Washing infected frogs in an anti-fungal drug bath reduced mortality rate and extended lifespan of population, and could buy valuable time to save species from extinction, research shows

Scientists have for the first time found a successful short-term treatment for amphibians infected with a deadly fungus in the wild.

Although the treatment would not save them from being reinfected and dying at a later date, it could “greatly extend” the time needed to save an amphibian population from extinction in the face of epidemic disease, according to the study led by scientists from the Zoological Society of London and published in the journal Biological Conservation.

A team of researchers on a last-ditch mission to save the critically endangered mountain chicken frog (Leptodactylus fallax) from extinction on Montserrat found that washing amphibians affected with chytridiomycosis in an anti-fungal bath reduced the mortality rate and extended the lifespan of the population by more than a year.

In early 2009, a global pandemic ravaging amphibian populations worldwide and caused by the fungus Batrachochytrium dendrobatidis reached Montserrat and reduced the population of tens of thousands to around 200 within months. In August the same year, scientists tried to save the last few hundred frogs - which are found only on the islands of Dominica and Montserrat - at their last remaining site in a ravine.

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Over 16 weeks, the team treated 80 frogs individually for five minutes at a time by washing them in a bag containing the anti-fungal drug itraconazole. Another group was washed with only water and another left untreated. The scientists were in the middle of assessing the impact of the treatment when the Soufriere Hills volcano erupted and the team was evacuated. When they returned, only two frogs were found to be remaining.

However, the scientists had managed to establish that while being treated, the anti-fungal bath stopped infection and increased the probability of survival. But as soon as the treatment was stopped and tagged frogs released back into the wild, the mortality rate returned as the frogs were reinfected by the presence of the fungus in the environment and by other amphibian species.

Michael Hudson, lead author of the paper, said this was the first time anything had been done in the field to slow the mortality rate. “This method represents a valuable addition to the currently sparse toolkit available to conservation scientists who are trying to combat the spread of chytrid in the wild.

“The treatment explored in this paper could be used to buy precious time in which to implement additional protective measures for at-risk amphibian species.”

Population modelling estimated that anti-fungal treatment would extend the time to extinction of the population from 49 to 124 weeks.

In this way, the paper says, in-situ treatment of individuals could be a useful short-term measure to augment other conservation actions for amphibian species threatened by chytridiomycosis or to facilitate population survival during periods of high disease risk.



“While the decrease in mortality looks modest, it’s actually very big in terms of the population life span,” Hudson said. “By treating these animals, an increase in survival rates means that if you applied it to the whole population, it would last an extra 75 weeks. This would would allow us to intervene and implement further conservation measures which might include other treatments and buy us extra time to take them into captivity.”

Chytridiomycosis has so far infected more than 600 amphibian species globally, causing population declines, extirpations or extinctions in more than 200 species. It spreads via spores and affects the skin of amphibians — through which many drink and breathe — leading to cardiac arrest.

Scientists have described it as “the worst infectious disease ever recorded among vertebrates in terms of the number of species impacted, and its propensity to drive them to extinction”.

While captive breeding programmes offer some hope, the International Union for Conservation of Nature currently estimates that even with the global efforts, only around 50 species could potentially be saved from extinction. Proven, field-based methods will therefore play a vital role in mitigating the risk posed by this disease, the paper says.

Last year US scientists made a breakthrough when they found it was possible for a certain type of toad to acquire immunity to the fungus. Another pioneering study used bacteria from frogs in Belize to test probiotic treatments.