Now, a new study led by first author and psychiatry resident Vincent Paquin from McGill University in Canada could help to explain this parasitic paradox – while going some way to identifying where the apparent dangers of T. gondii may actually lie.
In addition to contaminated food or water (including undercooked meat), the protozoan parasite T. gondii can be transmitted to humans through exposure to feces from an infected domestic cat, with a study in 1995 first observing a link between cat ownership in childhood and risk of mental illness later developing in adulthood.
However, other studies since have failed to replicate the cat ownership link, suggesting there might be more to the association than simply owning a cat.
"Domestic cats generally become infected with the parasite by feeding on rodents, and will only be contagious during the days or weeks that follow," Paquin and his co-authors write in their new paper.
"Hence, specifying whether the cat was known to hunt rodents might provide a better proxy for probable exposure to T. gondii compared to cat ownership alone."
In other words, as Paquin explains, cats themselves do not guarantee parasitic exposure, but rodent-hunting cats (i.e., cats that are allowed outside, as opposed to indoor-only pets) would probably be more likely to come into contact with T. gondii in the outdoor environment.
Hypothetically speaking, they might then transmit the infection to children, who could go on to develop psychological issues in adulthood, as identified in some studies, potentially through effects on the immune system.
To examine this hypothetical chain of transmission, the researchers surveyed approximately 2,200 participants in Montreal, asking them questions about childhood cat ownership, and measuring their frequency of psychotic experiences, alongside other questions about their personal history, such as how much they moved house during childhood, experiences with head trauma, history of smoking, and so on.
In analyzing the responses, the team observed that male participants who had owned a rodent-hunting cat during childhood showed an increased risk of having psychotic experiences in their adulthood; female respondents did not have the same link.
People who owned indoor-only cats during childhood (or no cat at all) did not show the same increased risk, which the team said was "consistent with our hypothesis based on the life cycle of T. gondii as the putative mechanism of this association".
However, other factors captured in the survey also seemed to influence the respondents' risk for psychotic experience, including smoking, frequency of residential moves in childhood or adolescence, and a history of head trauma, which Paquin says suggests "synergistic effects of these factors", beyond just parasitic infection alone.
While the study has a number of limitations – including that all the data from the survey were self-reported – the team says their findings illustrate the importance of examining interactions among different kinds of environmental exposures, which may in the future help us to identify with greater accuracy where problems from T. gondii exposure are more likely to arise.
"These are small pieces of evidence but it's interesting to consider that there might be combinations of risk factors at play," Paquin told Medscape Medical News.
"And even if the magnitude of the risk is small at the individual level, cats and T. gondii are so present in our society that if we add up all these small potential effects then it becomes a potential public health question."
The findings are reported in Journal of Psychiatric Research.