Is cat poop making us crazy?

When a woman who owns cats finds out she is pregnant, she will probably be warned to stop cleaning out the litter box. This is because cat feces can harbor a parasite called Toxoplasma gondii, which can cross the placenta and infect an unborn fetus. The infection can increase the risk of premature birth and low birth weight, and is associated with anemia, deafness, hydrocephalus, and mental retardation in the child after birth. Sometimes, if these problems aren't apparent at birth, they can develop later in life.

T. Gondii is not only found in cat owners, however. Up to a ⅓ of the population in the developed world might harbor the parasite. And for a parasite that seems to possess some potent mind control capabilities, that’s a scary thought.

T. Gondii is a protozoan parasite that likes to live inside the intestines of cats. In the world of parasites, the host where the parasite is able to thrive and reproduce is known as its definitive host; cats are the definitive host for T. Gondii. T. Gondii infection in American cats has been found to range from 16-80%, and worldwide it’s thought to be around the same as what is seen in humans: 30-40%.

Often parasites need help getting into their definitive host, and so they will utilize what is known as an intermediate host to get that help. Cats infected with T. Gondii will shed oocysts (sort of a protozoan parasite version of an embryo) in their feces. The parasite can then infect other cats if they come in contact with an infected cat’s feces.

But rodents also often come in contact with cat feces. In fact, they have a habit of digging undigested food out of cat and dog feces to make a sort of recycled dinner for themselves. Thus, they often end up ingesting T. Gondii, and that’s where things start to get interesting.

It is believed that T. gondii has evolved a way to manipulate the behavior of the rodents that it infects. After being ingested, T. gondii makes its way to their brains. Then, through a mechanism that isn’t yet understood, it does something remarkable.

Rats are born afraid of the smell of cat urine. Even if you take a laboratory rat that has never been outside of his cage--a rat that comes from hundreds of generations of other laboratory rats that never saw or smelled a cat before--and expose him to cat urine, he will exhibit a defensive and aversive response.

T. gondii, however, has found a way to influence the behavior of a rat so as to essentially erase that inborn fear. T. gondii-infected rats are much less averse to cat urine, and don’t avoid it any more than they do the urine of other rats. This change in behavior doesn’t appear to be due to a loss of smell, and it seems to be specific for cat urine.

How does this work in T. gondii’s favor? A rat that is not afraid of cat urine is more likely to put itself in situations where a cat might be nearby. Thus, it is more likely to be eaten by a cat (one of its natural predators), allowing T. gondii to end up right where it wants to be: in a cat’s intestines. Many thus believe that this is an example of T. gondii manipulating rat behavior to find its way back into its definitive host.

Rats, however, are perhaps not the only mammals whose behavior can be manipulated by T. gondii. Mounting evidence suggests that T. gondii may also affect human behavior.

In the middle of the twentieth century, some studies started to link T. Gondii infection to schizophrenia. Since then, a number of studies have also found this association. One study that used magnetic resonance imaging (MRI) to look at the brains of schizophrenic patients found that reductions in gray matter (which are a recognized, but not understood, hallmark of schizophrenia) were only seen in patients who also tested positive with T. Gondii.

What the link between T. Gondii infection and schizophrenia means isn’t clear. But a number of studies have suggested links between T. Gondii infection and other psychiatric morbidities as well. For example, there are indications that T. Gondii infection is associated with suicide. In a Danish cohort of almost 46,000 women followed over a period of more than 10 years, researchers found a 1.5-times increased risk of self-directed violence in infected mothers. There are indications of a T. Gondii association with traffic accidents and homicides as well. Curiously, one study even found that T. Gondii infected men rated the odor of cat urine as more pleasant than non-infected men (although in women, the opposite relationship was seen).

How and why T. Gondii would influence human behavior is unclear. It’s possible that T. Gondii just treats rodent and human brains similarly, and any effect on human behavior is simply an unintentional byproduct of the similarities our brains have with other mammals. We have just begun to scratch the surface of T. Gondii’s effects on humans, however. If this infection truly is influencing human behavior, then perhaps we have to begin considering microbial influence as a significant factor in behavior, especially abnormal psychiatric behavior.

Flegr J (2013). How and why Toxoplasma makes us crazy. Trends in parasitology, 29 (4), 156-63 PMID: 23433494

YOUR BRAIN, EXPLAINED

Sleep. Memory. Pleasure. Fear. Language. We experience these things every day, but how do our brains create them? Your Brain, Explained is a personal tour around your gray matter. Building on neuroscientist Marc Dingman’s popular YouTube series, 2-Minute Neuroscience, this is a friendly, engaging introduction to the human brain and its quirks using real-life examples and Dingman’s own, hand-drawn illustrations.

Dingman weaves classic studies with modern research into easily digestible sections, to provide an excellent primer on the rapidly advancing field of neuroscience. - Moheb Costandi, author, Neuroplasticity and 50 Human Brain Ideas You Really Need to Know
...a highly readable and accessible introduction to the operation of the brain and current issues in neuroscience... a wonderful introduction to the field. - Frank Amthor, PhD, Professor of Psychology, The University of Alabama at Birmingham, author, Neuroscience for Dummies
An informative, accessible and engaging book for anyone who has even the slightest interest in how the brain works, but doesn’t know where to begin. - Dean Burnett, PhD, author, Happy Brain and Idiot Brain
Reading like a collection of detective stories, Your Brain, Explained combines classic cases in the history of neurology with findings stemming from the latest techniques used to probe the brain’s secrets. - Stanley Finger, PhD, Professor Emeritus of Psychological & Brain Sciences, Washington University (St. Louis), author, Origins of Neuroscience

Order Now

BIZARRE

This book shows a whole other side of how brains work by examining the most unusual behavior to emerge from the human brain. In it, you'll meet a woman who is afraid to take a shower because she fears her body will slip down the drain, a man who is convinced he is a cat, a woman who compulsively snacks on cigarette ashes, and many other unusual cases. As uncommon as they are, each of these cases has something important to teach us about everyday brain function.

Bizarre is a collection of stories of how the brain can create zombies, cult members, extra limbs, instant musicians, and overnight accents, to name a few of the mind-scratching cases. After reading this book, you will walk away with a greater appreciation for this bizarre organ. If you are a fan of Oliver Sacks' books, you're certain to be a fan of Dingman's Bizarre. - Allison M. Wilck, PhD, Researcher and Assistant Professor of Psychology, Eastern Mennonite University
A unique combination of storytelling and scientific explanation that appeals to the brain novice, the trained neuroscientist, and everyone in between. Dingman explores some of the most fascinating and mysterious expressions of human behavior in a style that is case study, dramatic novel, and introductory textbook all rolled into one. - Alison Kreisler, PhD, Neuroscience Instructor, California State University, San Marcos
Dingman brings the history of neuroscience back to life and weaves in contemporary ideas seamlessly. Readers will come along for the ride of a really interesting read and accidentally learn some neuroscience along the way. - Erin Kirschmann, PhD, Associate Professor of Psychology & Counseling, Immaculata University
Through case studies of both exceptional people as well as those with disorders, Bizarre takes us on a fascinating journey in which we learn more about what is going on in our skull. - William J. Ray, PhD, Emeritus Professor of Psychology, The Pennsylvania State University, author, Abnormal Psychology