A parasite reprograms rats to find cats attractive
Toxoplasma gondii needs to get inside a cat to reproduce. Its solution is to rewrite the rat's brain.
Place a healthy rat in a box with a corner that smells of cat urine, and it will avoid that corner and raise its stress hormones. Infect the same rat with Toxoplasma gondii—a single-celled parasite—and something different happens: the rat no longer avoids the corner. In some experiments the infected animal's neural activity shifts in ways consistent with attraction to feline odours.
The mechanism is not random brain damage. Toxoplasma encodes its own version of an enzyme involved in dopamine synthesis. Infected dopaminergic cells in rats release around 350 percent more dopamine than uninfected ones. The parasite has built what amounts to a remote control out of mammalian neurochemistry.
It does this because cats are the only hosts in which Toxoplasma can sexually reproduce. A fearless rat is more likely to be caught and eaten. When the cat consumes the rat, the parasite completes its life cycle. A rat that successfully avoids cats is an evolutionary dead end for Toxoplasma.
An estimated quarter of the world's human population carries Toxoplasma. For most people the infection appears asymptomatic. Researchers have found statistical correlations in large human cohorts—in personality profiles, in accident rates among infected drivers—suggesting the parasite may not be entirely neutral in humans either. The full implications remain contested and studied. Carl Zimmer introduced many readers to this territory in Parasite Rex, which mapped the strange ecosystem living inside us.