Killer skin fungus mapped in all frog hotspots in India

Something wicked this way comes for amphibians in India?

The chytrid fungus, which has spelt doom for amphibian populations across the globe, has been detected in frogs across all major biodiversity hotspots in India, including the Himalayas where it was found to be more pervasive, a study has reported.

The killer skin fungus Bd, short for Batrachochytrium dendrobatidis has driven more than 200 amphibian species to extinction or near-extinction across the world.

Bd is a notifiable disease as per World Organisation for Animal Health (OIE) and every country has to report on the status of this aquatic disease.

Spread around the globe primarily through the pet trade, the pathogen is notorious for causing massive die-offs such as those in El Cope in Panama in 2004 where “dead frogs littered the forest floor.” The fungus invades amphibians’ skin, which the animals use to breathe and take up water.

Bd’s origin has been traced to the Korean Peninsula, where it likely evolved in the beginning of the 20th century.

A countrywide survey

Indian scientists have documented for the first time the status of the disease in India. Through genetic analyses they have shown that Bd is found in frog populations across India – in the Western Ghats, Eastern Ghats, Himalayas, hills of the northeast and Andaman and Nicobar Islands.

Using samples collected over a period of six years, representing around 25 percent of India’s total frog species, they found that Bd fungus is present at low levels across the hotspots, but showing high diversity of genetic variants.

“We found that there are 57 haplotypes (genetic variants of the pathogen) and estimate that there could be over 150 haplotypes of Bd in frog populations in India. Highest prevalence of Bd was recorded from the Himalayas and it also had the highest number of haplotypes,” Karthikeyan Vasudevan, lead author of the study and scientist in-charge at LaCONES, told Mongabay-India.

LaCONES is the Laboratory for the Conservation of Endangered Species under Centre for Cellular and Molecular Biology, located at Hyderabad in Telangana.

The study emphasised that “Bd fungus is present in all the frog hotspots in India,” and it needs to be matched with a heightened level of surveillance in the region.

The study said: “Trade of frogs in India ceased after a ban imposed on the collection and export of frogs. There is no known record of the import of frogs into India. Migratory birds, crayfish, and movement of researchers are some of the possible sources that might have seeded Bd fungus into India.”

The fungus is most prevalent in the Indian dancing frog family Micrixalidae (24 species), night frogs family Nyctibatrachidae (36 species) and leaping frogs family Ranixalidae (14 species), all of which contain species that are all narrowly endemic to different parts of the Western Ghats.

But Asia being a ‘coldspot’ (as opposed to a hotspot with high diversity) for Bd infection, the frogs have low grade infections and do not succumb to them.

A delicate equilibrium

The high number of genetic variants combined with low prevalence of infection presented a puzzle that prompted the researchers to examine the different push and pull factors acting on Bd, from an evolutionary point of view.

The researchers reckon that it is a case of one organism trying to dodge the other.

“When there is a pathogen and there is a host that is trying to escape the pathogen infection, then the pathogen will try to mutate and change. There is something prompting the fungus to constantly change so unless there is a pressure why would the pathogen be constantly in such different forms? If one form is effective, they will survive as one effective form,” Vasudevan explained.

Vasudevan argued if the ‘selection pressure’ hypothesis holds true then Asia might harbour several endemic strains of Bd that might provide clues to why frogs do not get infection and die in this part of the world.

“With this knowledge, the infections that cause mortality can be tackled to save several species of frogs that are on the decline or at the brink of extinction,” he said.

The data highlights the need for sensitive and specific assays for detection as conventional assays show limitations in detecting different haplotypes.

“We need to be more careful about quarantine measures. We need surveillance and we need quarantine. We need better assays and methods,” said Vasudevan.

Not just frogs and toads

It has been described as “a peculiar twist of fate”, that regions that are home to the world’s greatest amphibian diversity are also most suitable for Bd to thrive.

Another study on the pathogen has reported “widespread but low intensity infection of Bd” in the Western Ghats and is the first record of the fungus in caecilians (limbless, serpentine amphibians) in India. The infection has also been observed in the critically endangered Amboli toad (Xanthophryne tigerina) and in the endangered white-lipped cricket frog (Fejervarya cf. sahyadris). 

Bd was first discovered in the Western Ghats biodiversity hotspot, designated one of the eight most important global hotspots and one of the three most threatened by population growth, in 2011. 

In comparison with earlier studies on the Bd fungus in the Western Ghats, the present study brings low elevation sites in the Western Ghats in its ambit. 

According to lead author Christopher J. Thorpe, a postgraduate research student in the University of Plymouth’s School of Biological and Marine Sciences, it is important for those writing conservation policy in India to know if the species already infected are rare or threatened.

The caecilian result is significant because given their habitat preferences one would consider them safe from the infection. 

“They live predominantly underground, although in the rain they can sometimes be found above ground at night. You might therefore expect them to be safe from infection if the transmission is by contact with an infected individual or if it is swimming in a pool of water containing spores,” Thorpe said. 

How they get infected is not known, but the information may be helpful in investigating how the pathogen is transmitted, he noted. 

The study also draws attention to the link between the infection and elevational temperature changes, thermal exclusion, inter-site connectivity and anthropogenic disturbance. Coastal plateaus may be ‘thermal refuges’ from Bd, it said.

Expanding on the observation, Thorpe said Bd has an optimum temperature range and temperature in the area above the Western Ghats escarpment is close to its preferred temperature range. 

“So the amphibians living above the escarpment are at greater risk if the Bd becomes lethal. This is also the area with most rare species (endemics). Our study supports this by finding higher infection intensity above the escarpment. Bd dies above a certain temperature, it also dies if it dries out (desiccates). We found lower intensity infection where the temperature was higher (in the coastal area),” Thorpe said. 

For a fungal infection to develop into the lethal chytridmycosis, it requires a high spore count on an individual. Amphibians in the coastal area may remain below the lethal threshold and survive.

The research also underscores the impact of landscape on amphibian distribution in the Western Ghats.

Rapid changes in elevation create barriers to amphibian movement. Even if the coastal area does create an amphibian refuge, it is likely it will only protect the species already found there – species found above the Western Ghats escarpment cannot migrate there across an elevation barrier.

“Migration is not really an option as the impact of a lethal infection may not cause migration and it is likely it would be too rapid for migration,” Thorpe said.

“How Bd is transmitted is not well understood but there is evidence that infection is higher near human settlements. Therefore, published information relating infection levels in a new place (India) is useful for the wider investigation of Bd transmission vectors,” he added.

Researcher Varad Giri who mainly works on the conservation of caecilians and lizards of the Western Ghats, and was not associated with the studies, stressed that the findings are significant as they shed light on the status of the fungal pathogen in India against the backdrop of discovery of new amphibian species in the country.

“As we discover new species we must also step up surveillance. Studies like these are extremely significant in understanding the status of a pathogen with respect to particular species. Species discovery is the beginning of a conservation process and monitoring vulnerabilities such as disease threats ties in with conservation,” Giri told Mongabay-India.

Echoing Vasudevan and Thorpe, Giri also batted for putting Bd spread in India under the scanner.

“We should not stop here. Based on the published data, we need to continuously monitor the spread of the pathogen strains. We should try to ascertain the impact of the fungus on populations now-whether it is restricted to certain population, which are the populations that are highly affected, so on and so forth. Many species are restricted to a small geographical location and if this Bd comes the they will be wiped out,” warned Giri.

CITATION:

Mutnale, M. C., Anand, S., Eluvathingal, L. M., Roy, J. K., Reddy, G. S., & Vasudevan, K. (2018). Enzootic frog pathogen Batrachochytrium dendrobatidis in Asian tropics reveals high ITS haplotype diversity and low prevalence. Scientific reports, 8(1), 10125.

Thorpe, C. J., Lewis, T. R., Fisher, M. C., Wierzbicki, C. J., Kulkarni, S., Pryce, D., … & Knight, M. E. (2018). Climate structuring of Batrachochytrium dendrobatidis infection in the threatened amphibians of the northern Western Ghats, India. Royal Society Open Science, 5(6), 180211.