Scientifically Reviewing the Link between Feline Toxoplasmosis and Human Schizophrenia

Introduction: this is an article by a respected enthusiast and cat expert, Sarah Hartwell, the owner and author of, in which she reviews the link between feline toxoplasmosis and human schizophrenia with particular reference to a recently well-quoted study, which has been so assiduously discussed in the online news media (including on TIME) and often in a biased or careless way; hence the need for this article.

By Sarah Hartwell

There’s been a lot in the news recently about a link between feline toxoplasmosis and human schizophrenia. The media reports have concentrated on the “cats make you mentally ill” angle. This reminds me of a maths proverb: A bad statistician uses statistics like a drunk uses a lamp-post – for support rather than illumination.


Two useful tags. Click either to see the articles:- Toxic to cats | Dangers to cats

Prevalence and Routes of Infection

Depending on region, around 20-60% of cats tested are infected. Because they get it from their food, Toxoplasma gondii (abbreviated here to Toxo), is more common in stray and feral cats than in pet cats. Toxo becomes infectious 1-5 days after it is shed in cat faeces, so clean the litterbox (and flowerbed) promptly. Cats acquire Toxo by eating infected meat or prey. They can then shed Toxo oocysts (eggs) for 2-3 weeks. Most cats shed oocysts only once (after first infection) and are then effectively immune. Surveys found that less than 1% of infected cats actually shed oocysts after infection.

First of all, Toxo, is found in soil and can be contracted in a variety of ways. The most common ways are handling or eating raw meat and handling or eating unwashed vegetables. It is also present in the faeces of recently infected cats so it’s wise to wear rubber gloves when emptying litter trays or gardening in soil that a cat routinely uses as an outdoor toilet. A no-brainer is to wash your hands after gardening and not transfer soil to your mouth. Sheep afterbirths, mutton and unpasteurised sheep/goat milk are also sources of infection.

Around 30% of UK adults tested have antibodies (i.e. have been infected in the past), but in France and Germany up to 80% tested positive. The most likely explanation is dietary habits. Most people exposed to Toxo never suffer any symptoms at all. People with poor immune systems may develop toxoplasmosis, which is dangerous to unborn babies and can cause flu-like illness in adults (serious illness is more rare). In those people, Toxo can form tissue cysts and stress later on can activate them into producing millions of toxoplasma parasites that can lodge in the brain and/or major organs.


Obstacles to Research Outside of a Laboratory

A good researcher CHALLENGES his hypothesis rather than just looking for data to support it. If his hypothesis stands up to challenge then he’s onto something. For challenging the link between Toxo and schizophrenia, you need at least 4 sample sets. These sets must be large – at least 100 people in each – and geographically widespread to rule out local variations such as sheep farming regions. There are also cultural habits to consider: eating unwashed vegetables, eating undercooked (rare!) meat or feeding uncooked meat to cats.

  • People who owned cats/had close contact with cats as a child and later developed mental illness.
  • People who owned cats/had close contact with cats as a child and has not developed mental illness.
  • People who didn’t own cats/have close contact with cats and have developed mental illness.
  • People who didn’t own cats/have close contact with and have not developed mental illness.

Also extend this to those who have had/have not had close contact with other domestic animals (e.g. dogs, which can carry contaminated soil into the home) instead of cats. That will help with figures for toxo picked up from non-feline sources.

Even with those sample sets there are problems. You need the medical histories of both parents to rule out any inherited conditions. You need to scan the brain for congenital abnormalities. Have the individuals been exposed to other potential sources of Toxo? Do they have a family history of mental illness? Have they suffered brain injury? Or serious childhood illnesses that can affect the brain? Used mind-altering drugs? Handled new-born lambs? With humans it is very difficult to eliminate all of these variables and still have large enough sample sets. If they’ve had close contact with cats, how many of those cats are indoor/outdoor and how many are indoor-only (less likely to be exposed to Toxo)? For a reliable conclusion you need to RULE OUT EVERY OTHER POSSIBLE SOURCE of Toxo infection.

Statistics Pitfall: If the sample set is small the data simply can’t be trusted. For example, if 1 person in a 4-person sample set has 6 fingers you can’t extrapolate this to mean that a quarter of all people have 6 fingers! The study also requires the following large and geographically widespread sample sets to compare the correlation between childhood cat contact & mental illness (either in general or a specific mental illness):

Recent Study

The study in question is “Is childhood cat ownership a risk factor for schizophrenia later in life?” by E. Fuller Torrey, Wendy Simmons, Robert H. Yolken. It is a peer-reviewed paper and was published online April 18, 2015. I don’t have access to the full article, but I do have the abstract. The CAPITALISED TEXT is mine and it shows that the link is still a hypothesis not a conclusion!

Torrey and Yolken have been studying the link between Toxo and schizophrenia for around 30 years, but they are still careful to word their papers with “possible” and “needs clarification” so even after 30 years of diligent research it’s not proven scientific fact. 30 years may sound like obsessive behaviour, but many scientists devote an entire lifetime to investigating a single issue. Since it’s unethical to deliberately infect children with Toxo and see what happens in later life (and rodents are not perfect substitutes for humans in research), the team compared 2 previous studies that linked childhood cat ownership to adulthood schizophrenia later in life, and an unpublished mental health survey from 1982. They concluded that childhood cat ownership MAY BE A risk factor for developing mental disorders. Note that they said “a risk factor” – there are many other risk factors as already mentioned.


“Abstract: Two previous studies suggested that childhood cat ownership is a POSSIBLE risk factor for later developing schizophrenia or other serious mental illness. We therefore used an earlier, large NAMI questionnaire to try and replicate this finding. The results were the same, suggesting that cat ownership in childhood is significantly more common in families in which the child later becomes seriously mentally ill. IF TRUE, an explanatory mechanism MAY BE Toxoplasma gondii. WE URGE OUR COLLEAGUES TO TRY AND REPLICATE THESE FINDINGS TO CLARIFY WHETHER CHILDHOOD CAT OWNERSHIP IS TRULY A RISK FACTOR FOR LATER SCHIZOPHRENIA.”

In a study (published in Acta Psychiatrica Scandinavica) by A.L. Sutterland from the Department of Psychiatry at the Academic Medical Centre in Amsterdam, researchers analysed the findings of 50 published studies to confirm a link between Toxo and mental illness. The wording of the news article is misleading – it makes it sound like the link is already established. It found an apparently “overwhelming” association that a person with Toxo infection was almost twice as likely to develop schizophrenia than an uninfected person. But it also said THE FINDINGS SHOULD BE APPROACHED WITH CAUTION.

Torrey’s press release said:

“Cat ownership in childhood has now been reported in three studies to be significantly more common in families in which the child is later diagnosed with schizophrenia or another serious mental illness.” Saying it is “more common” is not the same as saying it is “cause and effect.” Torrey also says “I like cats. Unfortunately, IF WE ARE CORRECT that they transmit infections . . . ”

Many people with mental conditions (inherited or acquired) adopt cats for companionship. Mental illness can lead to cat ownership, but cat ownership doesn’t invariably lead to mental illness. Cat companionship is beneficial to the vulnerable or mentally ill people.


I have also looked at some of the historical studies and these are the main findings relevant to cat owners. These high publicity scare stories came from the Czech Republic and were picked up by tabloid newspapers.

Temporary (12 weeks) behavioural changes are seen in mice infected with toxoplasmosis (Hrda et al 2000, Webster 2001). The mice were less active, especially when infection was at its peak in their brains. This made them more vulnerable to predation by cats and completes the lifecycle. Infected mice had increased levels of dopamine in their brains which led to the controversial suggestion that toxoplasmosis could cause of schizophrenia in humans (Flegr et al 2003). Test-tube studies indicated that drugs used to control schizophrenia also affected the toxoplasmosis parasite. Researchers suggested this explained how schizophrenia drugs worked. Sceptics point out that the drugs might affect the parasite as a side-effect, rather than the primary effect.

A Czech study suggested latent toxoplasmosis infection causes slower reaction times (Havlicek et al 2001) that could put infected people more at risk of road traffic accidents (Flegr et al 2002). The higher the antibody count, the greater the risk. These smaller-scale studies should be interpreted cautiously. A study involving 857 Czech military conscripts, reported decreased IQ and verbal intelligence in people who had had toxoplasmosis (Flegr et al 2003). A different study found that pregnant women that had previously had toxoplasmosis were more intelligent (Flegr & Havlicek 1999). Czech researchers suggested that toxoplasmosis makes women reckless and friendly and make men jealous, suspicious, withdrawn and morose. They suggested that toxoplasmosis more than doubled a woman’s risk of causing a traffic accident. None of this has been corroborated, but they suggested the IF IT DID TURN OUT TO BE TRUE, toxoplasmosis is responsible for up to one million road deaths worldwide, making it second only to malaria in deadliness. It could also be responsible for domestic problems between infected parties. Thankfully, the rest of the scientific world was highly sceptical since the causes of domestic disputes and road accidents are far too varied to be pinned down on a single cause.

These are the studies I have referenced:

  • Flegr J, Havlicek J (1999) Changes in the personality profile of young women with latent toxoplasmosis. Folia Parasitologica (Praha) 46, 22-8.
  • Flegr J, Havlicek J, Kodym P, Maly M, Smahel Z (2002) Increased risk of traffic accidents in subjects with latent toxoplasmosis: a retrospective case-control study. BMC Infectious Diseases 2, 11.
  • Flegr J, Hrda S, Tachezy J (1998) The role of psychological factors in questionnaire-based studies on routes of human toxoplasmosis transmission. Central European Journal of Public Health 6, 45-50.
  • Flegr J, Preiss M, Klose J, Havlicek J, Vitakova M, Kodym P (2003) Decreased level of psychobiological factor novelty seeking and lower intelligence in men latently infected with the protozoan parasite Toxoplasma gondii. Dopamine, a missing link between schizophrenia and toxoplasmosis? Biological Psychology 63, 253-68.
  • Havlicek J, Gasova ZG, Smith AP, Zvara K, Flegr J (2001) Decrease of psychomotor performance in subjects with latent ‘asymptomatic’ toxoplasmosis. Parasitology 122, 515-20.
  • Hrda S, Votypka J, Kodym P, Flegr J (2000) Transient nature of Toxoplasma gondii-induced behavioral changes in mice. Journal of Parasitology 86, 657-63.
  • Webster JP (2001) Rats, cats, people and parasites: the impact of latent toxoplasmosis on behaviour. Microbes and Infection 3, 1037-45.

19 thoughts on “Scientifically Reviewing the Link between Feline Toxoplasmosis and Human Schizophrenia”

  1. What I find ironic is that, far from presenting a contradictory argument, Galen T is actually agreeing with what I’ve said: with toxo present in the environment and oocysts dispersed in several ways, close contact with cats is not necessary in order to become infected.


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