What parasite, spread by cats, was found in a December 2025 study to make people more impulsive and aggressive?

The parasite (Review) identified in recent studies, including those published around December 2025, that has been linked to increased impulsivity and aggression in humans is Toxoplasma gondii. This microscopic organism is remarkably common, estimated to infect a significant portion of the global population, often without causing obvious physical symptoms in healthy adults. However, a growing body of research suggests its subtle influence on our brains can have notable behavioral consequences.

Toxoplasma gondii has a complex life cycle, with cats serving as its definitive host where it sexually reproduces. Infected felines shed oocysts in their feces, which can then contaminate soil, water, and food. Humans can become infected by inadvertently ingesting these oocysts, often through contact with cat litter, contaminated produce, or by eating undercooked meat from infected intermediate hosts like livestock. Once in the human body, the parasite forms cysts, predominantly in muscle and brain tissues, where it can lie dormant for years.

The mechanism behind the behavioral changes is thought to involve the parasite's ability to manipulate neurotransmitters in the brain, particularly dopamine. Studies highlight that Toxoplasma gondii can increase dopamine production, a "feel-good" hormone associated with reward and motivation. While dopamine is crucial for normal brain function, an imbalance can lead to altered behavior. This manipulation is believed to contribute to poor impulse control, heightened aggression, and even riskier decision-making in infected individuals. This understanding underscores the intricate ways parasites can influence host behavior, even in complex organisms like humans.