ALSPAC says no to cat ownership - psychosis risk hypothesis but...

"While pregnant women should continue to avoid handling soiled cat litter, given possible T. gondii exposure, our study strongly indicates that cat ownership in pregnancy or early childhood does not confer an increased risk of later adolescent PEs [psychotic experiences]."

So said the findings reported by Francesca Solmi and colleagues [1] (open-access) who brought a smile to any reader of the title of their paper: "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." For those who might not be aware of the hypothesis, cat ownership has been previously linked to 'adverse' psychological outcomes (see here) tied into some peer-reviewed evidence on one possible environmental factor linked to psychosis and conditions manifesting psychosis: Toxoplasma gondii.

ALSPAC - Avon Longitudinal Study of Parents and Children - brought it's quite significant scientific prowess to bear on the question of whether "cat ownership in pregnancy and childhood (ages 4 and 10 years) was associated with psychotic experiences (PEs) in early (age 13, N = 6705) and late (age 18, N = 4676) adolescence, rated from semi-structured interviews." Having a cat in the house was not the only question asked by Solmi et al as the presence of other pets were also investigated: "dogs, rabbits, rodents, birds (all waves), and tortoises and fish (from 21 months)." PEs were assessed at approximate ages of 13 and 18 years old via responses to the "psychotic-like symptoms interview (PLIKSi), a semi-structured interviewer-rated screening assessment for PEs." Various other variables were also factored into the examination of any effect or not.

Results: well, as per the opening sentence to this post, cat ownership did not seem to be related to later PEs. The potential caveat being that in some of their analyses there did seem to be a possible association - "Owning a cat at age 4 years was associated with higher odds of having PEs at age 13 years in univariable models" - but the significance of this association disappeared when adjustments for other potentially confounding variables were made. Obviously this kind of study can't control for every single potentially confounding variable but they did at least try.

Why the disparity between these results and the previous ones suggestive of a possible connection between childhood cat ownership and later adverse psychological health? Well, an important point is made by Solmi and colleagues: "Our study was based on PEs in early and late adolescence, unlike other studies which were based on a clinical diagnosis of schizophrenia." In other words. psychotic experiences might be part and parcel of schizophrenia but not necessarily all that schizophrenia encompasses and not necessarily just enough to merit a diagnosis of schizophrenia. They do also go on to highlight how the previous report on the association may also not have included the range of potentially confounding variables that were included and controlled for in the current study as another possibility for the differences reported. Having said that [2]...

Does the T. gondii - schizophrenia hypothesis fall as a result of the Solmi results? Probably not. Solmi et al hint that even though cat ownership probably isn't related to PEs, they do not totally debunk the idea that there may be a connection. They did not for example, look for the presence of contact with T. gondii in this particular study (others have) as per serological examination of participants. I say this bearing in mind that not every moggy is necessarily infected with T. gondii or anything else. Sweeping generalisations on all cats are not required.

Music to close, and containing the lyric 'Caringosity killed the Kerouac cat', a chirpy little number from a band with quite a contentious name...

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[1] Solmi F. et al. 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. Psychol Med. 2017 Feb 22:1-9.

[2] Fuller Torrey E. et al. The antecedents of psychoses: a case-control study of selected risk factors. Schizophr Res. 2000 Nov 30;46(1):17-23.

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Solmi F, Hayes JF, Lewis G, & Kirkbride JB (2017). 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 PMID: 28222824