Major depressive disorder (MDD) should be re-conceptualized as an infectious disease, according to Turhan Canli, PhD, Associate Professor of Psychology and Radiology at Stony Brook University. In a paper published in Biology of Mood & Anxiety Disorders, Dr.Canli suggests that major depression may result from parasitic, bacterial, or viral infection. He presents examples that illustrate possible pathways by which these microorganisms could contribute to the etiology of MDD.

MDD remains highly prevalent disease with some 15 to 20 percent of the population experiencing MDD at some point. Recurrence is common, and pharmacological treatments have not changed. Because the causal aspects of the disease are not clearly defined, research to find causes remains paramount to help improve treatments.

"Given this track record of MDD, I propose reconceptualizing the condition as some form of infectious disease," said Dr. Canli, who is also Director of Stony Brook's SCAN Center, a member of the Program in Neuroscience, and a Senior Fellow in the Center for Medical Humanities, Compassionate Care, and Bioethics. "Future research should conduct a concerted effort search of parasites, bacteria, or viruses that may play a causal role in the etiology of MDD."

In the paper, Dr. Canli presents three arguments why reconceptualizing MDD as an infectious disease may be a fruitful endeavor.

First, he points out that patients with MDD exhibit illness behavior such as loss of energy, and that inflammatory biomarkers in MDD also suggest an illness-related origin. Second, he describes evidence that parasites, bacteria and viruses that infect humans in a way that alters their emotional behavior. Thirdly, Dr. Canli brings the notion of the human body as an ecosystem for microorganisms and the role of genetics.

Based on these points, Dr. Canli suggests a major research step would be to conduct large-scale studies with depressed patients, controls, and infectious-disease related protocols to determine the association or causal nature of infectious disease and depression.