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- LONDON - Scientists have
discovered a parasite that inhabits rats and makes them feel a suicidal
attraction for cats. The parasite, which infects as many as one in five
rats, can also affect humans.
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- The parasite, nicknamed the love bug but scientifically
known as Toxoplasma gondii, an intracellular protozoan, infects the rodent's
brain, inducing an effect similar to Prozac so it becomes less fearful
of cats.
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- Once the infected rat is eaten by the cat, the parasite
is successfully transmitted to its definitive host, which ensures the completion
of the parasite's life cycle.
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- Humans may also be influenced by the parasite, which
is transmitted through eating raw infected meat or contact with cat feces,
according to the latest issue of Royal Society's Proceedings: Biological
Sciences.
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- "We believe that these results may explain the reports
of altered personality and IQ levels in some humans," said Dr. Manuel
Berdoy, who made the discovery along with Joanne Webster, a doctor, and
David Macdonald, a professor.
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- "Although we clearly represent a dead-end host for
the parasite, these symptoms represent the outcome of a parasite evolved
to manipulate the behaviour of another mammal," Dr. Berdoy said.
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- In Britain, 22% of the population have been found to
be host to the parasite. The problem is worse in France, where the infection
rate is up to four times higher.
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- Although speculative, Dr. Webster said other work had
linked the parasite to decreased IQ, hyperactivity and altered personality
profiles.
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- "Perhaps you can see a side effect in other hosts,"
Dr. Webster said, adding it was not known if the bug encouraged people
to be fond of their felines.
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- Nature contains other bizarre examples of mind control
by parasitic invaders. Dr. William Eberhard, of the Smithsonian Tropical
Research Institute in Costa Rica, described a wasp living inside a spider.
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- The parasitic wasp larva spends two weeks inside the
live spider, feeding on its body fluids. When the wasp is ready to pupate,
the spider builds a web consisting of a few lines that support a central
platform from which the wasp will eventually hang in a cocoon. Dr. Eberhard
believes the larva produces a drug inducing its host to weave the supportive
web, before killing and eating the spider.
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- One classic example of this kind of mind control concerns
Dicrocoelium dendriticum, the lancet liver fluke parasite that inhabits
the bile duct of cattle.
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- The parasite's eggs are released in cattle feces, where
they are ingested by a land snail. Once inside a snail, the parasite then
develops into a stage called cercaria, eventually released in mucus "slime
balls" on vegetation.
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- There the parasite gains entry into a second host, the
ant Formica fusca, which dines on slime balls. Within the ant, most of
the parasites head for the abdomen but a few make for the head, where they
tinker with the ant's behaviour, causing it to commit suicide.
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- When the temperature drops as evening approaches, infected
ants do not return to their nests. Instead, they climb atop grass blades
and other vegetation.
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- The ants wait to be eaten by browsing cattle, which prefer
to eat late in the evening or in the early morning. Then the parasite's
life cycle is completed.
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