“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.
“Our study population,” explain the paper’s authors, “contains mothers born in Denmark who gave birth between May 15, 1992, and January 15, 1995, and whose child was screened for T. gondii (N=45,788).”  Because children don’t develop antibodies to T. gondii until they’re about three months old, the researchers were confident that “antibodies measured in the blood from the child were maternal in origin.” 
Using data from the Danish Cause of Death Register and Danish Psychiatric Central Research Register, Pedersen et al. examined the relationship between T. gondii infection and suicides, violent suicide attempts, and self-directed violence.
The study’s results are compelling for a number of reasons—among them, the large sample size and lengthy follow-up period (14 years). But again, no causal link was demonstrated.
Nor was any connection made to cats; indeed, it’s not clear from the study how these women became infected. As the Centers for Disease Control and Prevention points out on its website, “people are more likely to get [toxoplasmosis] from eating raw meat or from gardening” than from cats.
Connections to Cats
The Danish study is, of course, just the latest in a string of studies published over the past few years that have grabbed headlines, implicating T. gondii in everything from brain cancer (or not) to schizophrenia and high-risk behaviors.
And yet, the connections are largely correlative. “The exact link between T. gondii and psychiatric diseases,” reports Christof Koch in the May 2011 issue of Scientific American Mind, “is tantalizing but remains murky.” 
“To explain where we are in Toxo research today,” says Robert Yolken, an infectious disease expert at Johns Hopkins University interviewed for the excellent feature in the March issue of The Atlantic, “the analogy I always give is the ulcer bacteria. We first needed to find ways of treating the organism and showing that the disease went away when you did that. We will have to show that when we very effectively treat Toxoplasma, some portion of psychiatric illness goes away.” 
All of which might help explain why scientists close to the research aren’t about to give up their cats. Yolken, a guardian of two cats, told NPR, simply: “the benefits outweigh the risks.”  And Jaroslav Flegr, an evolutionary biologist at Charles University in Prague, who’s spent the past 20 years or so exploring the possible connections between T. gondii infection and human behavior—and the father of two young children—shares his home with two indoor-outdoor cats.
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Although T. gondii stories tend to become fodder for TNR opponents eager to portray cats as Public Enemy No. 1, all of this attention is, perhaps, not without its benefits. As Kathleen McAuliffe explained in her piece for The Atlantic “until solid proof exists that Toxo is as dangerous as some scientists now fear, pharmaceutical companies don’t have much incentive to develop anti-Toxo drugs.” 
In the meantime, I’ll be taking the CDC’s advice, washing my “hands thoroughly with soap and running water after touching cat feces.”
* “Toxoplasma gondii is a widespread zoonotic protozoan that infects most, if not all, species of birds and mammals. As the definitive hosts for this organism, felines [wild and domestic] are the only animals that pass oocysts in their feces.” 
1. Hamilton, J. (2012) A Parasite Carried By Cats Could Increase Suicide Risk. Shots (NPR Health BLog_ http://www.npr.org/blogs/health/2012/07/02/156142214/a-parasite-carried-by-cats-could-hurt-humans-sanity Accessed July 3, 2012.
2. Pedersen, M.G., et al., “Toxoplasma gondii infection and self-directed violence in mothers.” Archives of General Psychiatry. 2012: p. 1-8. http://dx.doi.org/10.1001/archgenpsychiatry.2012.668
3. Koch, C., “Protozoa Could Be Controlling Your Brain.” Scientific American Mind. 2011. May. http://www.scientificamerican.com/article.cfm?id=fatal-attraction
4. McAuliffe, K., “How Your Cat Is Making You Crazy.” The Atlantic. 2012. March. http://www.theatlantic.com/magazine/print/2012/03/how-your-cat-is-making-you-crazy/8873/
5. Elmore, S.A., et al., “Toxoplasma gondii: epidemiology, feline clinical aspects, and prevention.” Trends in Parasitology. 2010. 26(4): p. 190–196. http://www.sciencedirect.com/science/article/B6W7G-4YHFWNM-1/2/2a468a936eb06649fde0463deae4e92f