17 July 2021 | New Scientist | 41they are evolving resistance to the fungus.
As well as trying to save frogs from chytrid,
people have been trying to determine where
it came from in the first place. This has
already revealed some surprises that may
help us fight the disease.
For a long time, the prevailing hypothesis
was that this fungus came from Africa and
spread globally in the mid-20th century.
Unlikely as it sounds, this idea was largely
based on the way that pregnancy tests worked
before the invention of pee-on-a-stick home
tests. Believe it or not, this involved frogs.
The story begins with a British zoologist
named Lancelot Hogben who studied
hormones by injecting them into frogs.
In 1930, he was working with the African
clawed frog, a species then found abundantly
in sub-Saharan Africa. He discovered that
this frog would begin to lay eggs if certain
hormones related to pregnancy were injected
under its skin. By the 1950s, Hogben showed
that the frogs could be used as a reliable
human pregnancy test: inject a woman’s
urine under the frogs’ skin and if the animal
ovulated, then the woman was pregnant.
Thanks to this discovery, the clawed frogs
were exported widely from Africa for thisthem. When they’re small, when they’re ill,
they’ll hide. They are often in very remote
places that you need helicopters to go to.”
If disinfecting ponds sounds like an extreme
measure, it is. The team only took this step
in Majorca because the pools are regularly
refilled with rainwater, so the disinfectants
don’t cause lasting harm.
A glimmer of hope is that some frogs,
such as the critically endangered mountain
chicken frog, which is endemic to the Caribbean
islands of Dominica and Montserrat, have
responded positively to conservation efforts.
These frogs had been driven to extinction by
chytrid in Montserrat but after being bred
elsewhere, were reintroduced to the island
in 2019. Chytrid doesn’t survive well at high
temperatures. So to keep the fungus away
from the reintroduced mountain chickens’
new semi-wild enclosures, conservationists
are attempting to make the frogs’ environment
hotter, by removing tree cover and heating
their ponds.
There is hope, too, in Panama. There,
12 species of frog had decreased in abundance
after chytrid arrived in 2004. But a 2018 study
showed that nine of these species were
beginning to bounce back, possibly because >But when they returned the following year,
the toads were all infected again.
It turned out the Bd fungus had a trick up its
sleeve. “The neat but scary thing about chytrid
is that it’s an alternate saprobe, ” says Kolby. In
other words, it can switch from being a parasite
to getting its nutrition from decaying organic
matter. This meant traces of chytrid could hide
out in rocky crevices and feed on detritus while
the pools were empty. When the toads came
back, it could reinfect them.
The first eradication
Undeterred, the researchers treated the
toads again, this time thoroughly disinfecting
the pools too. In 2015, after monitoring the
toad population for a few years, the team
announced it had worked. Chytrid had been
eradicated from the rocky mountain pools.
That first eradication has been the only one,
so far. The midwife toads live in a small area.
Many other frog populations are dispersed
across huge distances, making them far more
difficult to catch and treat. “Frogs don’t call in
sick,” says Matthew Fisher at Imperial College
London, who was part of the team working in
Majorca along with Garner. “You’ve gotta find
“ Frogs don’t call in sick. They’re
often in very remote places that
you need a helicopter to get to”
Majorca midwife
toads (left) are
the only wild
population of
amphibians in
which chytrid has
been eradicated.
Biologists are
testing other
species, like this
dart frog (right), to
monitor the spread
of the disease QUEN
TINMART
INEZ/BIOSPH
OTO/ALAMY