Mind Control by Parasites

By Bill Christensen

Half of the world's human population is infected with Toxoplasma, parasites in the body—and the brain. Remember that.

Toxoplasma gondii is a common parasite found in the guts of cats; it sheds eggs that are picked up by rats and other animals that are eaten by cats. Toxoplasma forms cysts in the bodies of the intermediate rat hosts, including in the brain.

Since cats don't want to eat dead, decaying prey, Toxoplasma takes the evolutionarily sound course of being a "good" parasite, leaving the rats perfectly healthy. Or are they?

Oxford scientists discovered that the minds of the infected rats have been subtly altered. In a series of experiments, they demonstrated that healthy rats will prudently avoid areas that have been doused with cat urine. In fact, when scientists test anti-anxiety drugs on rats, they use a whiff of cat urine to induce neurochemical panic.

However, it turns out that Toxoplasma-ridden rats show no such reaction. In fact, some of the infected rats actually seek out the cat urine-marked areas again and again. The parasite alters the mind (and thus the behavior) of the rat for its own benefit.

If the parasite can alter rat behavior, does it have any effect on humans?

Dr. E. Fuller Torrey (Associate Director for Laboratory Research at the Stanley Medical Research Institute) noticed links between Toxoplasma and schizophrenia in human beings, approximately three billion of whom are infected with T. gondii:

* Toxoplasma infection is associated with damage to astrocytes, glial cells which surround and support neurons. Schizophrenia is also associated with damage to astrocytes.
* Pregnant women with high levels of antibodies to Toxoplasma are more likely to give birth to children who will develop schizophrenia.
* Human cells raised in petri dishes, and infected with Toxoplasma, will respond to drugs like haloperidol; the growth of the parasite stops. Haloperidol is an antipsychotic, used to treat schizophrenia.

Dr. Torrey got together with the Oxford scientists, to see if anything could be done about those parasite-controlled rats that were driven to hang around cat urine-soaked corners (waiting for cats). According to a recent press release, haloperidol restores the rat's healthy fear of cat urine. In fact, antipsychotic drugs were as effective as pyrimethamine, a drug that specifically eliminates Toxoplasma.
Are parasites like Toxoplasma subtly altering human behavior?

Still not sure that parasites can manipulate the behavior of host organisms? Consider these other cases:

* The lancet fluke Dicrocoelium dendriticum forces its ant host to attach to the tips of grass blades, the easier to be eaten. The fluke needs to get into the gut of a grazing animal to complete its life cycle.
* The fluke Euhaplorchis californiensis causes fish to shimmy and jump so wading birds will grab them and eat them, for the same reason.
* Hairworms, which live inside grasshoppers, sabotage the grasshopper's central nervous system, forcing them to jump into pools of water, drowning themselves. Hairworms then swim away from their hapless hosts to continue their life cycle.