‘Crazy cat lady syndrome’: From medical myth to potential cancer vaccine

By John Murphy, MDLinx
Published September 10, 2018

Key Takeaways

You may know her from depictions on TV, such as the Crazy Cat Lady character on The Simpsons. Or you may know her as a neighbor, or even as a patient. We all know at least one person who has an obsession with cats. But some people, men and women, can go from obsession to madness. Is there a medical link between cat ownership and psychosis? Is there a real medical diagnosis for ‘crazy cat lady syndrome’?

A good deal of research links infection by the parasite Toxoplasma gondii with increased risk for psychiatric disorders, including schizophreniaand bipolar disorder. One study even found an association between T. gondii infection and suicidein women of postmenopausal age.

Now, guess where this psychosis-inducing parasite comes from?


House cats are the primary hosts of T. gondii. Humans can pick up the parasite by cleaning the litter boxes of infected cats or by accidental ingestion of soil-based oocysts found on unwashed vegetables and fruits or in contaminated drinking water. This parasite also uses other animals as secondary hosts, so humans can be infected by eating undercooked meat from affected livestock.

T. gondii has a particular proclivity for muscle and brain tissue, where it forms cysts that remain throughout life, establishing a chronic infection.

However, a 2017 study published in Psychological Medicine concluded that having a cat doesn’t increase the risk of psychotic symptoms after all.

Researchers at University College of London (UCL), in London, UK, looked at nearly 5,000 children born in 1991 or 1992 and followed them until age 18. The researchers asked the children’s parents about having a pet cat while the mother was pregnant and the children were growing up.

“Since T. gondii infection is proposed to increase psychosis risk by affecting early life neurodevelopment, we restricted cat ownership to the prenatal and early childhood period (at age 4 years), although we also performed additional analyses using cat ownership at age 10 years to better align with previous research,” the authors noted. “Psychotic experiences in adolescence are an established risk factor for later schizophrenia, particularly with respect to psychotic symptoms which emerge or persist in late adolescence.”

After adjusting for several sociodemographic and socioeconomic factors, the researchers found no evidence in their large population-based cohort that cat ownership during pregnancy or childhood was associated with psychotic experiences in early and late adolescence.

“The message for cat owners is clear: there is no evidence that cats pose a risk to children’s mental health,” said lead author Francesca Solmi, PhD, MSc, research fellow, UCL Division of Psychiatry. “In our study, initial unadjusted analyses suggested a small link between cat ownership and psychotic symptoms at age 13, but this turned out to be due to other factors. Once we controlled for factors such as household over-crowding and socioeconomic status, the data showed that cats were not to blame. Previous studies reporting links between cat ownership and psychosis simply failed to adequately control for other possible explanations.”

Then again, Dr. Solmi and fellow researchers conceded that the association between T. gondii infection and future risks of psychosis are supported by good evidence. However, they appear to draw the line at owning a cat as the crucial link in that association, at least in children.

“From a public health perspective…it is perhaps reassuring that data from our prospective longitudinal study were not consistent with the hypothesis that cat ownership in pregnancy or early childhood is a risk factor for later psychosis,” they concluded.

The power of the parasite

In other T. gondii news, researchers at Dartmouth College’s Geisel School of Medicine, Lebanon, NH, are researching how to turn the parasite’s immunological power against cancer tumors, with the goal of making a cancer vaccine.

Researchers have known that T. gondii infection strongly induces a Th1 immune response. They’ve also known that this particular immune response is strongly associated with increased cancer survival rates.

“We know biologically this parasite has figured out how to stimulate the exact immune responses you want to fight cancer,” said investigator David J. Bzik, PhD, professor, Microbiology and Immunology.

But it wasn’t until recently that the researchers figured out a way to bioengineer T. gondii to prevent it from replicating if injected into humans. When they tested the vaccine in a mouse model, it provoked a strong antitumor response and also rejected new tumors from taking root.

“By gaining preferential access to the inside of powerful innate immune cell types, our mutated strain of T. gondii reprograms the natural power of the immune system to clear tumor cells and cancer,” said Barbara Fox, senior research associate, Microbiology and Immunology.

Further investigation is required before the vaccine can be tested in clinical trials, the researchers noted.

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