A parasite called Toxoplasma gondii has a unique mechanism to help it spread: “tricking” rats into delighting in the smell of its predator, cats. This is an important adaptation since this parasite can only sexually reproduce in the gut of cats, so the parasite needs to get the infected rats into the cats’ gut. Yeah, the cat eats the rat, and the parasite lives happily ever after.
Normally rats have a strong aversion to the scent of cat urine, but infected rats completely lose their aversion to cat pee but retain all their other fears and phobias.
“Toxoplasma affects fear of cat odors with almost surgical precision,” Vyas concluded. “A large number of other behaviors remain intact.”
What happens is that Tomoplasma infection induces cysts to form in the brain, specifically in the amygdala. This region of the brain is involved in fear response, and the cysts disrupt that normal response. In fact, it goes even further–the rats are actually *attracted* to the cat urine. The team is now examining how the parasite can target just one behavior so specifically.
Below: Representative scatter plots depicting the location of a control rat (C) and an infected rat (D) during trial, with respect to bobcat and rabbit urine.
Vyas et al. 2007. Behavioral changes induced by Toxoplasma infection of rodents are highly specific to aversion of cat odors. PNAS. Early Edition April 5th. (http://www.pnas.org/cgi/reprint/0608310104v1)
For more on this, check out Carl Zimmer’s post on The Loom.