Abstract
In this article, I argue for a reconceptualization of major depressive disorder (major depression) as an infectious disease. I suggest that major depression may result from a parasitic, bacterial, or viral infection and present examples that illustrate possible pathways by which these microorganisms could contribute to the etiology of major depression. I also argue that the reconceptualization of the human body as an ecosystem for these microorganisms and the human genome as a host for non-human exogenous sequences may greatly amplify the opportunity to discover genetic links to the illness. Deliberately speculative, this article is intended to stimulate novel research approaches and expand the circle of researchers taking aim at this vexing illness.
Highlights
My first argument is that patients with major depressive disorder (MDD) exhibit sickness behavior
I propose that future research should conduct a concerted search for parasites, bacteria, or viruses that may play a causal role in the etiology of MDD
The literature implicating the immune system in MDD [61] can be read as suggesting that the immune response itself is the causal mechanism in depression
Summary
Despite decades of substantial research efforts, major depressive disorder (MDD) remains among the most common mental disorders, with a 16.6% lifetime prevalence rate [1]. Pharmacological treatment approaches have not changed during this period, targeting primarily receptorligand interactions [2]. These types of antidepressants may bring relief to patients with severe symptoms but are not clinically more effective than placebos in mild to moderate cases [3]. Recurrence rates of 50% for firstepisode patients and of 80% for second-episode patients [4] suggest that the core of the illness goes untreated. Given this track record, I argue that it is time for an entirely different approach. I have outlined the idea in much greater detail elsewhere [5], but will highlight some key points here
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