Parasites, hosts, and behavioral manipulation
A cunning little larva has finally reached adolescence and at last emerges from her watery home. She slithers along until she comes upon a big green monster that leaps and bounds over vast stretches of earth, boisterously singing as he gambols about. The bouncing behemoth swallows her and she soon takes residence in a large dark void inside his stomach.
There she harbors a grave malice matched in wickedness by few other creatures.
Her sinister moment comes only when she has grown old enough and large enough — she hastily releases her noxious poison into the beast, corrupting its mind and sending it into a suicidal frenzy. The creature’s once graceful leaping – now reduced to clumsy hurdling – sends it to its doom in a nearby watering hole, where it drowns in seconds. Our slithering protagonist exits from the rear of the spongy corpse, and lays its eggs in water that will soon bear a future generation of these dogged, brain-washing larvae.
The above may sound like a bad sci-fi movie or a Disney cartoon gone wrong; however, the phenomenon depicted (albeit somewhat melodramatically) is a real, natural occurrence. The parasitic worms Spinochordodes tellinii use crickets and grasshoppers as their hosts, and after developing in the host’s gut, the parasite manages to manipulate the insect’s behavior, against all natural instinct, causing a suicidal leap into a body of water, where the parasite ultimately reproduces. How does a tiny parasitic worm like S. tellinii manage to hijack the nervous system of a cricket and send it on a suicidal death leap?
It turns out that parasites often have a lot more to do with their host’s nervous system than we think. Though the exact mechanisms behind parasite-induced behavioral manipulations remain undiscovered, examples of this phenomenon abound.
One of the hottest new behavior manipulators is the infamous protozoan parasite Toxoplasma gondii. T. gondii is best known for its toxic effects on developing fetuses, hence the fear most pregnant women rightfully have of anything having to do with cat excrement (where the parasite lives after reproducing in the cat’s gut). Besides causing nasty birth defects, T. gondii has been shown to cause some very peculiar behavioral effects in some of its “secondary” hosts (felines are the “primary” hosts).
Ground breaking research in England and at Stanford has shown that when a rodent – a lab rat in these studies – is infected with the parasite, it not only shows reduced fear of cat urine (a powerful innate fear that most rodent species have), but a slight attraction to it!
Somehow, the parasite manages to float around in the rodent’s blood stream, get off at the right stop in the fear center of the rodent’s brain (the amygdala in this case), deactivate the “fear-of-cat-urine” module, and in a bizarre encore, pop over to the dopaminergic pathway and turn on the attraction switch when the rat smells cat urine, ultimately turning a deep, innate fear into a slight attraction. Of particular interest is the fact that diminished aversion to cat odor was found to be the only behavioral pattern affected by T. gondii infection (Vyas et al, 2007).
In other words, when the parasite infects an animal preyed on by felines it is able to alter a specific behavior in the animal in order to increase its chance of getting to the fertile reproducing ground that is the feline gut.
This would be akin to you finally shaking a life-long, debilitating fear of heights because of a microscopic one-celled organism you accidentally ingested in your morning cereal.
Interestingly, some recent stirrings do point to effects of T. gondii on human behavior. Most of this work concerns T. gondii’s “subtle effects on personality and psychomotor performance.” Differences between infected and uninfected individuals have been found relating to one’s ability to concentrate, superego strength, and even the likelihood of getting into automobile accidents. Furthermore, a statistical correlation between T. gondii infection and schizophrenia has been found in recent research. This correlation has been supported by research revealing that Toxoplasma gondii can increase dopamine levels in its host’s brain; dopamine is thought to be a central player in schizophrenia.
Though these data are still young, a possible underlying mechanism of behavioral manipulation by Toxoplasma gondii in both rats and humans may soon be found, and further research would need to explain how Toxoplasma gondii cysts “know” how to migrate to their host’s amygdala and “know” how to turn off some behaviors without affecting many other ones.
Ultimately, parasites may play a larger role in our psyche then we want to believe. A humbling thought, though not a very new one – massive steps in animal evolution have been attributed to a constant struggle against parasites (including the creation of the sexes), and the sheer magnitude of the immune system is a good indicator of this 3-billion-year-old reality.
“Neuro-Parasitologist” may very well be a new title in the ’10s. They certainly have their work cut out for them.