A few weeks ago, I wrote an article debunking a persistent myth that suggests pregnant women with cats should get rid of them due to the danger of contracting a disease called toxoplasmosis from cat feces.
To ease any fears you may have, while it's true that cats are the primary hosts for the Toxoplasma gondii parasite, most humans in the U.S. are exposed not through infected cat poop, but through raw or undercooked meat, unwashed fruits and vegetables, and contaminated water or soil.
And while it's also true that toxoplasmosis can be devastating to women who are pregnant, employing a few common sense precautions is all that's required to stay safe.
So, there's one myth about the "dangers" of cat ownership demystified, but there's another bit of speculation about cats and toxoplasmosis floating around out there, animated by sensational headlines like "Cats Can Literally Make You Crazy"1 that also deserves a closer look.
Toxoplasmosis Infection Rates in Cats and Humans
Adult cats typically acquire the parasite and develop toxoplasmosis by eating infected rodents, birds and other animals. Estimates are that about 40% of cats in the U.S. are infected,2 however, most show no symptoms because their immune system responds appropriately.
Once inside a cat's intestines, T. gondii produces millions of resilient, thick-walled oocysts that complete their life cycle in the gastrointestinal (GI) tract and re-enter the environment in cat poop. Oocysts can live more than 18 months in soil and water.
According to Science magazine, while only about 11% of people in the U.S. are infected with T. gondii, rates are much higher in regions of the world where sanitation is poor, or people eat more raw meat. For example, over 90% of people in some parts of Europe and South America are infected.3
Toxoplasmosis Causes Brain Alterations in Rodents
Toxoplasma gondii is actually a brain parasite, and scientists have long wondered if it plays a role in mental illness, especially schizophrenia. According to Science magazine, while over 100 studies4 have found a link between toxoplasmosis and schizophrenia, not one has concluded the parasite is the definitive cause.5
Scientists suspect toxoplasmosis may cause mental illness based on the effect it has on rodents, who seem to lose their fear of the odor of cat urine and have been observed walking right into the mouths of waiting cats. The scientists' theory is that the parasite alters brain function by forming cysts in regions that process fear and decision-making and may also influence behavior by increasing levels of the neurotransmitter dopamine, which is involved in reward-seeking and risk-taking.
Scientists also suspect the infection may cause permanent alterations in the brain, since affected rodents remain without fear of cats long after the parasite has been cleared from their bodies. The parasite also forms cysts inside human neurons. From Science magazine's Emily Underwood:
"In people with HIV or other immune-weakening conditions, the cysts can grow and replicate, causing deadly brain inflammation, dementia, and psychosis. Although scientists have long assumed the cysts are benign in healthy people, a growing body of data suggests T. gondii infection can alter personality and increase the chance of developing schizophrenia and other mental illness.
Even without directly infecting the brain, a chronic T. gondii infection can ramp up inflammation, and inflammation has been linked to mental disorders such as schizophrenia, autism, and Alzheimer's disease."6
Does T. Gondii Exposure Cause Mental Illness in Humans?
While studies seem to show that toxoplasmosis causes "mental illness" in rodents, testing the hypothesis in humans is extremely difficult according to Duke University geneticist Karen Sugden, Ph.D.
In a 2016 study, Sugden looked at 200 New Zealanders in their late 30s infected with T. gondii and concluded that, "On the whole, there was little evidence that T. gondii was related to increased risk of psychiatric disorder, poor impulse control, personality aberrations or neurocognitive impairment."7
However, she doesn't dismiss the idea that the parasite may cause schizophrenia. She says to test the theory, researchers would need to know if study participants were exposed to the parasite as children or teens, before the typical age of onset of schizophrenia (late teens/early 20s).
Sugden's study, like others, used small sample sizes. Schizophrenia only occurs in about 1% of the population, so to obtain realistic statistical results, researchers would need to follow tens or even hundreds of thousands of people over long periods, testing for T. gondii exposure and mental illness at regular intervals to determine which came first.
A recent large-scale Danish study looked at 80,000 blood donors.8 The number of donors with schizophrenia was quite small (151), however, the researchers found that people exposed to T. gondii had a 47% increased risk of being diagnosed with schizophrenia. Of the 151 donors with schizophrenia, 28 were determined to have tested positive for T. gondii before being diagnosed with schizophrenia. The researchers found that these individuals were 2.5 times more likely to develop the disease post-exposure.
The Danish study results are similar to the results of other large studies that have also uncovered about a 2.5-fold increase in the chance that people with a toxoplasmosis infection will be diagnosed with schizophrenia. However, because the incidence of schizophrenia in the general population is so small, T. gondii exposure increases the odds only slightly.
Dr. Robert Yolken, a virologist at the Johns Hopkins University School of Medicine in Baltimore, Maryland, and one of the authors of the Danish study, and other researchers suspect that toxoplasmosis alone may not cause mental illness, but that the parasite "interacts with genetic variants that make some people more susceptible."
Do Cat Parents Need To Be Concerned About Developing Schizophrenia?
If you happen to be one of the 11% of U.S. residents (1 in 3 people worldwide) who contracts a T. gondii infection, according to the most recent research, your risk of developing schizophrenia as a result is as low or lower than other risk factors for the disorder you probably aren't even aware of (e.g., living in a city).9
If someone in your family suffers from schizophrenia or another mental illness, research actually suggests pet ownership can be tremendously beneficial. If you're a cat parent, and especially if you're expecting a child, your goal should be to avoid potential exposure to the parasite. This is easy to do by simply following a few common sense precautions:
- If you're pregnant and have one or more kitties, if possible, hand off litterbox chores to someone else in the family for the duration of your pregnancy
- Wear disposable gloves to clean the litterbox, as well as a face mask if you're immunosuppressed
- Keep the litterbox in pristine condition; the longer infected cat poop sits in there, the higher the risk that the eggs of the parasite will become infective
- If you also have a dog, make sure he doesn't snack on cat poop
- Don't allow your cat to roam freely outdoors; in good weather, either walk him with a harness and leash, or give him access to a secure outdoor enclosure
- Freeze meats for several days before thawing to feed your cat (or cooking them for your family); peel or wash fruits and vegetables before eating
- As always, use soap and hot water to wash cutting boards, dishes, utensils, counters and hands after they come in contact with raw meat, poultry, seafood and unwashed fruits or vegetables
- Cover outdoor sandboxes when not in use to keep kitties from using them as litterboxes
- Wear gloves when gardening or doing yard work, or whenever you may come in contact with soil, sand or water that could be contaminated with cat feces; wash your hands thoroughly afterwards
- Avoid handling or adopting stray or unknown cats while pregnant, and keep in mind kittens are at an especially high risk of shedding T. gondii oocysts.
