it could reinfect them.
The first eradication
Undeterred, the researchers treated the toads again, this time thoroughly disinfecting the pools too. In 2015, after monitoring the toad population for a few years, the team announced it had worked. Chytrid had been eradicated from the rocky mountain pools.
That first eradication has been the only one, so far. The midwife toads live in a small area. Many other frog populations are dispersed across huge distances, making them far more difficult to catch and treat. “Frogs don’t call in sick,” says at Imperial College London, who was part of the team working in Majorca along with Garner. “You’ve gotta find them. When they’re small, when they’re ill, they’ll hide. They are often in very remote places that you need helicopters to go to.” If disinfecting ponds sounds like an extreme measure, it is. The team only took this step in Majorca because the pools are regularly refilled with rainwater, so the disinfectants don’t cause lasting harm.
A glimmer of hope is that some frogs, such as the critically endangered mountain chicken frog, which is endemic to the Caribbean islands of Dominica and Montserrat, have responded positively to conservation efforts. These frogs had been driven to extinction by chytrid in Montserrat but after being bred elsewhere, were reintroduced to the island in 2019. Chytrid doesn’t survive well at high temperatures. So to keep the fungus away from the reintroduced mountain chickens’ new semi-wild enclosures, conservationists are attempting to make the frogs’ environment hotter, by removing tree cover and heating their ponds.
There is hope, too, in Panama. There, 12 species of frog had decreased in abundance after chytrid arrived in 2004. But a 2018 study showed that nine of these species were beginning to bounce back, possibly because they are evolving resistance to the fungus.
As well as trying to save frogs from chytrid, people have been trying to determine where it came from in the first place. This has already revealed some surprises that may help us fight the disease.
For a long time, the prevailing hypothesis was that this fungus came from Africa and spread globally in the mid-20th century. Unlikely as it sounds, this idea was largely based on the way that pregnancy tests worked before the invention of pee-on-a-stick home tests. Believe it or not, this involved frogs.
“Frogs don’t call in sick. They’re often in very remote places that you need a helicopter to get to”
The story begins with a British zoologist named Lancelot Hogben who studied hormones by injecting them into frogs. In 1930, he was working with the African clawed frog, a species then found abundantly in sub-Saharan Africa. He discovered that this frog would begin to lay eggs if certain hormones related to pregnancy were injected under its skin. By the 1950s, Hogben showed that the frogs could be used as a reliable human pregnancy test: inject a woman’s urine under the frogs’ skin and if the animal ovulated, then the woman was pregnant. Thanks to this discovery, the clawed frogs were exported widely from Africa for this use, and it seemed likely that chytrid came with them.
However, more recent evidence points to a different origin. By studying the DNA of chytrids, it has become clear that they have been around for millions of years. Various strains have long existed all around the world and amphibians lived more or less happily alongside their local strain – long exposure enabling them to build up resistance and avoid any potential harms caused by the fungus.
Amphibian apocalypse
We know that amphibians in South-East Asia are generally more resistant to the most dangerous strain of Bd, the one responsible for the panzootic, than amphibians are elsewhere. This suggests the panzootic strain originally came from South-East Asia. “We believe amphibians have been living with this particular chytrid for at least 40 million years,” says Fisher. “So, a very long time, but only in one part of the planet.” Frogs from this region were probably then exported, both to be pets and for lab experiments. This spread the South-East Asian strain of Bd to places where local amphibians had no resistance to it, and kicked off all the trouble.
The strain circulating most widely now appears to be a highly virulent hybrid between that initial Asian strain and another chytrid. In 2019, yet another new strain of Bd was discovered. There could be more. The parallels with the covid-19 pandemic, where travel has spread new virus variants around the world to devastating effect, are clear.
This suggests a course of action that could slow chytrid’s spread: tight controls or a ban on the export of amphibians. Although chytrid is found worldwide, there are pockets of clean habitat. For instance, Madagascar, a hotspot of amphibian diversity, is still thought to be free of the fungus. Bsal, the chytrid that mostly infects salamanders, seems to be confined to Europe for the moment. The UK, US, Canada and the EU all have legislation to control the movement of salamanders. According to the North American Bsal Task Force, . “There are increasingly global bans on movement of amphibians that are known to be vectors of chytrids, and that appears to be a very powerful way of stopping further spread of Bd and Bsal,” says Fisher.
Export bans won’t have much effect in places where chytrid is already present though. That’s why people are also trialling new methods of controlling the fungus to halt the further decline of amphibian populations. But there are no easy options.
One disease control strategy is to cull animals so that population density is reduced and disease can’t spread as rapidly. This could be applied to groups of frogs that are especially susceptible to chytrid to stop them passing it to other species. at the University of North Carolina, Asheville, and his colleagues have modelled how this might play out. They found that . Not a promising strategy, then.
A better idea could be to give frogs probiotics. We have discovered that . Some conservationists have mooted the idea of giving vulnerable frogs a dose of these defensive bacteria. This might confer more enduring protection than provided by antifungals, but it would still involve the difficult task of finding and treating frogs in the wild.
A fighting chance
Another line of defence involves genetics and amphibian immunology. We know that certain chytrid-resistant frogs gain some level of protection from their genes, but don’t fully grasp how this works. at the University of Central Florida is one of those trying to find out. She is conducting gene-editing experiments to tease out what genetic changes might give the animals more of a fighting chance.
“We’re finding out what happens when you remove large numbers of frogs from an ecosystem. It isn’t pretty”
Savage has found that the frogs worst affected by Bd have an unexpectedly strong immune reaction to it. “The frogs that are dying are the ones that have super cranked up immune responses,” she says. “The ones that are surviving are actually pretty much shutting down their immune cell expression.”
There is a sense in which this is hopeful. It shows that we don’t understand much about how amphibian immune systems work, so research could unearth new, helpful insights. Savage points out that frogs seem to get infected with diseases very often. Indeed, it isn’t just chytrid that is ailing our frogs. News broke in June that . “Maybe there’s something fundamentally different about amphibians’ immune systems that we need to understand to really figure out why they are suffering from diseases at a higher rate than most other organisms,” says Savage.
For Kolby, the most worrying thing about the chytrid panzootic is that it could be just the first example of a disease outbreak created by blithely moving animals around the world. “It took us decades to even realise frogs were suffering from a pandemic.” He reckons it could already be happening again. We certainly know there are other dangerous animal diseases out there (see “Animal outbreaks“, opposite).
Meanwhile, we are finding out what happens when large numbers of frogs are lost from an ecosystem. It isn’t pretty. Disease-causing mosquitoes and other insects aren’t being eaten as much. With less prey, animals like snakes and birds are starting to face declines as well. “You know,” says Savage, “a world without frogs would be a very sad world.”
Animal outbreaks
Amphibians aren’t the only animals battling a panzootic disease – one that has spread around the world
PAST THREATS
Rinderpest This viral disease caused the deaths of millions of cattle and buffalo around the world, resulting in famines in the 20th century. Following the development of a vaccine in 1960, there was an international effort to get rid of rinderpest. It was declared eradicated in 2011.
PRESENT THREATS
Avian influenza One strain of “bird flu”, called H5N1, is panzootic. It causes respiratory problems in birds, including domestic poultry and has a high mortality rate. It can also infect humans – 700 people are known to have contracted it since 2003 and about 60 per cent have died.
White-nose syndrome This disease, which affects bats, is caused by a fungus called Pseudogymnoascus destructans. Millions of bats have died of it in North America. It is also prevalent across Europe, although bats there are thought to have developed some resistance.
Newcastle disease Another panzootic that affects birds, including pigeons, the Newcastle disease virus is highly contagious. So far, however, there have been no cases of people catching it.
FUTURE THREATS
African swine fever Acute forms of this viral disease cause fever, depression and loss of appetite in pigs. It spreads easily and has recently been seen across Africa, Asia and Europe. Some think it could go global.
Sarcoptic mange A skin infection reported in thousands of individuals across almost 150 wild and domestic mammal species. It is caused by the mite Sarcoptes scabiei, which is also behind the itchy condition known as scabies in people. Sarcoptic mange may be the newest emerging panzootic in wildlife.
