Is schizophrenia hiding in your cat litter? Spoiler alert: nope

You might have heard of that particular study: it's of course a little more complicated than that, but it has been found that the onset of schizophrenia might be linked to getting into contact with... cat pee. But no need to panic and kick out your best friend (yes, I am firmly convinced of the supreme nature of the friendship between cats and humans) without a warning. It is only one factor among many others intertwining to cause schizophrenia to appear in somebody.

Chiefly, cats aren't actually to blame. Rather, it's really about the one and only toxoplasma gondii, a parasite responsible for an infectious disease called toxoplasmosis. By the way, this disease is perfectly benign (and most of the time people don't even show any symptoms), except for pregnant people and people with a weakened immune system. Plus, once you've contracted the disease you get immune to it for life, and the parasite can't be transmitted from human to human.

This parasite's unique course of action is actually pretty impressive: it gets to the brain of the animal it has infected (preferably a warm-blooded one) such as a mouse, and once there, exercises control that will make the animal actively look for its predators of choice -- namely, cats. Why so? Because cats are among the only animals in which this parasite can reproduce. So the mouse gets eaten by the cat and bam, the parasite lands right where it wants and can make baby parasites. Just how incredibly clever is that?!

Some studies had already established probable links between humans contracting common infectious parasites and exhibiting certain behaviors -- some of which constitute acknowledged symptoms of particular mental illnesses. But saying that toxoplasmosis causes mental illness is a big step... that this latest study does take indeed.

Featuring over 11,000 participants, this study is the largest to date on the subject.

It notices that among people diagnosed with schizophrenia, the toxoplasma gondii parasite is detected at a much higher rate than in people without the mental illness. It thereby demonstrates causal links specifically between toxoplasmosis and schizophrenia. The link between infection by this virus and schizophrenia is actually even more significant than that of genetic and environmental factors, which we already know participate in the onset of the illness -- making this new study incredibly interesting. This is also why it is now possible to speak of causal link instead of mere correlation. The study, importantly, notes that infection by this parasite is not correlated to the onset of other mental illnesses than schizophrenia.

In this same study, the research team also demonstrates strong correlation links between infection by CMV (cytomegalovirus, which is a herpes family virus, also benign except for pregnant people and people with a weakened immune system) and the onset of any given severe mental illness (especially mood disorders).

So what's the catch? Well, we don't know the exact nature of these links. How does the virus responsible for toxoplasmosis increase the risk of having schizophrenia? The same goes for CMV and the rest of mental illnesses: what could be going on in the human body, when reacting to the presence of these viruses, so that illnesses such as bipolar disorder or depression would appear? We still don't have an answer to this question, although it is suspected that changes in the production of dopamine by the brain might be responsible.

Nonetheless, the results of this study are extremely useful: thanks to this new knowledge, we could for instance put a new detection system in place for schizophrenia, one with better targeting and earlier intervention. It would be based on the presence or absence of antibodies for the toxoplasma gondii parasite.

The takeaway message is: don't panic. First of all, an indoor cat that doesn't hunt (and therefore doesn't eat small rodents) will not become infected with toxoplamosis.

Second of all, the parasite gets transmitted via contact with the feces, possibly urine, of said cat -- not by simply petting it.

Last but not least, as I mentioned before, contracting toxoplasmosis is merely one among a myriad of other factors that can contribute to schizophrenia: there's little chance that you would need to be concerned if you have no family history of the illness or if you haven't been exposed to one of the known environmental factors.

We're talking more a tool for prevention and detection here: for instance, if you plan on carrying a child in your womb, and all the more if you have family history of schizophrenia, then it would be good for the well-being of your child that you first determine whether you've ever been in contact with the toxoplasma gondii virus (if not, you have to be careful not to get infected by it during your pregnancy). A simple blood test will answer that question. And then, knowing that 75% of mental illnesses appear during puberty and before the age of 25, the best course of action remains to watch out for symptoms in people at risk (meaning people who have already contracted toxoplasmosis AND have been exposed to genetic and/or environmental factors), so that they can be cared for by a medical team as soon as schizophrenia first manifests.

The sooner the person gets treatment, the easier it will be for them to build a normal and happy life for themselves. Because yes, you can have schizophrenia and still enjoy a good life!

Going further

• Here's some great and very practical advice on how to to help a loved one manage a psychotic episode (in French only)