People who are hospitalized for infections may face an increased risk of dying from suicide, according to a new study that may suggest a biological basis for some suicidal behavior.

The researchers examined data in a Danish national registry, following 7.2 million people from 1980 to 2011. They found that people who were hospitalized for infections during that period had a 42 percent increased risk of dying by suicide compared with people not hospitalized for infection.

Though the study cannot prove causality, researchers found that 1 in 10 suicides could be linked to this infection-related risk, according to the findings published Aug. 10 in the journal JAMA Psychiatry. [5 Myths About Suicide, Debunked]

The research bolsters an idea that has been gaining more weight among psychiatrists, the hypothesis that suicide is linked to bodily inflammation.

"To find evidence confirming some of the current edge-cutting hypotheses in psychiatric research in an epidemiological study of this scale is highly significant," said Lena Brundin, a neurobiologist at the Van Andel Research Institute and Michigan State University, who was not involved in the research but wrote an editorial to accompany it in the journal. "This is a very impactful study."

How inflammation could affect behavior

Over the years, small studies have found a link between inflammation — which is essentially the result of the immune system going into attack mode — and suicidal behavior. A 2015 review published in the Annals of Gastroenterology found that a third to half of patients treated with drugs that spur inflammatory responses develop depression as a side effect of treatment. Inflammatory chemicals called interleukin-1β and interleukin-6 show up at high levels in the blood and brain tissue of people who have died by suicide, according to a 2015 paper published in Biological Psychiatry. And Brundin and her colleagues have found that the more suicidal a patient is, the higher the levels of quinolinic acid in their cerebrospinal fluid. Quinolinic acid is generated by inflammation and can interfere with brain signaling.

But no one had really addressed the origin of this inflammation, Brundin told Live Science. In the new study, a team led by researchers at Copenhagen University Hospital turned toward infections, an obvious immune-system trigger, as a possible cause of inflammation.

The researchers used the Danish Civil Registration System to gather data on the psychiatric diagnoses, hospitalization history and cause of death for residents of Denmark who were 15 or older in 1980. This cohort of more than 7 million people was followed until December 2011.The Danish registries are an unusually detailed data source, offering detailed information on aspects of people's lives such as unemployment, marital status and socioeconomic status, that can be difficult to gather in large studies.

During the study period, 809,384 people were hospitalized with infections. There were also 32,683 deaths by suicide. Of the people who had died by suicide, 7,892, or 24.1 percent, had been hospitalized with an infection.

The role of infections

The vast majority of people who had an infection serious enough to require hospitalization did not die by suicide. And likewise, the majority of people who did die by suicide did not have a history of serious infection. Nevertheless, after controlling for demographics, socioeconomics and other factors that might influence suicide rate, the researchers found that people who were hospitalized for infections were 42 percent more likely to die by suicide than people who were not.

"The numbers indicating an increased risk for suicide after severe infections are high, perhaps surprisingly high even for us working with this subject," Brundin told Live Science.

The longer or more serious a person's infection history, the higher their risk of dying by suicide. A hospital stay of four days or fewer translated to a 42 percent increased risk, but those who spent more than 94 days hospitalized had a 138 percent increase in risk of suicide. Likewise, a single infection raised the risk of suicide by 34 percent, but people who had seven or more infections had nearly three times the risk of suicide than those who had none. [7 Devastating Infectious Diseases]

The association held true for all infections except for ear infections or pregnancy-related infections. The risk was highest in patients with hepatitis (an infection of the liver) or HIV/AIDS, which come with their own risks of depression and suicidal ideation.

The association also held when the researchers limited their analysis only to people who'd never had a psychiatric diagnosis. Even without a mental health diagnosis, being hospitalized for an infection was linked to a 21 percent increase in suicide risk.

There are many mechanisms that could link infections and suicide, study leader Helene Lund-Sørensen, of Mental Health Centre Copenhagen, told Live Science. The team controlled for factors like cohabitation, sex, age and the presence of chronic diseases to try to narrow their focus on psychiatric disorders, but weren't able to account for the fact that some psychiatric disorders might go undiagnosed. There might also be self-care issues that cause people with depression and suicidal behaviors to be more likely to come down with an infection.

However, Lund-Sørensen and her colleagues wrote, the new findings fit with other results from the smaller psychiatric studies on inflammation and suicidal behavior, the researchers wrote.

"Provided that the association between infection and the risk of death by suicide was causal, identification and early treatment of infections could be explored as a public health measure for prevention of suicide," they wrote. "Still, further efforts are needed to clarify the exact mechanisms by which infection influences human behavior and risk of suicide." [7 Absolutely Horrible Head Infections]

Lund-Sørensen and her colleagues are planning a study to investigate whether less severe infections that don't require hospitalization, might be linked with suicide.

There are many ways an inflammation-suicide link could affect how health care providers approach patients, Brundin said, such as increasing their focus on psychiatric symptoms when patients are in the hospital with an infectious disease. Clinical trials could attempt to eradicate chronic infections with antibiotics, Brundin said, and anti-inflammatory drugs could reduce inflammation. Currently, patients with suicidal behavior are excluded from most clinical trials, Brundin added, which gives a skewed view on how pharmaceuticals might affect them.

"Novel treatment studies in suicidal and depressive patients are highly warranted," she said.

Suicide is complex, study co-author Teodor Postolache, a professor of psychiatry at the University of Maryland School of Medicine, told Live Science. And translating the research on inflammation to treatment is similarly complex: Some patients may be vulnerable to certain inflammatory triggers, like allergies, that don't affect others. Other environmental factors, like sunlight exposure or even low oxygen levels from living at a high elevation, have been shown to be associated with suicide, too. And, of course, there are traditional risk factors for suicide, such as having an untreated mood disorder and access to firearms, which makes suicide attempts more deadly.

"Every layer is going to be important," Postolache said. "This is such a resilient epidemic worldwide."

The number for the National Suicide Prevention Lifeline is 1-800-273-8255.

Original article on Live Science.

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