17 July 2021 | New Scientist | 43America. “There are increasingly global bans
on movement of amphibians that are known
to be vectors of chytrids, and that appears to
be a very powerful way of stopping further
spread of Bd and Bsal,” says Fisher.
Export bans won’t have much effect in
places where chytrid is already present
though. That’s why people are also trialling
new methods of controlling the fungus to halt
the further decline of amphibian populations.
But there are no easy options.
One disease control strategy is to cull
animals so that population density is
reduced and disease can’t spread as rapidly.
This could be applied to groups of frogs that
are especially susceptible to chytrid to stop
them passing it to other species. Brian
Drawert at the University of North Carolina,
Asheville, and his colleagues have modelled
how this might play out. They found that
culling enough animals to significantly
curb the spread raises the probability of
population collapse to intolerable levels.
Not a promising strategy, then.
A better idea could be to give frogs
probiotics. We have discovered that some
chytrid-tolerant frogs have a mix of bacteria
on their skin that can fight off the fungus.
Some conservationists have mooted the
idea of giving vulnerable frogs a dose of
these defensive bacteria. This might confer
more enduring protection than provided
by antifungals, but it would still involve
the difficult task of finding and treating
frogs in the wild.
A fighting chance
Another line of defence involves genetics
and amphibian immunology. We know that
certain chytrid-resistant frogs gain some
level of protection from their genes, but
don’t fully grasp how this works. Anna
Savage at the University of Central Florida
is one of those trying to find out. She is
conducting gene-editing experiments to
tease out what genetic changes might give
the animals more of a fighting chance.
Savage has found that the frogs worst
affected by Bd have an unexpectedly strong
immune reaction to it. “The frogs that are
dying are the ones that have super cranked up
immune responses,” she says. “The ones that
are surviving are actually pretty much shutting
down their immune cell expression.”
There is a sense in which this is hopeful.
It shows that we don’t understand much
about how amphibian immune systems
work, so research could unearth new, helpful
insights. Savage points out that frogs seem
to get infected with diseases very often.
Indeed, it isn’t just chytrid that is ailing our
frogs. News broke in June that severe perkinsea
infection, a deadly disease that causes bloating
in tadpoles, has been identified for the first
time in the UK. “Maybe there’s something
fundamentally different about amphibians’immune systems that we need to understand
to really figure out why they are suffering
from diseases at a higher rate than most
other organisms,” says Savage.
For Kolby, the most worrying thing
about the chytrid panzootic is that it could
be just the first example of a disease outbreak
created by blithely moving animals around
the world. “It took us decades to even realise
frogs were suffering from a pandemic.” He
reckons it could already be happening again.
We certainly know there are other dangerous
animal diseases out there (see “Animal
outbreaks”, opposite).
Meanwhile, we are finding out what happens
when large numbers of frogs are lost from an
ecosystem. It isn’t pretty. Disease-causing
mosquitoes and other insects aren’t being
eaten as much. With less prey, animals like
snakes and birds are starting to face declines
as well. “You know,” says Savage, “a world
without frogs would be a very sad world.” ❚Krista Charles is an intern
at New Scientist“ We’re finding out what
happens when you
remove large numbers of
frogs from an ecosystem.
It isn’t pretty”
ROLF^
NUSSBA
UMER
/NATU
RE
PLBullfrogs, like
this one in Texas,
can be killed by
chytrid fungus,
especially when
they are young