Parasites on the Brain

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Parasites on the Brain

Posted by Cynthia Wood on February 8th, 2006 at 10:54 am

 

Toxoplasma gondii may be the most prevalent human parasite. As many as

50% of humans worldwide, and up to 80% in urban areas, have been

infected with it at some time in their lives. An estimated 60 million

people in the US have active cases at any given time. It's a single

celled parasite whose favored host is cats. However it can infect and

live in a host of other creatures including rats and humans. Most

infected people, and most infected rats, show no particular signs of

illness when infected. They continue on with their daily life and work

completely unaware they've been parasitized. But they may not be as

unaffected as they seem.

 

Rat and CatThere are some interesting studies showing striking

behavior differences between rats that have been infected with

Toxoplasma and those that haven't. Normal rats are very reactive to

the smell of cat urine – an unsurprising survival instinct. If they

encounter cat urine in their environment they have an extreme fear

reaction, and they will avoid that spot thereafter. Rats infected with

Toxoplasma don't do this. They have no fear reaction to the smell of

cat pee; they don't avoid the areas where they smell it. In fact some

of the studied rats preferentially returned to the sites where they

had smelled the urine. It's hard to see how this could benefit the

rat, but easy to see how it could benefit Toxoplasma, which could

return to its preferred host to complete its life cycle if the rat

gets eaten.

 

Now consider that statistic from the first paragraph again. Up to one

half of humans worldwide are, or have been, infected with Toxoplasma.

Can something that affects the behavior of one mammalian host so

drastically have no effect on the other?

 

The answer has always been no for some people. A small minority of

people have strong psychological effects from toxoplasmosis, including

delusions, hallucinations, and paranoia. The Toxoplasmamajority of the

infected however, show no such drastic symptoms. Most people with

toxoplasmosis have no idea they're infected, but that doesn't mean

that they're unaffected. At Charles University in Prague,

parasitologist Jeroslav Flegr administered psychological tests to

people infected with Toxoplasma, and compared them to a control group.

He found alterations in the psychology of the infected individuals

that seemed to be gender-based. Infected men appeared more jealous and

suspicious. Infected women appeared more warm-hearted and outgoing.

Both sexes seemed to be more self-reproachful than the control group.

 

Those results are fairly subtle. E. Fuller Torrey of the Stanley

Medical Research Institute has found evidence of some that may be more

drastic. Toxoplasma is associated with damage to the brain's

astrocytes – glial cells that function as an interface between

neuronal and non-neuronal tissues. Astrocyte damage has also been

associated with schizophrenia. Now add in that pregnant women with

high levels of Toxoplasma antibodies are more likely to have children

who later develop schizophrenia, and you have something to give most

people pause. Torrey's study also found that some of the drugs used to

treat schizophrenia have an inhibiting effect on Toxoplasma growth.

 

What exactly the connection is between Toxoplasma and schizophrenia

has not been determined, though it seems clear there is one. What

alterations Toxoplasma makes to human psychology in general is even

more unclear. What is abundantly clear is that whatever those

alterations are, they affect a huge number of people. Can we afford to

be as ignorant of them as we currently are?

 

Further Reading:

Torrey article

 

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