Testing positive for a common parasite that lives in the bodies of 10 - 20% of Americans is linked to a sevenfold higher risk of attempted suicide according to new research.

This was the main finding of a study appearing in the August issue of The Journal of Clinical Psychiatry that supports growing evidence linking infection by the Toxoplasma gondii parasite to suicide attempts.

A Common Parasite

T. gondii is a common protozoa (parasite) that once ingested travels to the brain, where it can cause subtle changes over time. It reproduces in the cells of its primary host, which is any member of the cat family.

Human sources of infection include any food or water contaminated with eggs from the parasite. Cat litter is another source, hence why it is often referrred to as the "cat litter parasite". T. gondii can also be picked up from eating undercooked food or raw meat from animals that carry the parasite.

Inflammation in the Brain

Lena Brundin, of Michigan State University, co-led the new study. She told the press that between one in ten and one in five people in the US carry the parasite, and while it was thought to lie dormant, it in fact appears to cause inflammation that produces metabolites that accumulate over time and can harm the brain.

"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," said Brundin, an associate professor of experimental psychiatry in the College of Human Medicine at MSU.

For example, last month saw the publication of a study that linked higher suicide risk in new mothers to T. gondii.

What the New Study Found

The study is the first to use a suicide assessment scale to assess risk in people infected with the parasite, including 54 who had attempted suicide and 30 controls. All patients were adults and were recruited between 2006 and 2010, and were tested for signs of the parasite. The 54 who had attempted suicide were inpatients at Lund University Hospital in Sweden, and the controls were randomly selected from the municipal population register in Lund.

The results showed testing positive for the parasite was significantly tied to higher scores on the scale, which would indicate a higher risk of a future suicide attempt.

"... we found that if you are positive for the parasite, you are seven times more likely to attempt suicide," said Brundin.

However, Brundin emphasized that most people infected with the parasite are unlikely to attempt suicide:

"Some individuals may for some reason be more susceptible to develop symptoms," she explained.

Implications

Estimates for the US in 2009 suggest one death every 14 minutes, nearly 37,000 in the year, was due to suicide.

These figures highlight what Brundin describes as a "major health problem" where nine out of ten 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," she urged.

Could Reduced Serotonin Be a Symptom, Not a Cause of Depression?

Brundin is no newcomer to looking at how inflammation in the brain might be linked to symptoms of depression, a major factor in suicide.

About ten years ago, after working on Parkinson's disease, she began investigating the role of the neurotransmitter serotonin, and a class of antidepressants called selective serotonin re-uptake inhibitors, or SSRIs, which have been the preferred treatment for depression.

SSRIs are thought to increase serotonin in the brain, but this only works in about half of patients with depression.

Through her work, Brundin is coming round to the view that reduction in serotonin in the brain is more of a symptom than a root cause of depression.

A plausible explanation could be that depression results from changes in brain chemistry produced by inflammation caused by an infection or a parasite.

Scope for New Treatments

"I think it's very positive that we are finding biological changes in suicidal patients," says Brundin, because "It means we can develop new treatments to prevent suicides, and patients can feel hope that maybe we can help them."

Studies like this one open up the scope for developing new treatments that target specific biological mechanisms, she adds.

Several organizations funded the research, including the Swedish Research Council and the American Foundation for Suicide Prevention.

Written by Catharine Paddock PhD