Monday, April 09, 2007

Self-Destruction Explained

The Boston Globe reported today (link) that the parasite Toxoplasma gondii (which causes toxoplasmosis in cats, mice... and humans - through changing litter boxes) modifies mice’s response to cats for its own benefit. Mice infected with this bacterium not only are less afraid of cats, they are positively attracted to cat urine.

This is because Toxoplasma gondii has a two-stage life cycle. It lives in mice, but can only reproduce in cats. The parasite can only spread when a cat eats an infected mouse. The bacterium

eliminates the mouse's instinctive fear for the smell of cat urine. But the Stanford researchers found that the process went still further -- transforming fear into active attraction. "It wouldn't be totally unbelievable if all it did was destroy the behavior, the aversion," said Robert Sapolsky, one of the study's authors. "But what it does, it creates a new behavior instead." The parasite preferentially targets the amygdala, the part of the brain that generates fear, and it reprograms the mice with remarkably few side effects. The effect applies only to cat urine and not other animals. The mice have normal levels of general anxiety and normal reactions to other instinctive fears, such as bright lights, ...

This story is astounding in its implications, and makes OOTJ wonder whether there are other parasites out there that do similar re-wiring. This parasite re-wires the mouse brain to improve the chance that the mouse will be killed and eaten by a cat. The parasite trumps the host’s interests in survival to improve its own chances of reproducing and spreading.

Hey, think how this could explain all sorts of self-destructive behaviors in people we know!

* Students who skip classes and textbooks and who put off studying until the night before the exam;
* Lawyers who procrastinate, file late, misappropriate and commingle funds
– maybe it’s the effect of a parasite that can only reproduce when the student is out of school of the lawyer has been disbarred. Perhaps the microbe can only thrive and spread through extra sleep and sunshine.

* People with drinking and drug problems
– maybe it’s being caused by a parasite that needs the toxins in the host’s bloodstream.

* Those of us who are a bit too plump for our own good
– maybe this is not a lack of will-power. Perhaps we are host to parasites that need a nice padding of fat to thrive. See?! It's not our fault!

The unnerving image of intestinal parasites is from the online journal Nature,

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