The Return of the Puppet Masters + Parasites brainwash grasshoppers into death d

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The Return of the Puppet Masters

by Carl Zimmer

_http://scienceblogs.com/loom/2006/01/the_return_of_the_puppet_maste.php_

(http://scienceblogs.com/loom/2006/01/the_return_of_the_puppet_maste.php)

 

 

Are brain parasites altering the personalities of three billion people?

The question emerged a few years ago, and it shows no signs of going away.

 

 

I first encountered this idea while working on my book Parasite Rex. I was

investigating the remarkable ability parasites have to manipulate the

behavior of their hosts. The lancet fluke Dicrocoelium dendriticum, for

example, forces its ant host to clamp itself to the tip of grass blades, where

a

grazing mammal might eat it. It's in the fluke's interest to get eaten,

because only by getting into the gut of a sheep or some other grazer can it

complete its life cycle. Another fluke, Euhaplorchis californiensis, causes

infected fish to shimmy and jump, greatly increasing the chance that wading

birds will grab them.

 

 

Those parasites were weird enough, but then I got to know Toxoplasma

gondii. This single-celled parasite lives in the guts of cats, sheddding eggs

that can be picked up by rats and other animals that can just so happen be

eaten by cats. Toxoplasma forms cysts throughout its intermediate host's

body, including the brain. And yet a Toxoplasma-ridden rat is perfectly

healthy. That makes good sense for the parasite, since a cat would not be

particularly interested in eating a dead rat. But scientists at Oxford

discovered

that the parasite changes the rats in one subtle but vital way.

 

 

The scientists studied the rats in a six-foot by six-foot outdoor

enclosure. They used bricks to turn it into a maze of paths and cells. In each

corner of the enclosure they put a nest box along with a bowl of food and

water. On each the nests they added a few drops of a particular odor. On one

they added the scent of fresh straw bedding, on another the bedding from a

rat's nests, on another the scent of rabbit urine, on another, the urine of a

cat. When they set healthy rats loose in the enclosure, the animals rooted

around curiously and investigated the nests. But when they came across the

cat odor, they shied away and never returned to that corner. This was no

surprise: the odor of a cat triggers a sudden shift in the chemistry of rat

brains that brings on intense anxiety. (When researchers test anti-anxiety

drugs on rats, they use a whiff of cat urine to make them panic.) The anxiety

attack made the healthy rats shy away from the odor and in general makes

them leery of investigating new things. Better to lie low and stay alive.

 

 

Then the researchers put Toxoplasma-carrying rats in the enclosure. Rats

carrying the parasite are for the most part indistinguishable from healthy

ones. They can compete for mates just as well and have no trouble feeding

themselves. The only difference, the researchers found, is that they are more

likely to get themselves killed. The scent of a cat in the enclosure didn't

make them anxious, and they went about their business as if nothing was

bothering them. They would explore around the odor at least as often as they

did anywhere else in the enclosure. In some cases, they even took a special

interest in the spot and came back to it over and over again.

 

 

The scientists speculated that Toxoplasma was secreted some substance that

was altering the patterns of brain activity in the rats. This manipulation

likely evolved through natural selection, since parasites that were more

likely to end up in cats would leave more offpsring.

 

 

The Oxford scientists knew that humans can be hosts to Toxoplasma, too.

People can become infected by its eggs by handling soil or kitty litter. For

most people, the infection causes no harm. Only if a person's immune system

is weak does Toxoplasma grow uncontrollably. That's why pregnant women are

advised not to handle kitty litter, and why toxoplasmosis is a serious

risk for people with AIDS. Otherwise, the parasite lives quietly in people's

bodies (and brains). It's estimated that about half of all people on Earth

are infected with Toxoplasma.

 

 

Given that human and rat brains have a lot of similarities (they share the

same basic anatomy and use the same neurotransmitters), a question

naturally arose: if Toxoplasma can alter the behavior of a rat, could it alter

a

human? Obviously, this manipulation would not do the parasite any good as an

adaptation, since it's pretty rare for a human to be devoured by a cat.

But it could still have an effect.

 

 

Some scientists believe that Toxoplasma changes the personality of its

human hosts, bringing different shifts to men and women. Parasitologist

Jaroslav Flegr of Charles University in Prague administered psychological

questionnaires to people infected with Toxoplasma and controls. Those infected,

he

found, show a small, but statistically significant, tendency to be more

self-reproaching and insecure. Paradoxically, infected women, on average,

tend to be more outgoing and warmhearted than controls, while infected men

tend to be more jealous and suspicious.

 

 

It's controversial work, disputed by many. But it attracted the attention

of E. Fuller Torrey of the Stanley Medical Research Institute in Bethesda,

Maryland. Torrey and his colleagues had noticed some intriguing links

between Toxoplasma and schizophrenia. Infection with the parasite has been

associated with damage to a certain class of neurons (astrocytes). So has

schizophrenia. Pregnant women with high levels of Toxoplasma antibodies in their

blood were more likely to give birth to children who would later develop

schizophrenia. Torrey lays out more links in this 2003 paper. While none is a

smoking gun, they are certainly food for thought. It's conceivable that

exposure to Toxoplasma causes subtle changes in most people's personality, but

in a small minority, it has more devastating effects.

 

 

A year later, Torrey and his colleagues discovered one more fascinating

link. They raised human cells in Petri dishes and infected them with

Toxoplasma. Then they dosed the cells with a variety of drugs used to treat

schizophrenia. Several of the drugs--most notably haloperidol--blocked the

growth

of the parasite.

 

 

So Fuller and the Oxford scientists joined forces to find an answer to the

next logical question: can drugs used to treat schizophrenia help a

parasite-crazed rat? They now report their results in the Proceedings of the

Royal Society of London (press release). They ran the original tests on 49 more

rats. Once again, parasitized rats lost their healthy fear of cats. Then

the researchers treated the rats with haloperidol and several other

anti-psychotic drugs. They found that the drugs made the rats more scared. They

also

found that the antipsychotics were as effective as pyrimethamine, a drug

that is specifically used to eliminate Toxoplasma.

 

 

There's plenty left to do to turn these results into a full-blown

explanation of parasites and personalities. For example, what is Toxoplasma

releasing into brains to manipulate its hosts? And how does that substance give

rise to schizophrenia in some humans? And even if the hypothesis does hold

up, it would only account for some cases of schizophrenia, while the cause of

others would remain undiscovered. But still...the idea that parasites are

tinkering with humanity's personality--perhaps even giving rise to cultural

diversity--is taking over my head like a bad case of Toxoplasma.

 

and

 

Parasites brainwash grasshoppers into death dive

_http://www.newscientist.com/article/dn7927_

(http://www.newscientist.com/article/dn7927)