Scientists hopeful of finding other amphibian species presumed to have been wiped out by chytrid fungus infection

Sightings of a toad thought to be extinct in Costa Rica have lead researchers to believe other isolated fragments of Central America's disappearing amphibians may survive in regions scourged by a deadly fungus.

The Costa Rican variable harlequin toad (Atelopus varius) was reported to have vanished completely from the country early last decade. But a small breeding population has been found hidden in a mountainous reserve.

A team of scientists led by José González-Maya studied the population between 2011 and 2013. Their report, recently published in the journal Amphibia-Reptilia, said the find was the fifth such "re-discovery" in Costa Rica and that more could be expected.

"Our findings fit into the emerging theory that species which have been presumed extinct are beginning to be re-discovered in Costa Rica and elsewhere."

González-Maya said that this pattern could redefine how scientists understood chytrid fungus, which has caused the extinction of over 200 amphibians and threatens hundreds more.

Map showing the spread of chytrid disease in Central America

"I think it is risky to say that many species are rebounding, I would say that there is a chance the fungus is changing and eventually some frog populations are also changing and somehow adapting or tolerating the infection."

González-Maya said it was likely other species currently considered extinct persisted in lonely relict populations. The small group of around 200 toads showed evidence that it was breeding and potentially recovering. The catchment had previously been infected by chytrid.

Little is known about the lifecycle of the fungus. González-Maya said the find may indicate it cannot persist in an area once it has eliminated the host population of frogs.

"We need to further investigate the natural history of the fungus in order to see how long it survives or how we can control it. If remaining populations are still out there oversighted by science, we may have a chance of recovering those if we understand how the fungus works."

Harlequin toads are among the amphibians most susceptible to the chytrid fungus, which is carried in water and soil. More than 90% of harlequins are considered threatened and three quarters are critically endangered.

Atelopus varius, known in the pet trade as the "clown frog", was once distributed across a wide area of Costa Rica and Panama. Before the 1990s, populations were hit hard by traders who collected large numbers of them for sale on the international pet market. Chytrid is now considered the main cause of their decline.

"In less than 20 years, known populations in Costa Rica declined from 100 to one," says the report. In Panama it has been a similar story, with only three remnant populations remaining.

Chytridiomycosis (the disease caused by the fungus) causes supple, complex amphibian skin to thicken and become impermeable to the transition of vital moisture and salts. The victims are, in effect, entombed in a coffin of their own skin. The International Union for Conservation of Nature has called the epidemic "the worst infectious disease ever recorded among vertebrates in terms of the number of species impacted, and its propensity to drive them to extinction."