Infectious disease physicians always like to imagine that every type of pathology is due to an infection. Cancer-causing viruses and ulcer-inducing bacteria are cases that illustrate the ubiquity of microbes as etiologic agents for conditions long thought to be non-infectious. Psychiatric illnesses have, thus far, not demonstrated a definitive link to infection. A growing body of fascinating new research, however, is beginning to change that.
Many science and evolutionary biology enthusiasts are familiar with the parasitic infection toxoplasmosis, an organism that often has pregnant woman avoiding kitty litter boxes. It is well established that toxoplasmosis infections in the brain of rodents change their behavior by attracting them to a substance they naturally avoid, cat urine. Such a behavior change renders them much more likely to be devoured by a feline allowing the Toxoplasma parasite to reach its beloved primary host.
Could this infection also influence human behaviors?
Perhaps it is behind the plague of humans who display a zombie-like obsession with endlessly watching cat videos on the internet?
About a quarter of humans are infected with toxoplasmosis and it is generally considered to not be a major problem except for in certain circumstances such as pregnancy, HIV, or transplantation. What really is striking is the fact that those with certain mental conditions are harbor the organism at a higher rate than the general public. For example, schizophrenics are 2.7 times more likely to be antibody positive than members of the general public. Associations have also been noted for generalized anxiety disorder and bipolar disorders.
Correlation is not causation and an infection cannot override the volitional nature of human consciousness. However many questions remain to be answered such as: is toxoplasmosis a surrogate marker for something else that lies behind these conditions, if the organism plays a causative role is it the infection or the post-infection immune response that is operative, would treatment change the course of illness?
It is also important to remember that not every case shows this association and multifactorial explanations for psychiatric illnesses are the norm.
Nevertheless that an infectious disease may play a partial role in some of these illnesses would represent an important breakthrough that could lead to new means of both prevention and treatment.