Can depressive disorder be the outcome of an infectious disease

One in five Australians aged 16-85 experience a mental illness in any year. The most common mental illnesses are depressive, anxiety and substance use disorder. These three types of mental illnesses often occur in combination. For example, a person with an anxiety disorder could also develop depression, or a person with depression might misuse alcohol or other drugs, in an effort to self-medicate. Of the 20% of Australians with a mental illness in any one year, 11.5% have one disorder and 8.5% have two or more disorders. Almost half (45%) Australians will experience a mental illness in their lifetime.

Recently Psychiatric News ran a story with at least one researcher asking if major depressive disorder is the outcome of an infectious disease? This question arises from the belief that a variety of infectious pathogens affect the central nervous.

Patients with depression exhibit sickness behavior. Also, depression is significantly associated with infectious agents, including viruses like Borna disease virus, herpes simplex virus-1, varicella zoster, and Epstein-Barr virus. Parasites like Toxoplasma gondii may play a role. And even if every case of depression isn’t caused by an infectious agent; given its prevalence, even a subset of patients would still add up to a large number of cases. We know, for example, infectious agents have been known to affect the brain and cause psychiatric disorders. Syphilis helped fill America’s mental asylums in the late 19th century.

It’s said that among patients with diagnosed major depression or bipolar disorder, those with a history of suicide attempt had higher Toxoplasma gondii antibody titers.

Another possibility is the “leaky gut” hypothesis, suggesting that cytokines increase intestinal-tract permeability to lipopolysaccherides from gram negative bacteria and that antibodies to the LPS are found at higher levels in depressed patients.

The key may not be a disease, however, rather an inflammatory reaction caused by disease given that higher levels of pro-inflammatory cytokines are found in people with depression. A recent report, drawing on data from the Avon Longitudinal Study of Parents and Children in Great Britain, found that children with the highest levels of the systemic inflammatory marker IL-6 at age 9 were more likely to be depressed at age 18.