Scientists find cat parasite that fuels risky behavior in humans. Almost 60% of us have it

Anyone who has watched horror movies can imagine how ghosts crawl out of the dark, possess human bodies, and then make them do strange things. In real life too, there are tiny ghosts that, when they enter human bodies, can make them exhibit strange behaviors, like aggression and high-risk decision-making. Dubbed "Toxoplasma gondii," these parasites may be influencing the brains and behavior of up to 60% of people worldwide. Blame the cats, whose feces can release the parasite's egg-like oocysts into the environment. Researchers are investigating whether the parasite may alter dopamine levels in the brain, potentially affecting behaviors such as risk-taking and impulsivity, according to a study published in Frontiers in Psychiatry on May 27, 2025.

'Effects go beyond physical changes'

Parasitic infections have mostly been underestimated by the medical community, but research suggests that their effects can go beyond just physical changes. T. gondii, in particular, targets a person’s nervous system and throws their dopamine balance into disorder. T. gondii can form tissue cysts in the body, including the brain, where the parasite may remain for years. Researchers are investigating whether these cysts and changes in neurotransmitter activity could contribute to subtle behavioral differences, including increased risk-taking and impulsivity. 

Chemistry thrown off-balance

Most people with healthy immune systems have no symptoms, although some may experience flu-like symptoms, swollen lymph nodes, muscle aches, or pain. But deep within the innards of their physiology, T.gondii quietly releases a deluge of “feel-good” hormone, dopamine, into the body, which triggers the same feeling that the person gets from sleeping, eating, social interaction, or sexual activity. Having more dopamine should be an exciting experience, but in this case, it often makes people go out of control, causing them to display behaviors like poor impulse control, sexual aggression, and risky decision-making.

How it works

The parasite itself would have no life unless cats had it in their intestines during their reproduction period. While the cats reproduce, they create an egg-like structure called “oocysts” inside their digestive tracts. When the cat poops or pees, the oocysts are released into the soil, water, and plants. Once the parasite propagates in the environment, it viciously makes its way into other animals’ bodies.

Dancing with the devil

In what may be called the animal version of “Dancing with the Devil,” the parasite alters the brain chemistry of the animal, floods their body with dopamine, and evokes “fatal attraction.” The excess chemical in their bodies reduces the fear, anxiety, and resistance, which causes them to stand bold in the face of the predator. Scientists termed this cycle the “manipulation hypothesis,” in which the parasite triggers behavioral changes in the prey’s brain, so it keeps on returning to the predator in a self-sabotaging mode. In turn, when the cat eats an infected rodent, such as a mouse, the parasite enters its body, and the life cycle continues, per ASM.

The effect is not limited to animals’ brains. Researchers found that, in humans too, T.gondii can reduce the fear response and increase risk-taking behavior, often landing them in trouble. In humans, infection can occur through contaminated food, particularly undercooked meat or unwashed produce, or through accidental ingestion of the parasite from cat feces or contaminated soil. According to the CDC, pregnant women, infants, and people with a weak immune system are more vulnerable to getting affected by this parasite. Nearly 40 million people are infected with T.gondii in the US alone.

There is no turning back

The parasite can remain in the body for life, although most healthy people do not require treatment. There is currently no established treatment that reliably eliminates the dormant parasite from the body. CDC advises people to follow stringent food precautions and hygiene practices like washing hands, washing fruits and vegetables, avoiding unpasteurized milk, cooking meat at optimal temperatures, and wearing gloves while gardening. Looking ahead, the study will enable researchers to raise awareness and design treatments for these psychopathologies to minimize the negative consequences for future generations. “In particular, the potential for parasitic infections to exacerbate aggressive and impulsive behavior raises important questions for understanding and preventing violence and behavioral disorders,” researchers concluded.

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