40 | New Scientist | 17 July 2021when animal diseases spill over into other
species. Luckily, we are making progress. We
know where this “frog pandemic” came from
and – although it will be difficult – we are in
the early stages of a global fightback.
We first realised frogs were in trouble in
1993, when several species started dying off
at once in Queensland, Australia. Researchers
quickly realised the frogs were all being killed
by the same mystery disease, which we now
call chytridiomycosis. It wasn’t until a few
years later that the culprit was identified.
In 1998, biologist Lee Berger at James Cook
University in Australia and her colleagues
described the microscopic fungus
Batrachochytrium dendrobatidis, or Bd,
and showed that it caused the disease.
Once we knew about chytridiomycosis,
we began finding more of it. Combing through
old tissue samples in labs around the world
revealed that the disease had been killing
frogs here and there since at least the 1960s.
We also began to see it pop up in more places
around the planet and in other amphibians.
It has now spread to every continent except
Antarctica, making it a panzootic, the animal
equivalent of a pandemic.
The offending fungus is part of a much
broader group of aquatic fungi called
chytrids. Most of them just feed on pollen
or algae. The ones that affect frogs are
a rare exception. Amphibians have thin,
water-permeable skin, which they breathe
through. The Bd^ fungus gets into the skin
and begins to attack. We don’t fully understand
why, but not every infected animal develops
chytridiomycosis. Those that do get it
experience problems with their nerves
and with regulating their water and oxygen
levels. “With amphibians, you have the full
spectrum,” says Kolby, who runs the Honduras
Amphibian Rescue & Conservation Center.
“Some species nearly can’t become infected,
they’re so resistant. Others absorb it like a
sponge and die in five days.”
It is difficult to determine exactly how the
disease causes death because infected frogs are
often in remote places. “The one case where
they’ve actually looked at a cause of death,
AR there’s heart arrhythmias. So, essentially a
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fungal infection of the skin causes a heart
attack,” says Trenton Garner at the Zoological
Society of London. After parasitising an
amphibian, the Bd fungus goes through its
reproductive cycle, releasing spores into the
water, where they can latch on to other animals.
A closely related fungus called
Batrachochytrium salamandrivorans, or Bsal,
which also causes chytridiomycosis, mostly
in salamanders and newts, was discovered in- The most comprehensive study we have,
published in 2019, found that Bd and Bsal have
affected more than 500 amphibian species,
90 of which have already gone extinct. Some
biologists say this makes them the most
destructive pathogens the world has ever seen.
Can we fight back? We have plenty of creams
that are effective at stopping fungal infections,
so one early idea was to use these. In 2008,
researchers from the National Museum of
Natural Sciences in Madrid, the Zoological
Society of London and Imperial College
London decided to try this out in Majorca.
They began capturing midwife toads and their
tadpoles and treating them with the antifungal
drug itraconazole. The researchers then
drained the pools to get rid of any spores and
allowed them to naturally refill with rainwater.
501
Number of amphibian species infected
with the fungal disease chytridiomycosis90
Number of amphibian species that have
been driven to extinction by this diseaseFire salamanders live in
Europe, where chytrid
fungus is spreadingCHRISM
ATTISON/ALA
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1
Number of wild frog populations in
which the disease has been eradicated
Source: doi.org/gfxkrt