2 November 2019 | New Scientist | 39to only the sickest animals, leaving healthier
ones to use their own immune system
to keep parasites in check. As a result, the
conservationists aren’t just working to save
one endangered species: a 2017 analysis
identified 36 species that called woylies
home. Eleven of these, including a previously
unknown tick called Ixodes woyliei, live
exclusively on woylies, and would blink out of
existence if their host were to become extinct.
The woylie efforts are part of a growing trend
among conservationists to be more mindful of
parasites. “We’re not just crazy nutcases – we
want to save the host species, too,” says
Hopkins. Teams trying to rebuild black-footed
ferret populations, for example, are recruiting
a single-celled parasite called Eimeria to help.
Others want to deliberately cultivate parasites
in captive Iberian lynxes. Finding the right
balance in different situations won’t be easy,
but if done correctly this new focus on parasite
conservation looks like a virtuous cycle. It
should help protect their hosts and entire
ecosystems, and the parasites.
Could this be the beginning of a makeover
for some of the world’s most despised and
misunderstood species? Nichols certainly
hopes so. “What we need are children’s books
about parasites, and pillows you sell for far too
much money on Etsy,” she says. “Parasites are
so stinking cool and gross.” ❚Carrie Arnold is a writer based
outside Richmond, Virginiasediment are less disturbed by burrowing
shellfish. The exposed cockles are rich pickings
for shorebirds and fish. And their shells offer
a new habitat for barnacles and other aquatic
species. By keeping cockle numbers lower
and more stable, C. australis does a huge
service to the ecosystem.
As well as disrupting ecosystems, the loss
of parasites could even be bad for their hosts.
When scientists gave anti-parasitic drugs to
Eurasian oystercatchers, they found that
treated chicks were more likely to die than
untreated ones. The researchers aren’t exactly
sure why, but they think the lack of parasites
may have interfered with the proper
functioning of the bird’s immune system.
Something similar is thought to happen in
humans. Known as the hygiene hypothesis,
some evidence shows that in our hyper-clean,
low-parasite modern world, our immune
system isn’t trained to deal with the worms
and bacteria with which our species evolved.
As a result, it overreacts to what should be
harmless targets, causing allergies and
autoimmune conditions. The presence of
parasites helps to redirect the immune system
away from the prolonged inflammation
indicative of these conditions, says Cizauskas.
The growing realisation that there is a
positive side to parasites has persuaded
some conservationists that we cannot simply
ignore such species and focus only on more
charismatic animals. Lafferty calls our current
attitude “taxonomic chauvinism”. It is much
easier to get donations to save cuddly pandas
and majestic elephants than stink bugs and
spiders, and parasites are at the bottom of the
list. “But there’s an intrinsic value to every
species. They’re all the result of millions of
years of evolution, and they have adaptations
that are remarkable,” he says.
Dobson notes that saving any species
means also saving its ecosystem – and you
can’t save that ecosystem without also saving
its parasites. But in the name of conservation,
we intentionally kill parasites. The California
condor louse isn’t an isolated example. Many
parasites have co-evolved to live exclusively
on a particular host and, in our efforts to save
other species including black-footed ferrets
and kakapos, we have deliberately driven
some of their parasites to extinction.
The answer isn’t as simple as forgoing anti-
parasitic drugs. Although parasites aren’t the
supervillains portrayed in many biology
textbooks, clearly they aren’t always heroes.
As we know to our cost, some are deadly.
Plasmodium, the parasite that causes malaria,
kills millions of people every year. So do
gastrointestinal parasites such as Giardia
and entamoeba. But we also know that most
parasites aren’t deadly. However, they are
most likely to kill a host that is undernourished
and stressed – conditions that a species
on the road to extinction often faces. So,
conservationists concerned about parasites
must tread a fine line, as those trying to help
the woylie have discovered.Good infestations
The number of woylies has plummeted as a
result of predation, habitat loss and climate
change. Andrew Thompson at Murdoch
University, Australia, suspects that these
external stressors have somehow altered
the marsupial’s relationship with its normal
parasites, explaining why woylies captured
for breeding programmes are often besieged
by multiple types of parasites, from intestinal
worms to itchy ticks and lice. For many
veterinarians, the instinct would be to blast
them with strong, anti-parasitic drugs to
eradicate these invaders. However, Thompson
and his fellow conservationists have
discovered that the right combination of
parasites can actually improve a woylie’s
chances of survival (see “Two parasites are
better than one”, left). Furthermore, they
worry that giving captive animals too much
anti-parasitic treatment might leave their
immune systems ill-equipped to handle
the normal range of parasites they encounter
if hey return to the wild.
So, instead of trying to expunge woylie
parasites altogether, the team doles out drugsFlukes that parasitise New
Zealand cockles are good for
mudflat biodiversity ...... but Giardia is far from
benign, killing millions of
people every yearSCOTT LEWIS/GETTY IMAGES AMI IMAGES/SCIENCE PHOTO LIBRARY