“There’s fresh evidence that cats can be a threat to your mental health,” according to a post on yesterday’s NPR health blog, Shots. The threat, reporter Jon Hamilton explains, is not the cats themselves by the Toxoplasma gondii parasite that some cats pass in their feces.*
“A study of more than 45,000 Danish women found that those infected with [Toxoplasma gondii] were 1.5 times more likely to attempt suicide than women who weren’t infected.” 
“Still,” Hamilton continues, “the absolute risk of suicide remains very small. Fewer than 1,000 of the women attempted any sort of self-directed violence during the 30-year study span. And just seven committed suicide.” 
In fact, it may well be that T. gondii infection has no bearing on the risk of suicide at all.
As the researchers themselves point out in a paper published in this month’s issue of the Archives of General Psychiatry, “we cannot say with certainty whether the observed association between T. gondii infection and self-directed violence is causal.”
“T. gondii infection is likely not a random event and it is conceivable that the results could be alternatively explained by people with psychiatric disturbances having a higher risk of becoming T. gondii infected prior to contact with the health system.” 
In other words, it’s possible that mental illness is a risk factor for T. gondii infection, rather than the other way around. Read more