Major Depressive Disorder (MDD) should be redefined as an infectious disease, argues Dr Turhan Canli, Associate Professor of Psychology and Radiology at Stony Brook University, New York.
Published in Biology of Mood & Anxiety Disorders, the work claims that the root cause of MDD could be parasitic, bacterial or viral infection and aims to stimulate novel research approaches in the fight against depression. Indeed, MDD is one of the most common mental disorders, with 16.6% of the population suffering at some point. Dr Canli argues that current treatments do not tackle the root cause of depression and speculates that research investigating a role of microorganisms could lead to treatment breakthroughs.
Dr Canli considers three main lines of evidence to support his view. Firstly, patients with MDD display physical symptoms. In particular, inflammatory biomarkers are elevated in depressed patients, indicating activation of the immune system which could arise in response to pathogens.
Secondly, pathogens can affect the emotional behaviour of humans. A prime example is the parasite Toxoplasma gondii which can form cysts in the brains of rats and affect their behaviour; importantly, one fifth of the US population may be infected. Of those diagnosed with depression, patients with a history of suicide attempts had higher levels of antibodies in response to this pathogen, supporting that infection with parasites can correlate with behaviour patterns.
Finally, Dr Canli considers the role of genetics; the work suggests that genetic differences could render some more vulnerable to depression than others and that this deserves further consideration.
Intentionally speculative, the work nevertheless provides clear rationale for investing future research efforts into investigating possible associations between infection and depression. Such efforts may reduce the stigma often associated with mental illness and, as Dr Canli concludes, may even represent a step towards vaccination against major depression.
