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  Deciphering the Tools of Nature’s Zombies By CARL ZIMMER Published:
December 5, 2012In the rain forests of Costa Rica lives Anelosimus octavius*
, *a species of spider that sometimes displays a strange and ghoulish habit*
.*

From time to time these spiders abandon their own web and build a radically
different one, a home not for the spider but for a parasitic wasp that has
been living inside it. Then the spider dies — a zombie architect, its brain
hijacked by its parasitic invader — and out of its body crawls the wasp’s
larva, which has been growing inside it all this time.

The current issue <http://jeb.biologists.org/content/216/1/1.full.html?etoc> of
the prestigious Journal of Experimental Biology is entirely dedicated to
such examples of zombies in nature. They are far from rare. Viruses, fungi,
protozoans, wasps, tapeworms and a vast number of other parasites can
control the brains of their hosts and get them to do their bidding. But
only recently have scientists started to work out the sophisticated
biochemistry that the parasites use.

“The knowledge that parasites can manipulate their hosts is old. The new
part is how they do it,” said Shelley
Adamo<http://myweb.dal.ca/sadamo/Adamo_Lab_-_Dalhousie_University/Home.html>
of
Dalhousie University in Nova Scotia, a co-editor of the new issue. “The
last 5 to 10 years have really been exciting.”

In the case of the Costa Rican spider, the new web is splendidly suited to
its wasp invader. Unlike the spider’s normal web, mostly a tangle of
threads, this one has a platform topped by a thick sheet that protects it
from the rain. The wasp larva crawls to the edge of the platform and spins
a cocoon that hangs down through an opening that the spider has kindly
provided for the parasite.

To manipulate the spiders, the wasp must have genes that produce proteins
that alter spider behavior, and in some species, scientists are now
pinpointing this type of gene. Such is the case with the baculovirus, a
virus sprinkled liberally on leaves in forests and gardens. (The cabbage in
a serving of coleslaw carries 100 million baculoviruses.)

Human diners need not worry, because the virus is harmful only to
caterpillars of insect species, like gypsy moths. When a caterpillar bites
a baculovirus-laden leaf, the parasite invades its cells and begins to
replicate, sending the command “climb high.” The hosts end up high in
trees, which has earned this infection the name treetop disease. The bodies
of the caterpillars then dissolve, releasing a rain of viruses on
unsuspecting hosts below.

David P. Hughes <http://ento.psu.edu/directory/dhughes> of Penn State
University and his colleagues have found that a single gene, known as egt,
is responsible* *for driving the caterpillars up trees. The gene encodes an
enzyme. When the enzyme is released inside the caterpillar, it destroys a
hormone that signals a caterpillar to stop feeding and molt.

Dr. Hughes suspects that the virus goads the caterpillar into a feeding
frenzy. Normally, gypsy moth caterpillars come out at night to feed and
then return to crevices near the bottom of trees to hide from predators.
The zombie caterpillars, on the other hand, cannot stop searching for food.

“The infected individuals are out there, just eating and eating,” Dr.
Hughes said. “They’re stuck in a loop.”

Other parasites manipulate their hosts by altering the neurotransmitters in
their brains. This kind of psychopharmacology is how thorny-headed worms
send their hosts to their doom.

Their host is a shrimplike crustacean called a gammarid. Gammarids, which
live in ponds, typically respond to disturbances by diving down into the
mud. An infected gammarid, by contrast, races up to the surface of the
pond. It then scoots across the water until it finds a stem, a rock or some
other object it can cling to.

The gammarid’s odd swimming behavior allows the parasite to take the next
step in its life cycle. Unlike baculoviruses, which go from caterpillar to
caterpillar, thorny-headed worms need to live in two species: a gammarid
and then a bird. Hiding in the pond mud keeps a gammarid safe from
predators. By forcing it to swim to the surface, the thorny-headed worm
makes it an easy target.

Simone Helluy of Wellesley College studies this suicidal reversal. Her
research indicates that the parasites manipulate the gammarid’s brain
through its immune system.

The invader provokes a strong response from the gammarid’s immune cells,
which unleash chemicals to kill the parasite. But the parasite fends off
these attacks, and the host’s immune system instead produces an
inflammation that infiltrates its own brain. There, it disrupts the brain’s
chemistry — in particular, causing it to produce copious amounts of the
neurotransmitter serotonin.

Page 2 of 2

Serotonin influences how neurons transmit signals. Dr. Helluy proposes that
the rush of serotonin triggered by the thorny-headed worms corrupts the
signals traveling from the eyes to the brain. Normally, an escape reflex
causes the gammarid to be attracted to the darkness at the bottom of its
pond. Thorny-headed worms may cause their host to perceive sunlight as
darkness, and thus swim up instead of down.

Whether humans are susceptible to this sort of zombie invasion is less
clear. It is challenging enough to figure out how parasites manipulate
invertebrates, which have a few hundred thousand neurons in their nervous
systems. Vertebrates, including humans, have millions or billions of
neurons, and so scientists have made fewer advances in studying their
zombification.

Most of the research on vertebrate zombies has been carried on a
single-celled parasite, Toxoplasma gondii. Like thorny-headed worms, it
moves between predators and their prey. Toxoplasma reproduces in the guts
of cats, which shed it in their feces.

Mammals and birds can pick up the parasite, which invade their brain cells
and form cysts. When cats eat these infected animals, Toxoplasma completes
its cycle. Scientists have found that Toxoplasma-infected rats lose their
fear of cat odor — potentially making them easier prey to catch.

Glenn McConkey of the University of Leeds and his colleagues have found a
possible explanation for how Toxoplasma wreaks this change. It produces an
enzyme that speeds the production of the neurotransmitter dopamine, which
influences mammals’ motivation and how they value rewards. Adding extra
dopamine might make Toxoplasma’s hosts more curious and less fearful.

But Ajai Vyas of Nanyang Technological University in Singapore has found
evidence that Toxoplasma simultaneously manipulates its hosts in other
ways. Infected male rats, he found, make extra testosterone. This change
makes the males more attractive to females, and when they mate the males
spread the parasite to females.

By causing male rats to make more testosterone, Toxoplasma may do more than
spread itself to other rats. Testosterone also tamps down fear. The
infected rats may thus become even less concerned when they pick up the
scent of a cat.

This research could potentially provide important clues about human
behavior. In the case of Toxoplasma, for example, humans can become hosts
if they handle contaminated cat litter or eat parasite-laden meat. Some
studies have linked Toxoplasma infection with subtle changes in
personality, as well as with a higher risk of schizophrenia.

Dr. Adamo, the co-editor of the journal’s new issue, thinks this new
science of “neuroparasitology” can offer inspiration to pharmaceutical
companies that are struggling to find effective drugs for mental disorders.
“A number of the big companies have given up on their neuroscience labs,”
she said. “Maybe the parasites can teach us something.”

She points out that the way parasites manipulate brains is profoundly
different from drugs like Prozac. “The way that a parasite goes about
changing behavior is not the way a neurobiologist would do it,” she said.

A typical drug focuses on just one type of molecule in the brain.
Parasites, on the other hand, often launch a much broader attack that still
manages to cause a specific change in their host. “Perhaps tweaking several
systems simultaneously might give better results than trying to hit one
particular system with a sledgehammer,” Dr. Adamo said.

But she added that she and other parasitologists barely understand those
zombifying tweaks. “All we know now,” she said, “is they have their own
ways.”

*To read the latest scientific information on how parasites manipulate
hosts (e.g., the fungi and flukeworms that turn ants into “zombies”), read
this special issue of The Journal of Experimental Biology: “Neural
parasitology: how parasites manipulate host
behavior<http://jeb.biologists.org/content/216/1?etoc>
.” *

*
*

*Martin*
–––––––––––––––––––––––––––––––––––––––––
Martin Weiss, PhD
Senior Scientist
New York Hall of Science
mweiss at nyscience.org
cell   347-460-1858
desk 718 595 9156

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