Amphibians around the world are dying in huge numbers and humans should be worried: Not because it threatens your favourite frog-leg delicacy, but because the die-offs can mess up the ecological balance and allow dengue and malaria carriers a free run.
On 5 May, the United Nations (UN) published a report stating that one million plant and animal species are on the verge of extinction, more so than at any other time in human history.
According to the report, amphibians, or animals such as frogs and salamanders that can live on both land and in water, are among the worst-affected.
Take the case of the Panamanian golden frog native to El Valle de Anton, a town in Panama, South America, renowned for its rich biodiversity.
These frogs are taxi-cab yellow with dark brown splotches, and have enough poison on their skin to kill more than a thousand mice.
However, despite this powerful adaptation, the frogs are being killed off in the thousands by a strange fungal disease called chytridiomycosis.
In 2004, around the town of El Cope, close to El Valle, corpses of the frog began showing up along hills and streams, killed by the same disease.
Other amphibians are also facing similar extinction threats: Nearly 41 per cent of known amphibian species face extinction due to human-induced factors like habitat destruction and the climate change crisis.
Herpetologist Simon Kärvemo of Sweden’s Uppsala University told ThePrint how the disappearance of amphibians, the first vertebrates on land, “could have a huge impact worldwide”.
With their presence on Earth for more than 365 million years, he said, amphibians have fit into integral roles in the ecosystem.
Apart from controlling agricultural pests, Kärvemo added, they serve as a food source for reptiles, birds and mammals, whose populations may dwindle as a result of a decline in amphibian populations.
Importantly for countries like India, amphibians are crucial in regulating insect populations, including those of mosquitoes, he said, thus helping reduce the incidence of malaria and other vector-borne diseases.
What happens when frogs die
In 1977, at the Animal Welfare Conference in New Delhi, then Prime Minister Morarji Desai announced that the export of frog products from the country would be banned amid growing concern over the large-scale killing of the amphibian for trade.
Frogs maintain the ecosystem in balance by consuming nearly their own body weight’s equivalence in insects. Naturalist Humayun Abdulali estimated in the 1980s that the earning of every 35 paise in India’s export of frog legs prevented the destruction of nearly a kg of agricultural pests.
He cautioned that nearly 9,000 tonnes of insects could survive annually due to the killing of frogs, which could entail increased use of chemical insecticides and pesticides, considerably reducing the quality of food.
The official ban on frog-leg trade took effect in 1987, leading to some relief, but chytridiomycosis has emerged as a new global threat that worsens the existential crisis facing frogs and other amphibians.
The killer fungus wiping out amphibians
Chytridiomycosis is caused by a fungus named Batrachochytrium dendrobatidis (Bd), also known as the ‘amphibian chytrid fungus’.
Last year, National Geographic reported that Bd had led to the extinction or near-extinction of nearly 200 amphibian species.
In March, a study published in the journal Science revealed that chytrid had caused the decline of more than 500 amphibian species across five continents — the Americas, Europe, Africa and Australia — between 1965 and 2015.
This makes these die-offs the largest ever attributable to a single disease — far greater in number than the human deaths caused by the bubonic plague.
The fungus kills by growing on these animals, and throwing off their skin’s balance of water and salt. Since amphibians breathe through their skin, the fungal growth collapses their respiration, and eventually causes heart failure.
In the battle against Bd, only 12 per cent of the affected species have shown promising signs of recovery, and about 40 per cent still continue to decline.
Origins and spread of chytridiomycosis
While there are several lineages of Bd across the world, the lethal Global Pandemic Lineage or BdGPL was found to be closely related to those from Asia — particularly to strains from the Korean peninsula.
Researchers now believe that BdGPL spread worldwide in the last 120-150 years through trade and transportation links established across the continents — perhaps most amplified during the Korean war in the early 1950s.
The fungus is a new inclusion to habitats in many parts of the world. However, it still remains a mystery why the Asian amphibians do not show signs of mass mortality from the fungus.
Even if Asian frogs seem to be safe now, there is no certainty that they will continue to be, as researchers fear Bd might evolve into a bigger threat.
“Things are bad. Many kinds of chytrids can hybridise, and they can invade new areas and cause additional losses,” said Karen Lips, professor of biology at University of Maryland, whose work established the link between chytrid and frog extinction.
In 2016, research published in the journal Molecular Ecology noted that the native Brazilian strain of Bd could hybridise with the killer BdGPL. Last year, researchers showed that the African lineage of Bd could do the same.
This puts countries under more pressure to find out what strains of Bd exist within their borders and to understand their relationship with other lineages.
The Indian effort
India has a list of nearly 400, and growing, species of frogs, and lies at the intersection of four global mega-biodiversity hotspots. So, researchers here have the opportunity to explore the relationship between the pathogen and the host.
A team led by Karthikeyan Vasudevan at Laboratory for Conservation of Endangered Species (LaCONES), Centre for Cellular and Molecular Biology (CCMB), in Hyderabad is particularly focused on surveying Indian frogs for Bd.
In a paper published last year in the journal Scientific Reports, Vasudevan and his team noted that the prevalence of chytridiomycosis in Asia was poorly understood, with the exception of the understanding gleaned from a large-scale survey involving 15 countries in Asia.
So far, this is the largest study on Bd’s prevalence in India with samples representing nearly 25 per cent of India’s frog species collected over six years. It revealed widespread occurrence of Bd in several parts of India.
We do not yet have a place-wise census for Indian frog species to determine Bd’s effect. All the frogs in multiple sites have to be surveyed over seasons.
Vasudevan explained to ThePrint how his team used capture-and-recapture methods to assess frog populations across India.
“We need to tag each individual frog in an area. We do this using tags that have small transponders. These are injected onto the body of the animal,” he said.
“The frogs carry it like an ID. Whenever these are captured again, we use a reader, and note the ID,” Vasudevan added.
“A subset of the marked animals will be recaptured. And from that you can find out what the population is like. Every time we mark an individual, we also swab them,” he said. “From the swabs, we test them for Bd, which tells us whether these are in the state of being infected or not. When we recapture, we can see if the population’s state has changed during recapture or not.”
By directly sequencing the DNA from the swabs, Vasudevan and team found several smaller genetic variants of Bd in India that are less prevalent among frogs. However, further research is needed to understand why there are no mass deaths here like in the other five continents.
Questions that remain
We need to particularly understand if the numerous native Bd types found on Indian frogs are outcompeting BdGPL, or if our frogs have never encountered the killer lineage before. Culturing the fungus in labs, and analysing its complete genome is the key to getting more insights.
“We may still have to culture the fungus growing on sampled frogs to identify the strains involved in infection in India,” said Vasudevan. “If we culture the Bd fungus in the lab, on a growth medium, it allows us to do better analysis.
“We could get the whole genome of the Bd fungus and then compare it with the genome already read from several Bd strains across the world,” he added.
“With this, we can even find the time period when the diversification took place. We can use the molecular clock to say that the Indian Bd has split from other more infectious strains at a certain point in time,” he said.
When researchers monitor animal populations for pathogens, they try to understand if a potential epidemic would be gradual or sudden.
In this scenario, it is important to know how quickly a frog population gets infected with Bd, how fast they recover, and how these two factors balance against each other.
Gayathri Sreedharan, a PhD candidate working on chytrid at CCMB-LaCONES, explained further.
“In disease ecology, there are two kinds of host-pathogen dynamics — the epizootic and the enzootic. The epizootic dynamic is one where a disease suddenly breaks out and sweeps through a population as an epidemic,” she said.
“And enzootic dynamic is usually because of a gradual host-pathogen mutual understanding that keeps sustaining in the system, but somehow starts killing the host when the balance tips.”
Sometimes, the infection could go through a population at a high rate but the recovery could be quick. This could be why the fungus was not detected in some surveys. But this balance may not always exist, and the disease could gradually break out, she said.
“And if the deaths are gradual, we may need a decade of close watch to confirm whether a decline is happening,” she added.
Sreedharan said we needed information on the prevalence of Bd among tadpoles in India, since the fungus is aquatic in one of its life stages. “Our surveys still don’t tell us how many of the tadpoles are surviving and metamorphosing into adults, and Bd may be more lethal at this stage,” she added.
As the extinction clock ticks, India is barely making small inroads in understanding chytridiomycosis, and needs to step up with more surveys and culture studies, say researchers.
“The rest of the world is actually looking at India for inputs, but very little information is what they seem to be getting from us,” Vasudevan added.
Vishwam Sankaran is a freelance science writer based out of Chennai.