There’s been a lot of discussion about toxoplasmosis and mental health over the past few weeks in response to a paper in Schizophrenia Research (Torrey et al 2015). Some internet sources are having a great time writing “crazy cat lady” and similar headlines.
But is there any substance to it?
Yes, and no.
The science in the paper that’s being cited is pretty limited. From my reading of it (and I doubt most people that have written about the paper actually read it), the paper wasn’t meant to be a definitive answer. It was meant to raise awareness.
It certainly did that.
The content relates to the protozoal parasite Toxoplasma gondii. Cats are the definitive host, meaning the adult stage of the parasite lives in the intestinal tract of cats. If infected, cats can pass eggs in their feces. After a few days outside the cat’s body, these eggs are able to infect people, if ingested. A reasonable percentage of healthy people have been exposed to Toxoplasma, potentially directly from cat poop, but more likely from general environmental exposures (e.g. gardens, sandboxes) or unwashed vegetables. Rarely does disease occur, and the main concerns are for severely immunocompromised individuals (who can get Toxoplasma encephalitis (i.e. inflammation of the brain)) and for pregnant women who have not been exposed to the parasite previously (in which the parasite can cause severe infection or death of the fetus).
There have been lingering concerns about the role of toxo in other diseases, schizophrenia being one of them. There have been two studies that suggested a link, but the evidence was relatively weak: basically they simply found that the incidence of cat ownership while young seemed to be more common in schizophrenia patients compared to a control group.
The Torrey study set out to look at that again, using existing data. They looked at data from a 1982 survey of families of schizophrenic patients, collected from people that attended the National Alliance for the Mentally Ill (NAMI) convention. They compared those numbers to a 1991 survey of the general public reported by the American Veterinary Medical Association. Cat ownership was 51% in the schizophrenia population and 43% in the AVMA survey. So, it’s suggestive.
However, these are two completely different studies of different groups in different decades using different methods. A control group has to be very well matched to a “case” group to make a valid comparison, something that may not be true here (there’s no way to know). Various confounding factors could be present. For example, the paper mentions that at the time of the 1982 survey, people with schizophrenia tended to be disproportionately middle and upper class. That would probably be biased even further by including only people that attended a convention (and had the money, time and motivation to go). Could that type of person also be more likely to own a cat? Could that have affected the results? Certainly, it must be possible.
A key point that’s being missed by most reports is the final paragraph of the paper.
It is important to ascertain whether or not cat ownership in child-hood is a risk factor for later schizophrenia since it is a risk factor which could be minimized. We therefore urge our colleagues in other countries to collect data on cat and other pet ownership, and a major goal of this paper is to encourage such research.
It’s important to ascertain….
More work needs to be done….
Not, ‘”cats cause mental illness.”
It’s good to pay attention to the potential link and properly explore it. At the same time, toxo avoidance measures are very easy for cat owners. They include:
- Cleaning the litterbox daily
- Avoiding contact with cat feces, especially old feces
- Handwashing after contact with the litterbox or feces
- Keeping your cat inside so it doesn’t get infected by eating infected rodents
- Keeping sandboxes covered to they don’t become neighbourhood cat litterboxes.
More information about toxoplasmosis can be found in on the Worms & Germs Resources – Pets page.