Pavlov's Couch

A Psychology Student's Mental Experience

The Parasite That Changes Your Personality

Image from Wikimedia

Meet toxoplasma gondii. This lovely looking fellow is a parasite who finds host in cats, however it is also found – in the asexual stage of it’s life cycle – in other warm blooded animals, most commonly rodents but also birds and – you guessed it – humans. Luckily the parasite is kept in it’s latency stage by the human immune system, although if you have a compromised immune system it can raise it’s ugly head a little. But for the majority of people infected with this parasite, they have absolutely no idea they have a stowaway in their body. Which is quite lucky really, since about 22%-84% (Desmonts, 1974) of the worlds population is thought to carry this beastie!

The lifecycle of toxoplasma gondii, in a nutshell, goes something like: Infected rodent is eaten by a cat > parasite reproduces in the cat > parasite excreted through the feces > infects another warm blooded animal, like a rodent > form cysts in the brain, heart, and other tissue > repeat. This is why pregnant women are told to keep away from cat litter trays – because the parasite can be passed on in-utero and would certainly do a baby some damage! Even if you are not a cat owner, do not consider yourself safe. It can be ingested from unwashed vegetables for example, since the oocysts that exist in cat faeces are as persistent as a two year old with an ice-cream van outside: “These oocysts contaminate the surrounding environment; a 20-g cat stool can contain between 2 and 20 million oocysts, and after faecal decomposition, the local soil contamination can be as high as 100 000 oocysts/g and remain infectious for more than 1 year”  (Webster, 2001). Also a study of meat samples in shops in the UK found up to 38% were infected with T. gondii! (Aspinall et al., 2002)

Image from Wikimedia

That’s creepy enough. Things got interesting, however, when it was discovered that this parasite actually changes the behaviour of rodents to make it more likely that they will get eaten by a cat, thus allowing the parasite to complete it’s lifecycle. Specifically it was found that rats infected with the parasite did not show the usual fear-reaction and aversion to the smell of cat urine and in some instances even seemed to be attracted to it! (Webster, 2001;  2007) Other studies have also show infected rats to have diminished memory and maze-learning abilities, were more active, spent less time grooming before exploring new environments, and “laboratory mice…have been observed to run in circles and have their heads bent to one side.” (Webster, 2007)

This then raised the question – if this parasite can do this to rodents, does it affect humans too? Well according the the research, yes. Take this rather bold statement for example:

“Previously it was thought to be completely harmless, but the parasite actually changes your personality. If you are infected, you become more daring and unconcentrated, for example it has been noted that people that have the infection, more often are involved in traffic accidents, and even is thought be make a person suffer from schizophrenia.”
(Bech-Neilsen, 2012)

Cats really can drive you mad!
Image from: SXC

While I suspect this statement is somewhat hyperbolic considering the studies into personality changes have only shown subtle (although statistically significant) differences, and the link to schizophrenia is not nearly as clear cut as seems to be suggest here, the quote makes the point well: Your personality, and your behaviour, may be being altered by a parasite in your body.

I’ll say that again. You may have a parasite in your body (22%-84%, remember?). This parasite may be affecting the way that you think and behave without you even realising it.

Scared yet? I am!

In a correlational study looking at on a number of personality trait studies performed using Cattell’s 16PF personality trait questionnaire, a correlation between was found between infection and lower superego strength  and higher vigilance in men, leading to increased tendency to boredom, jealousy, suspiciousness, and temper in men as well as a disregard for social rules. Women, in contrast, were shown to have higher superego strength and higher warmth ratings, “suggesting that they were more warm hearted, outgoing, conscientious, persistent, and moralistic.” It has also been said to be linked to higher anxiety levels in both genders. Concentration and psychomotor skills deficits were also reported (Flegr, 2007). T. Gondii has been claimed to be responsible for 9% of intellectual disability cases in some regions. (da Silva and Langoni, 2007)

Things get really interesting when you look at T. Gondii with mental health. The link between the parasite and schizophrenia has been considered for over 50 years now, and studies have shown that individuals with schizophrenia have increased amount of T. Gondii antibodies, as well as identifying exposure to cats in childhood as a risk factor for schizophrenia (Torrey, 2003). Moreover, it has been found that many antipsychotic drugs used in the treatment of schizophrenia actually stop reproduction of the parasite in laboratory cultures. This has brought up the hypothesis that many anti-psychotic and mood-stabilising medication may work, wholly or partly, by inhibiting the T. Gondii parasite, giving support to the idea that the parasite may play a role in development of schizophrenia. To test this, a study was carried out where infected rats were treated with antipsychotic medications or water. The control (water medicated) rats exhibited the suicidal attraction to cats and other modified behaviours as described above, however the rats treated with antipsychotics did not exhibit these changes (Webster, 2007).

It’s all pretty interesting food for thought, and one thing is for sure: I’m steering clear of cats and I’m making sure my salads are very thoroughly washed from now on. Although the odds say that I’m probably already infected.If that is the case…I wonder how different I would be as a person if I wasn’t?

(References at bottom of post)


Aspinall, TV., Marlee, D., Hyde, JE., Sims, PFG., 2002. Prevalence of Toxoplasma gondii in commercial meat products as moni- tored by polymerase chain reaction—food for thought? Int J Epidemiol. 32:1193–1199.

Bech-Nielsen, S., 2012. Toxoplasma gondii associated behavioural changes in mice, rats and humans: evidence from current research. Preventive veterinary medicine, 103(1), pp.78-9.

da Silva, R.C. & Langoni, H., 2009. Toxoplasma gondii: host-parasite interaction and behavior manipulation. Parasitology research, 105(4), pp.893-8.

Desmonts G., Couvreur J., 1974. Congenital toxoplasmosis. A prospective study of 378 pregnancies, N. Engl. J. Med. 290, pp.1110–1116

Flegr, J., 2007. Effects of toxoplasma on human behavior. Schizophrenia bulletin, 33(3), pp.757-60.

Torrey, E., 2003. Toxoplasma gondii and schizophrenia. Emerging infectious diseases, 9(11), pp.1375-1380.

Webster, J.P., 2001. Rats, cats, people and parasites: the impact of latent toxoplasmosis on behaviour. Microbes and infection / Institut Pasteur, 3(12), pp.1037-45.

Webster, J.P., 2007. The effect of Toxoplasma gondii on animal behavior: playing cat and mouse. Schizophrenia bulletin, 33(3), pp.752-6.

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3 thoughts on “The Parasite That Changes Your Personality

  1. Paradigm on said:

    Interesting, although I’ve heard that Schizophrenia measured as a spectrum (which is the right way imo) has a heritability of 80 percent. Also, most research suggest that personality changes in adults are slow and gradual and follow a similar pattern – less impulsive, more agreeable and conscientious.

    Still, given that the environmental influence on personality is largely unknown, this could be an important part of the puzzle.

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