Toxoplasmosis, Schizophrenia, and the Crazy Cat Lady

Throughout medical school, parasite infections are a popular exam answer.  With a patient history of changing kitty litter or eating undercooked meat (steak tartare anyone?), Toxoplasmosis is a popular and common parasitic infection.

As a member of the acronym (TORCH) for important congenital infection during pregnancy, medical students and doctors are well aware of the disease caused by this parasite.  Lesser known, are the effects of Toxoplasma gondii on schizophrenia, mental health, and sex drive.


Toxoplasmosis, Schizophrenia, and the Crazy Cat Lady.  Is there a connection?

Recently I’ve read/heard two mentions of the effects Toxoplasma gondii has on behavior, particularly with cat owners AKA the ‘Crazy Cat Lady.’  The first, was while listening to the popular podcast Radiolab‘s episode, The Scratch.  The second, was in an article published by the rising media source, Vice News.  Both sources, illuminated the effects of this disease, beyond traditional flu symptoms and congenital effects.

“There is strong psychological resistance to the possibility that human behavior can be influenced by some stupid parasite,” he says. “Nobody likes to feel like a puppet.” – Dr. Jaroslav Flegr

I have always been fascinated with the mysterious Crazy Cat Lady.  Before now, there has never been validity to their strange behavior.  Dr. Jaroslav Flegr, a parasitologist, author of the book Frozen Evolution, and biology professor at Charles University in Prague has recently published research on the effects of Toxoplasmosis on human behavior.  Other renowned scientists such as the neuroendocrinologist Dr. Robert Sapolsky of Standford University and Dr. E. Fuller Torrey, a psychiatrist and executive director at the Stanley Medical Research Institute, have suggested associations between Toxoplasma and mental health disorders.

Science behind the Crazy Cat Lady

toxoplasma_lifecycleSo here’s the scoop on Toxoplasma.  The parasite can only reproduce inside the stomach of a cat, which is then shed through the cat feces.  Small animals consume the cat feces as a nice afternoon snack, then they become the cat’s afternoon snack.  The circle of life.

Here’s the problem/evolutionary genius of Toxoplasma.  Rats know their natural predators and do everything they can to avoid being eaten.  Rats can even smell a near-by cat, but once the parasite enters the rat, the parasitic cysts travel and embed themselves in the brain of the animal, specifically the rat’s amygdala.  The amygdala is considered the seat for basic instinctual emotions such as fear, anxiety, and sexual arousal.  The brilliance of the parasite is this: it crosses the wires of the rat’s brain making the rat attracted to cat urine and the smell of cats (1, 2).  Now the rat seeks out the cat, the cat has a nice snack for dinner, and the parasite is allowed to reproduce again in the cat’s stomach.

Now humans are a “dead end” host for Toxoplasma, but think about its effect in our brain if we happened to ingest this parasite?  What would it do to our amygdala?

Lets discuss the emerging research in this area.

The most interesting emerging research about the behavioral and mental health effect of Toxoplasma is with the disease Schizophrenia. There have been over 54 studies about Toxoplasmosis and schizophrenia, along with other psychoses.  Schizophrenia was rare up until the late 18th century.  Around this time, people started taking cats as pets, which grow throughout the 19th century, along with the incidence of schizophrenia.  Currently, this epidemiological association is only a theory, but a quite interesting one that may have some validity.

Beyond the association of schizophrenia and Toxoplasma, there has been published research on the association of Toxoplasmosis and car accidents.  The research states that, people infected with Toxoplasma are 2-4x more likely of dying in car accidents (3, 4).

The bigger question that comes from this idea is if one parasite can affect our behavior is a way that more or less relinquishes some of our free will, are there other parasite or infection that can produce the same or similar effect?  Research states up to 30-60% of the world could be infected with a toxic parasite that modifies brain activity (5, 6).  And where does this leave us in the question of free will?  We will save these questions for another post.  If you are interested in reading further about free will, check out Sam Harris‘ book Free Will.

For more information on the effect of Toxoplasmosis on the Crazy Cat Lady  follow the links below:

Robert Sapolsky Interview: Toxoplasmosis


(1) Predator Cat Odors Activate Sexual Arousal Pathways in Brains of Toxoplasma gondii Infected Rats

(2) Behavioral changes induced by Toxoplasma infection of rodents are highly specific to aversion of cat odors

(3) Increased risk of traffic accidents in subjects infected with Toxoplasmosis: a retrospective case-control study

(4) Increased incidence of traffic accidents in Toxoplasma-infected military drivers and protective effect RhD molecule revealed by large-scale prospective cohort study

(5) Effect of Toxoplasma on Human Behavior

(6) Toxoplasma gondii: a potential role in the pathogenesis of psychiatric disorders