Depression may be like an infectious disease

Depression has long been a misunderstood disorder. Even its classification remains tenuous: It has been characterized as a disease, an emotional disorder and a dysfunction of the brain, among many other labels. And the affliction itself continues to resist understanding.

Turhan Canli, a professor in the departments of psychology, radiology and genetics and neuroscience at Stony Brook University, suggests that we should view depression as a disease that does not necessarily come upon a single person as an isolated event, but as something that results from an infection by a parasite, bacteria or virus.

Depression is a complex illness, affecting nearly 20 percent of all people over the course of their lives. It has been interpreted through the perspectives of psychology, biology, neuroscience and other disciplines. What depression has seldom been viewed as, however, is an infectious disease.

In Canli’s paper, “Reconceptualizing major depressive disorder as an infectious disease,” published in the journal Biology of Mood and Anxiety Disorders, Canli presents methods by which microorganisms might contribute to the manifestation of symptoms of depression. Canli cites the paper as intentionally speculative, a paper intending to stimulate new approaches to researching major depressive disorder (MDD).

In the first part of the paper, Canli notes that patients with MDD often exhibit behavior associated with illness. He corroborates this claim with the fact that there are elevated levels of inflammatory markers in patients with MDD, according to a meta-analysis of 24 studies, which Canli performed. The presence of the inflammatory markers, he reasons, could have been justified by a bodily immune response to some kind of pathogen.

Canli also notes that parasites, bacteria and viruses have already been known to alter the emotions of humans. He most pertinently cites a study in which elevated levels of Toxoplasma gondii, a parasite which is thought to affect one-fifth of the population of the United States, are correlated to rates of suicide in the United States. Bacteria, Canli suggests, may also cause MDD. The mechanism by which researchers suggest that bacteria may cause depression is called the “leaky gut” hypothesis: Cytokines may cause the intestines to become permeable to liposaccharides from bacteria, activating the immune system. Additionally, Canli cites a correlation between several viruses and depression, including the Borna disease virus, herpes simplex virus-1, varicella zoster virus, and Epstein-Barr virus.

The benefit of thinking of depression as a disease caused by parasites, bacteria and viruses, is that such a mode of thought may help researchers eventually find the genetic roots of the disease. Canli notes that though researchers have long claimed that susceptibility to MDD was genetic, searches for specific genes linked to MDD have proven fruitless. Using the infectious disease mindset, he suggests that researchers look into retroviral insertions into the human genome.

Canli’s research into Major Depressive Disorder has already provoked articles in the New York Times and fierce discussion on message boards dedicated to depression. Many have disagreed, and even more have sympathized with his opinions. No matter what the general consensus on depression as an infectious disease is, the fact that we are interpreting depression from such a bevy of angles is an important step in achieving the mindset that will help researchers ultimately find a cure.