Toxoplasma gondii rewires a mouse's brain, eliminating its fear of cats and making it an easy target for predation.

Toxoplasma gondii is a protozoan parasite that requires the digestive tract of a felid—such as a domestic cat—to undergo sexual reproduction. Research published in journals like PLOS ONE and Nature has demonstrated that the parasite forms cysts in the mouse brain, specifically targeting the amygdala, the region responsible for processing fear and emotional responses. By altering neurotransmitter levels, particularly dopamine, the parasite effectively reprograms the mouse's behavior.A landmark study led by Wendy Ingram at the University of California, Berkeley, revealed that this loss of fear persists even after the parasite is cleared from the body, suggesting the parasite causes permanent structural or epigenetic changes in the host's brain. This phenomenon is known as the "manipulation hypothesis"—a strategy where parasites alter a host's phenotype to increase their own transmission success. While the mouse ultimately loses its life, the parasite successfully returns to a feline host to reproduce and lay its eggs, or oocysts.These oocysts are shed in cat feces and can survive in the environment for months. Humans can also become infected through contact with cat litter or contaminated soil, a condition called toxoplasmosis. While typically harmless to healthy adults, it poses significant risks for pregnant women and immunocompromised individuals. Some controversial studies have investigated whether the parasite influences human personality traits or risk-taking behaviors, though these links remain hotly debated within the scientific community.