The Parasite That Manufactures Its Own Dopamine Inside Your Brain
Scientists have spent decades wondering how Toxoplasma gondii rewires the brains of its hosts. In December 2025, a team led by Professor Joanne Webster at the Royal Veterinary College answered the question definitively: the parasite produces its own dopamine.
Published in Nature Communications, the study identified a parasite-encoded protein called tyrosine hydroxylase (TgTH) that directly synthesizes dopamine inside infected brain tissue. This is not the parasite nudging your neurochemistry. It is the parasite running its own dopamine factory inside your neurons.
The Rat Experiments
The research team at the Royal Veterinary College, working with Imperial College London and the University of Leeds, infected rats with three different T. gondii strains: wild-type, a genetically modified strain with moderate TgTH expression, and a modified strain with high TgTH expression.
The results followed a clean dose-response curve. Rats infected with high-TgTH strains spent significantly more time in zones scented with cat urine. The behavioral shift -- walking toward the animal most likely to eat you -- scaled directly with how much dopamine the parasite pumped out. They called it the "Fatal Feline Attraction test."
Female rats were generally more active and exploratory than males across all groups, but the parasite's dopamine production overrode normal survival behavior in both sexes.
It Gets Worse: Brain Cell Communication Collapses
Six months before the RVC findings, a team at UC Riverside led by parasite immunologist Emma Wilson published equally disturbing results in PLOS Pathogens. They found that neurons infected with T. gondii release fewer extracellular vesicles -- the tiny packets brain cells use to communicate with each other.
The content of those vesicles was altered too. Astrocytes receiving signals from infected neurons showed increased inflammatory markers and decreased glutamate transporters. Excess glutamate accumulation is associated with seizures and neural damage.
The Schizophrenia Connection
Dopamine dysregulation is central to schizophrenia. The RVC study's funding came from the Stanley Medical Research Institute, which has spent years investigating the link between T. gondii infection and psychotic disorders. The parasite's ability to independently produce dopamine in brain tissue gives that connection a direct biochemical mechanism.
Remember: cats are the only definitive host. The parasite evolved this dopamine manufacturing capability specifically to make prey animals walk toward cats. When it ends up in a human brain instead of a rat brain, it does not stop producing. It just keeps pumping dopamine into tissue that was never meant to receive it.
The study was published December 8, 2025, with funding from the Stanley Medical Research Institute and the Medical Research Council. The DOI is 10.1038/s41467-025-66139-3.