About 10-20 percent of people in the United States have Toxoplasma gondii, or T. gondii, in their bodies, but in most it was thought to lie dormant. . . . In fact, it appears the parasite can cause inflammation over time, which produces harmful metabolites that can damage brain cells. "Previous research has found signs of inflammation in the brains of suicide victims and people battling depression, and there also are previous reports linking Toxoplasma gondii to suicide attempts. . . . In our study we found that if you are positive for the parasite, you are seven times more likely to attempt suicide."From here (emphasis added).
If these numbers are right, infected people make up about 15% of Americans, but about 55% of all Americans who attempt suicide. (The study itself was conducted in Sweden with 54 probands and 30 controls.)
This is huge. To be clear, the study itself looked only at non-fatal suicide attempts and not completed suicides. But, the linked article notes that there were 36,909 suicides in 2009 in the United States. Thus, if the percentage of suicide attempts resulting in death is similar for both infected and non-infected individuals, this parasite may be the statistical proximate cause of more than 14,000 suicides a year in the United States; 40% of the total number of U.S. suicides.
The only other infectious agent with a comparable mortality impact in the United States is influenza. But influenza deaths, which are predominantly among infants, people with compromised immune systems, and the elderly, while suicide is found in appreciable frequencies at all ages from adolescence onward.
Also, "90 percent of people who attempt suicide have a diagnosed psychiatric disorder. If we could identify those people infected with this parasite, it could help us predict who is at a higher risk."
The mere fact that one has a parasite that infects 45,000,000 people in the U.S. of whom 20,300 a year commit suicide, does not itself create an overwhelming individualized risk, and some of those people who were infected with the parasite (statistically) would have committed suicide anyway.
But, the presence of this infection is a much more powerful screening tool when combined with the stastically powerful screen of a previously diagnosed psychiatric disorder (and, in fact, only some diagnosed psychiatric disorders, not all of them, are associated with elevated suicide risk), which is probably mostly independent of the incidence of infection with this common parasite. There are tens of millions of Americans who have diagnosed psychiatric disorders as well, but the intersection of the two categories is probably small enough to justify increased vigilance for the individuals of the appropriate ages (i.e. not young children) who have both risk factors.
Also, knowing that this parasite has a profound mortality impact, at least in certain populations, may make finding a way to treat this infection, previously viewed as mostly harmless, a major new public health priority. An effective diagnosis and treatment for this parasitic infection, if affordable, could be the biggest advance for public health since the invention of vaccines again common viral diseases and antibiotics against bacterial diseases.
The underlying study is:
Yuanfen Zhang, Lil Träskman-Bendz, Shorena Janelidze, Patricia Langenberg, Ahmed Saleh, Niel Constantine, Olaoluwa Okusaga, Cecilie Bay-Richter, Lena Brundin, Teodor T. Postolache. Toxoplasma gondiiImmunoglobulin G Antibodies and Nonfatal Suicidal Self-Directed Violence. The Journal of Clinical Psychiatry, 2012; DOI: 10.4088/JCP.11m07532