New Research Challenges Alleged Links Between Cat Ownership and Mental Illness

Writing last week for Scientific American, Yale School of Medicine research fellow Jack Turban waded into the controversy surrounding cat ownership, Toxoplasma gondii, and the parasite’s alleged role in mental illness—asking (and answering) the question, “are cats really to blame for psychotic behavior?” As it turns out, not so much.*

“In the largest and best-controlled study to date, the researchers showed that those exposed to cats were at no increased risk of psychosis after controlling for a number of other variables (including ethnicity, social class, and dog ownership—to control for exposure to animal stool).”

But wait—what about all those scary headlines…?

As the study’s lead author Francesca Solmi explains, “previous reports of positive associations between cat ownership and schizophrenia” might be attributable to “small sample sizes and lack of control for confounders inherent to some studies.”

“Earlier studies also relied on retrospective recall, and hence the potential of recall bias, of cat exposure and did not distinguish between ownership in infancy v. later childhood, making it impossible to attribute risk to specific periods of cat ownership over the early life-course.” [1]

All of which ought to come as welcome news to the American Bird Conservancy, given that organization’s very public “concern” for “the likelihood of infection with Toxoplasma gondii.” And The Wildlife Society, whose recommendation (PDF) for the “removal of free-ranging cats and elimination of feral cat colonies” rests in part on the parasite’s potential for “significant effects on the health of humans.”

And let’s not forget about Peter Marra, who considers toxoplasmosis “perhaps one of the most significant” of all zoonotic diseases—sufficient to warrant unprecedented action from the Centers for Disease Control and Prevention.

“Advertising campaigns underwritten by various government entities have helped discourage smoking. Perhaps billboards warning of the potential maladies brought on by cats carrying Toxoplasma gondii could similarly impact public behavior. As free-ranging cats become recognized as a serious threat to public health—and thus become viewed as a public-health issue—there is hope that more resources can be devoted to managing the problem.” [2]

Or, you know, maybe not.

Hat-tip to Louise Holton at Alley Cat Rescue and Bob Weedon, DVM, MPH.

*Unfortunately, readers have to drill down five paragraphs before they find the answer to the question posed by the article’s headline, “Are Cats Responsible for ‘Cat Ladies’?” (“your cat friends are just fine”). It’s disappointing to see Scientific American resort to the sort of click-bait move Ryan Holiday describes in Trust Me, I’m Lying: Confessions of a Media Manipulator. (“When you take away the question mark, it usually turns their headline into a lie… After the reader clicks, they soon discover that the answer to the ‘question’ in their headline is obviously, No, of course not.”)

Literature Cited

  1. Solmi, F., Hayes, J. F., Lewis, G. & Kirkbride, J. B. Curiosity killed the cat: no evidence of an association between cat ownership and psychotic symptoms at ages 13 and 18 years in a UK general population cohort. Psychological Medicine. 1–9 (2017).
  2. Marra, P. P. & Santella, C. Cat Wars: The Devastating Consequences of a Cuddly Killer. (Princeton University Press, 2016).



Disappointed but not surprised that you deleted Marian Brown’s very factual scientifically supported comments on this subject on your facebook page. Pretty embarrassing to be debunked on your own blog. Unfortunately I didn’t save the whole thing, but here is what I got. I dare you not to delete it. Hopefully Ms. Brown will find this and repost her comments.

Cat ownership is the wrong metric, since the cat parasite Toxoplasma gondii oocysts are ubiquitous in the environment now and owning a cat is not the main risk factor for infection with Tg. The Boyer article is a good reference, with 78% of the women studied infected by oocysts, but they conclude, “Although it is beneficial for physicians to advise pregnant women to avoid exposure to cat feces, not all exposure is from recognized sources; most appeared not to be. Thus, although education may help to prevent some cases of toxoplasmosis, it will not be sufficient to prevent a substantial proportion of these infections, because acquisition can be undetected and recognized risk factors are frequently absent.” Cat ownership was one specific risk factor studied in the Boyer article. The data reveal that ownership of a cat is not necessary to acquire T. gondii, because it has been shown that oocyst exposure is not always associated with cat ownership or with recognition of risk factors. Cat ownership is simply the wrong metric, when correlating psychosis and infection with Toxoplasma gondii. Also, psychosis isn’t really what has been studied, rather it has been bipolar disorder with psychosis, various forms of schizophrenia, and harming behavior (self or others). Depression is not associated with Tg. The psychiatric disorders associated so far with Tg are affected by dopamine levels, which is the neurotransmitter that the encysted Tg parasite facilitates (two copies of the tyrosine hydroxylase enzyme) while encysted in the human (and rat) brain.

And the Boyer article link –

Clay Johnston: I’m tempted to ignore your comment for the same reason I deleted Marian Brown’s repeated comments on the Vox Felina Facebook page: although there is lots of information here, your main point is unclear. Specifically, this VF post was in response to previous claims that prior cat ownership is related to mental illness later in life—that’s it. Your response, on the other hand, is all over the map.

Clay Johnston: I’m not sure if you were aware, but the degree of anti-cat hate that has afflicted certain sectors of the conservation concern has in fact led at least a few individuals to suggest that mere cat ownership is in fact a risk factor for developing a mental illness or several. Have your internet travels acquainted you with the wit and wisdom of a fellow who goes by the label TNR Researcher? That might give you an idea.

The study discussed here quite clearly debunks such nonsense. Whether or not the T. Gondi parasite as such is a causal factor in the development of mental illness of any kind is a separate matter.

On a separate but related note, the exact circumstances under which people acquire a pathogen should- quite obviously- have implications for public health policy. The best evidence currently available would seem to indicate that improving sanitation around our food and water supply (as opposed to mass cat-killing) is a fairly effective means of combating T. Gondii infection. At least in the US, seroprevalence would seem to be declining.

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