Cats contract toxoplasmosis gondii from eating infected prey, and then infect other animals with it, causing changes in rodents brains that see them lose their fear of cats. But what effect is toxoplasmosis having on the brains of cat owners?
Cats are the “definitive host” for the infection toxoplasmosis gondii, which means that whilst the infection can live in other animals it can only reproduce inside a cat’s gut. In order to ensure its survival it has to make sure that it will enter a cat’s gut and so it can also infect and live in rodents and other prey animals. Its effect on rodents though is unique – it changes their behaviour and makes them less risk averse around cats. It actually makes rodents easier to catch through making them attracted by the odour of cats instead of being repelled.
Recent research has shown that these effects remain in rodents after any toxoplasmosis infection has cleared, leading to the conclusion that it must be due to a permanent change in their brain structure. As up to one third of people around the world are also infected questions have been asked about the potential effects on their brain structure and mental health.
Partly this sprang from the brains of infected mice showing increased levels of dopamine, which suggested to some researchers toxoplasmosis might cause of schizophrenia in humans. This was seen as supported by further research that demonstrated that drugs used to treat schizophrenia directly affected the toxoplasmosis parasite.
As far back as 2001 toxoplasmosis infection was suggested as a risk factor for road traffic accidents, due to it allegedly delaying people’s reaction times and making them behave more recklessly. However the results of these comparatively small studies have not been convincingly reproduced, and a further study by the same group reported toxoplasmosis infection caused withdrawal, suspicion and rigid thinking and behaviour – not the attributes of increased appetite for risk.
Much more recently a paper was published in April 2015 entitled “Is childhood cat ownership a risk factor for schizophrenia later in life?” This led to headlines in the popular press about cats causing mental illness, a finding that was reported much more conclusively than it was in the original paper. In fact the paper concluded that whilst “cat ownership in childhood is significantly more common in families in which the child later becomes seriously mentally ill” the link remains a hypothesis, and toxoplasma gondii only one possible vector. It’s also worth pointing out here that any correlation between cat ownership and mental health is not the same thing as establishing that cause and effect exist – I’ve written elsewhere about our propensity to see patterns and relationships.
The major stumbling block in establishing a link between toxoplasmosis gondii and human mental health are the huge range of and genetic factors related both to toxoplasmosis and mental illness. Toxoplasmosis gondii is present in soil, and spores can be windborne for great distances. In addition poorly cooked and undercooked meat is a source of infection, with as strong a correlation seen between the consumption of “rare” or “blue” meat and levels of toxoplasmosis as with childhood cat ownership.
Any hypothesis that could allow for these environmental factors would still have to encompass and explain all the childhood cat owners and people infected with toxoplasmosis gondii who do not go on to develop schizophrenia and other mental illness. Essentially, despite over thirty years of study and reporting in the media, there is no direct link established between cat ownership and schizophrenia.
The potential physical effects of contracting toxoplasmosis gondii are not in dispute, especially in pregnant women who contract the disease for the first time whilst pregnant. They may be asymptomatic but their baby may have birth defects including blindness and brain damage. This is uncommon, since most pregnant women at risk will already have been exposed to toxoplasmosis and so do not become re-infected. Infection can be prevented through good hygiene, and avoiding undercooked meats, unpasteurised dairy products, and contact with litter trays.
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