By Rachel Nuwerjan, Janauary 4, 2016, Science Daily
After a six-year effort, researchers on the Spanish island of Majorca have rid several groups of Majorcan midwife toads
of the pathogen Batrachochytrium dendrobatidis — better known as
chytrid fungus, or B.d. It’s the first time the disease, which is devastating amphibians worldwide, has been eradicated in a wild population.
“This
is proof of principle that you can go out there and mitigate infections
and that the method doesn’t need to be that complex,” said Trenton
Garner, a biologist at the Zoological Society of London, who reported the findings with his co-authors in the journal Biology Letters.
Described in 1998
following mass die-offs of frogs in Australia and Panama, B.d.
colonizes cells on the outer layer of an amphibian’s moist skin, causing
the skin to thicken and interfering with electrolyte transport. The
infection eventually leads to cardiac arrest.
An
extreme generalist, the fungus infects many types of frogs,
salamanders, newts and toads. It has been confirmed in 700 species on
six continents. Several are presumed to have been driven to extinction,
while many others have suffered catastrophic population declines.
Researchers
do not know why the fungus is so virulent; around 1,000 chytrid species
exist in the wild, but only two are known to infect vertebrates. “It’s
not a biologically sound strategy for a parasite to cause its host
communities to go into serious decline or drive its host to extinction,
but that is the case for this fungus,” Dr. Garner said. “It’s very
worrisome.”
Some
zoos and research labs have managed to clear up the infection in
captive populations, but until now no one has done so in the wild. The
lucky recipient of this experiment is the Majorcan midwife toad, a
species once thought to be extinct that was rediscovered in the 1980s in
several isolated ponds in the island’s limestone outcrops.
A
successful captive breeding effort allowed conservationists to expand
the native frog’s range (despite its name, Majorcan midwife toads are
not true toads). But in 2007, wildlife managers found that chytrid had
sneaked into the wild populations — likely introduced via the
captive-bred individuals meant to save the species.
In
2009, Dr. Garner and his colleagues began their efforts to clear the
frogs of the fungus. They focused on the tadpoles, which readily pick up
the infection in ponds where they live, but are not killed by it. When
the tadpoles metamorphose into frogs, however, mass mortality ensues.
The researchers removed thousands of tadpoles from five ponds and then
drained the ponds almost completely, hoping that the sun’s warmth would
kill the chytrid, which is sensitive to temperature.
Back
in the lab on Majorca, they bathed the tadpoles in antifungals and kept
them in captivity for months, until rains replenished the ponds.
Crossing their fingers, the researchers returned the fungus-free
tadpoles. But within a year, chytrid was back.
The
researchers were not dissuaded. They tried again in 2012, but instead
of just removing and treating the tadpoles, they also applied low
concentrations of a common commercial disinfectant to some of the ponds
and the rocky crannies around them.
The
following year, tadpoles and frogs in the three ponds that were
disinfected and whose tadpole residents were treated did not have any
signs of B.d. Three years on, those three ponds are still fungus free,
and the researchers have gone on to apply the treatment to the remaining
two ponds as well.
“They
set out to eliminate a major threat to the survival of a very special
frog, and they were successful,” said David Wake, an amphibian
specialist and evolutionary biologist at the University of California,
Berkeley, who was not involved in the research. “It took years, but it
worked — so far.”
The
hope now is that this success will raise interest in mitigation efforts
beyond Majorca. “The method is really cheap and easy to use, so why not
try it in other places?” said Jaime Bosch, a senior research scientist
at the National Museum of Natural Sciences in Madrid and one of the paper’s authors.
“We can’t just stand still and do nothing, watching amphibian after amphibian go extinct.”
Conditions
on Majorca, however, are particularly well suited to such
interventions, Dr. Wake said. Not only is the island isolated, but the
frogs breed in satellite sites largely cut off from one another, and
without other amphibian species hopping about that could reintroduce the
pathogen or complicate tadpole treatment.
“Doing such a study almost anywhere else might be dauntingly difficult,” Dr. Wake said.
The
special circumstances on the island, and the threat B.d. posed to the
protected Majorcan midwife toads, justified the use of fungicides in the
environment, said Deanna Olson, a research ecologist at the United
States Forest Service’s Pacific Northwest Research Station, who was not involved in the work. But it’s a strategy that some other scientists see as extreme.
“That
would be considered very controversial in a wider application,” Dr.
Olson said. “Eradication of B.d. in the wild has been discussed, but
implementation has been stalled due to the widespread effects of
antifungals on an extremely important component of ecosystems: fungi.”
More
work is needed to devise strategies for treating amphibians living in
different settings and to address concerns about antifungals, but Dr.
Garner believes that the new method could be tailored to more complex
environments.
“Arguing
that these methods could only apply with these chemicals and conditions
on Majorca limits thinking,” he said. “There are other ways to mitigate
fungal infections on large geographical scales, as we regularly show
with livestock, agriculture and human pathogens.”
B.d., however, is just one of a suite of amphibian worries. A new species of chytrid fungus,
recently detected in Britain and Germany, infects only salamanders and
newts, but appears to be extremely virulent, causing rapid population
decline. And a group of new viruses in Europe appears to kill all amphibians, and potentially reptiles, in its path.
Habitat
loss, however, remains the biggest threat, while climate change
exacerbates the problems caused by both disease and increasingly small
ranges. “Amphibians are confronting a lot of issues now,” Dr. Garner
said.
A
version of this article appears in print on January 5, 2016, on page D3
of the New York edition with the headline: Reprieve for Fungus-Battered
Frogs. Order Reprints| Today's Paper|Subscribe
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