The chorus is dying

A virulent fungal disease is killing frogs in what might be the sixth mass extinction

By Gayathri Vaidyanathan
Published: Monday 30 November 2009

Green tree frog THE frogs sit, serene and impassive, inside a tiny rainforest in the American Museum of Natural History in New York. A choir of croaks, whistles and chirps from the collection of brilliantly-hued amphibians gives the darkness a voice. But this chorus is dying.

In 2004, the International Union for Conservation of Nature (iucn) found 32.5 per cent of the amphibian species endangered. Frogs have been facing catastrophic declines over the past decades in what is being termed as the biggest extinction event in progress since the dinosaurs disappeared 65.5 million years ago.

This is a crisis situation. About a third of the amphibians are threatened or extinct, said Jamie Voyles, a researcher at James Cook University in Australia. Along with factors like habitat destruction and climate change, the drastic decline in frog populations is also attributed to the fungus Batrachochytrium dendrobatidis (Bd).

Voyles and her colleagues discovered the mechanism by which the Bd fungus operates. In a study published in the October 23 issue of Science, they identified the frogs skin as the key target. Frogs breathe and drink through their skin. When the fungus colonizes the skin, it chokes the frogsthe disease is chytridiomycosis.

A fatal skin infection
The fungus works by disrupting the electrolytic balance across the skin in green tree frogs (Litoria caerulea). The moisture-laden skin of amphibians is necessary for survival. A network of pumps allows the exchange of electrolytes such as sodium and potassium to maintain a chemical balance and help the skin breathe. When scientists measured the electrolyte balance in infected frogs, they found it had gone awry. The levels of sodium in blood and urine fell 50 per cent, and that of potassium went down 20 per cent. This led to cardiac arrest and subsequently the frogs died.

When Voyles placed infected frogs in an electrolyte-rich solution, she found the chemical balance got restored gradually. This was apparent as the sick frogs became more active even though the fungus was still there. Results suggested the disease was a skin infection that led to death.

Skin infection as a cause of death is unusual which is why it took so long for the mechanism to be identified. Normally skin infections are not lethal, but here there are no other problems except for infection of the skin, said Voyles. Earlier studies had looked at the mechanism of disease by autopsy but this is the first study that finds out how the fungus works in real time.

Questions unanswered
The fungus was discovered for the first time in the South African clawed frog (Xenopus laevis) in the 1930s. Researchers looking at large-scale frog deaths in Queensland in Australia traced the cause, posthumously, to the chytrid Bd fungus in 1993. There have been several such incidents in the last two decades endangering frogs around the globe. Until now, no one knew how the fungus killed. That question has finally been answered. Several, however, remain.

This fungus can attack a broad range of hosts. Whether it is the old fungal strain that has achieved a new level of virulence or a completely new strain is unknown. Why the frogs have suddenly become so susceptible to the fungus cannot be answered either, said Voyles. The fungus could be acting with other factors, but there are no toxins in the environment that can be detected, she said. This is not to say that toxins are not present for it could be that one is not looking in the right places, she added. Funding into frog research is limited and that hinders insight into the disease.

Practically speaking
It is difficult to protect frogs from the Bd fungus in the wild. The only remedy is bathing infected frogs in an anti-fungal medicine. Collecting each and every frog is not possible.

In Panama, the disease wiped out 90 per cent of the frogs in six months. They were saved from extinction because a team from Amphibian Ark (a collaboration of iucn and World Association of Zoos and Aquaria) captured a few and brought them back to a zoo in Atlanta in usa. Conservation efforts are concentrated on maintaining species in captivity. But there are several species that are endangered. Removal of frogsan essential part of the food chainwould spell doom for other animals such as snakes. As tadpoles eat algae, their disappearance would also lead to increased algal growth in water which would, in turn, choke other plant species.

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