The cat parasite that rewires rats, and the part about you that keeps not showing up. · published 28 September 2026
The celebrated ’fatal attraction’ finding — a parasite surgically deleting one specific fear — did not survive its 2020 follow-up: infected mice were simply less anxious and bolder across the board, with the size of the change tracking brain cyst load, meaning the manipulation is real but crude rather than clever, and the human version of the story fails the best prospective cohort test anyone has run.
These are the working notes, unedited. Everything the video
states as fact comes from here, and nothing else may be stated as fact.
The sections on limits and on what the finding does not show are
part of the brief, not an afterthought — see
the method.
The finding everyone repeats. Toxoplasma gondii can only complete its life cycle inside a cat. In 2000, Berdoy, Webster and Macdonald published “Fatal attraction in rats infected with Toxoplasma gondii” in Proceedings of the Royal Society B: rats have an innate aversion to the smell of cat, and infected rats lost it - in some cases the aversion turned into attraction. A parasite appearing to steer its host into the mouth of its next host is about as good as biology gets as a story.
It got better, and then worse. In 2007 a PNAS paper reported that the effect was highly specific: the behavioural change was aimed at aversion to cat odour and left other fear responses intact. That specificity is what made it a hijacking rather than a side effect, and it is the version that spread.
The correction, 2020. Boillat and colleagues, “Neuroinflammation-Associated Aspecific Manipulation of Mouse Predator Fear by Toxoplasma gondii”, Cell Reports. Infected mice showed lowered general anxiety and more exploratory behaviour, and their predator aversion was altered with no selectivity toward cats at all. The size of the behavioural change tracked cyst load - that is, the amount of inflammation in the brain. Their own framing is that this refutes the idea of a selective loss of cat fear and points to broad immune-related changes instead.
So the behaviour is real. The elegance is not. A parasite that makes a rodent generally braver and less anxious will incidentally get more of its hosts eaten, which is a duller mechanism than a targeted hijack and does not require the parasite to know what a cat is.
The human claim. The popular version says latent toxoplasma makes people reckless, raises the odds of schizophrenia, causes car crashes, and explains the “crazy cat lady”. The strongest test of it is Sugden and colleagues, PLOS ONE, 2016, using the Dunedin birth cohort - 837 people followed since birth, tested for antibodies. They found no significant association with schizophrenia, and little evidence of any link to depression, suicidal behaviour, criminality, personality aberration or neurocognitive impairment. Essentially nothing, across the board.
Limits that must be stated, and they cut both ways. One cohort of 837 is not the last word, and other work does report associations - a meta-analysis in Psychological Medicine found links with suicide attempts and traffic accidents. So “toxoplasma does nothing to humans” is not established either. What is fair to say is that the confident popular claim runs well ahead of both bodies of evidence, and that the best-designed prospective study found nothing where it should have been easiest to find.
What this does not show
Not that toxoplasma is harmless - it is a serious risk in pregnancy and for immunocompromised people, and that is not in dispute. Not that owning a cat is the main way people get it; undercooked meat and soil matter more. And not that the rat work was wrong: infected rodents do behave differently, which is the part that survived.