Sovereign Health/Shutterstock
Source: Sovereign Health/Shutterstock

What if depression were an infection? You feel some of the early symptoms first: for instance, fatigue, loss of interest and sleep disturbances. As your symptoms get more serious, your friends urge you to see a doctor. When you arrive at the doctor’s office, the doctor evaluates your symptoms, listens to your heart and gives you a quick blood test. After the visit, you receive a prescription to help treat the infection and reduce some of the inflammation associated with it. In a week, you’re feeling healthy again — the depression is gone.

It might sound like a pipe dream, but scientists suspect this scenario isn’t very far from the truth.

The link between infections and mental illness

When people are depressed, they feel sick. They lose their appetite. They feel tired all the time. Their joints ache.

Although most of us focus on the psychological symptoms associated with depression (for instance, hopelessness and low mood), these physical symptoms are no less real. In fact, according to a study that examined over 25,000 patients across 14 countries, roughly 69 percent of people with depression first seek treatment because of their physical symptoms — not their mental symptoms.

Since mood disorders often result in symptoms not unlike those found in physical illnesses, a group of Danish scientists sought to determine whether mental and physical illnesses were closer than previously thought. In this study, scientists examined the medical records of over 3 million people born between 1945 and 1996, comparing the incidence of infections such as sepsis and hepatitis with the incidence of mood disorders such as depression and bipolar.

The results? Patients with a history of hospitalization for infection had a 62 percent increased risk of developing a mood disorder later in life. In other words, having a serious infection increases a person’s risk of developing a mental illness more than twofold.

This study was far from the only one to find a link between mental illness and infections. Toxoplasmosis, a parasitic infection, has been known to evoke psychological symptoms not unlike those found in schizophrenia. In certain instances, strep throat (a bacterial infection) has been found to trigger obsessive-compulsive disorder in children, a condition known as pediatric autoimmune neuropsychiatric disorders associated with streptococci (PANDAS). Multiple studies have found that depression is associated with increased inflammatory markers such as interleukin 6 (IL-6) and tumor necrosis factor alpha (TNF-a), biological indicators that the body is fighting an infection.

As scientists perform more and more research, it seems clear: Mental illness and physical illness are linked, at least sometimes. The question is: Why? And what can we do about it?

The immune system: A double-edged sword

When people get sick, their body reacts by triggering the immune system. The immune system’s job is to fight off harmful stimuli including bacteria, viruses and foreign objects. One of the ways the immune system fights illness is through triggering inflammation, a healing process that also causes redness, swelling and warmth. Even though inflammation has an overall positive effect on the body, it can sometimes act as a double-edged sword. Inflammation can gum up cellular functions (due to the swelling), hinder mobility (also due to the swelling) or cause an unnecessary amount of pain (because the swelling irritates nerves). When a person struggles with an autoimmune disease, such as lupus or Crohn’s disease, inflammation occurs because the body is attacking itself — not because it’s fighting an injury or infection.

Scientists suspect that — as in autoimmune disorders — the symptoms associated with mental illness are actually the result of inflammation, not a specific infection. In other words, mental illness isn’t caused by a physical illness — instead, it results from the body’s immune response to that illness.

There is ample evidence that mental illness is linked to inflammation and the immune system. According to the large-scale Danish study on mental illness and physical illness, developing an infection wasn’t the only thing that increased the risk of developing a mental illness — patients with autoimmune diseases were 45 percent more likely to have a mood disorder as well. One study found that individuals with severe depression had increased levels of quinolinic acid — a byproduct of inflammation — in one of the brain areas responsible for emotional regulation. Another study found that healthy patients actually experience symptoms of depression if they’re given infusions designed to trigger the immune system. Yet another study found that people with overactive immune systems were more likely to develop schizophrenia later in life.

Not only has inflammation been linked to mental illness, but reducing inflammation is associated with treating mental illness. Research indicates that selective serotonin reuptake inhibitors (SSRIs), a common kind of antidepressant, don’t just increase serotonin levels — they also decrease inflammation, which may explain why they reduce symptoms so effectively. Another form of depression medication (tricyclic antidepressants) reduces inflammation so successfully that it can be used to treat inflammatory bowel disease.

What about people who develop mental illness after experiencing a traumatic event? According to research, psychological stressors such as abuse, grief and loneliness can also activate the immune system.

What does all this mean?

Depression and other mental illnesses are unlikely to be infections. Scientists suspect, however, that the body’s immune response to an infection might trigger mental illness in some individuals. Other individuals might experience this same immune response when faced with psychological trauma, whereas still others might struggle with a naturally overactive immune system. In all cases, the body’s immune response somehow interrupts the brain’s usual functioning, causing severe psychological symptoms.

(As an aside, heart disease is also linked to an overactive immune system, which may be why people with depression often suffer from poor cardiovascular health. By understanding what causes mental illness, we may also be able to help people avoid heart disease as well!)

Researchers are currently attempting to determine whether treating inflammation can help people struggling with mental illness. In the future, we might look back on our current mental health treatments and laugh. Maybe one day, all we’ll need is a doctor’s visit and a week of medication to get healthy again.

Contributed by Courtney Lopresti, M.S. neuroscience

About the Author

Ralph Ryback M.D.

Ralph Ryback, M.D., has taught at many institutions including Harvard Medical School. 

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