Scientists believe infection causes depression

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Apart from genetic characteristics, changes in hormonal levels, stress, grief, and substance abuse, a growing body of studies suggest that certain infections can actually trigger depression.

NPR.org reports that in 2014, associate professor of psychology and radiology at Stony Brook University Turhan Canli published a paper stating that depression should be considered as an infectious disease.

Canli has observed that patients diagnosed with depression also manifest non-psychological symptoms. "Depressed patients act physically sick," says Canli. "They're tired, they lose their appetite, they don't want to get out of bed."

The theory that infection can cause neurological and psychological conditions isn't something new. Abnormal immune activity creates autoantibodies, or antibodies that attack its own tissues. The presence of autoantibodies in schizophrenia patients has been reported way back in the 1930s.

Canli states that viruses such as Toxoplasma gondii can lead to emotional disturbances similar to mental disorders. Other bacteria associated with mental disorders include the Borna disease virus, Epstein-Barr, certain strains of herpes such as varicella zoster, which causes chickenpox and shingles.

A 2013 Danish study looked into medical histories of more than three million people and found that infection-related hospitalizations were linked to a 62 percent risk of developing conditions such as depression and bipolar disorders.

With these findings, tests are being conducted to determine if anti-inflammatory medication can be a possible treatment for depression. The results have been mixed so far.

A recent study has confirmed that supplementing Prozac with low-dose aspirin reduces the risk of depression while ibuprofen supplements lowers the risk of psychiatric symptoms.

On the other hand, diclofenac or celecoxib, anti-inflammatory medicines used to treat arthritis, was seen to increase the risk of psychiatric conditions.

"The truth of the matter is that there is probably a subset of people who get depressed in response to inflammation," says Dr. Charles Raison, a psychiatry professor at the University of Arizona. "Maybe their bodies generate more inflammation, or maybe they're more sensitive to it."

The connection between infections and depression is still being investigated, but psychiatrists agree it's a promising place to start looking for alternative treatments that will address the disease.

"I'm not convinced that anti-inflammatory strategies are going to turn out to be the most powerful treatments around," says Raison. "But I think if we really want to understand depression, we definitely have to understand how the immune system talks to the brain. I just don't think we've identified immune-based or anti-inflammatory treatments yet that are going to have big effects in depression."

According to biotechin.asia, major depression affects one in 6 people worldwide, surpassing cardiovascular and respiratory diseases, cancer, and HIV/AIDS combined.