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How a Family Pet Can Decrease Risk for Mental Disorders

Analysis by Dr. Karen Shaw Becker

early exposure to dogs reduce risk of schizophrenia

Story at-a-glance -

  • A recent study suggests exposure to a dog in the home early in life may lower the risk of developing schizophrenia as an adult
  • The study results show that people exposed to a family dog before age 13 are as much as 24% less likely to be diagnosed with the disorder later in life
  • Past studies have suggested that exposure to the toxoplasmosis parasite, of which cats are a primary host, may increase the risk of developing schizophrenia; however, most people in the U.S. acquire the parasite by other means (e.g., undercooked meat)
  • Fortunately, it’s a simple matter to avoid contact with the Toxoplasma gondii parasite by following a few commonsense precautions

It seems almost every day we learn something new and amazing about the ways in which dogs benefit the lives of humans. Today's lesson comes from a recent study that suggests exposure to a canine companion as a child may lower the risk of developing schizophrenia as an adult.1

Past studies have identified pets in the home as environmental factors that appear to alter the human immune system, impacting, for example, allergic responses and stress reduction effects on brain chemistry. Some scientists theorize that the immune modulation effect of pets may also influence the risk of developing psychiatric disorders to which a person is genetically or otherwise susceptible.

"Serious psychiatric disorders have been associated with alterations in the immune system linked to environmental exposures in early life," lead study author Robert Yolken, M.D., of the Johns Hopkins Children's Center said in a news release, "and since household pets are often among the first things with which children have close contact, it was logical for us to explore the possibilities of a connection between the two."2

Study Looked at People Diagnosed With Schizophrenia or Bipolar Disorder

For their study, Yolken and researchers at Sheppard Pratt Health System in Baltimore examined links between exposure to a family pet (cat or dog) during a child's first 12 years and a later diagnosis of schizophrenia or bipolar disorder. The team looked at a total of 1,371 men and women between 18 and 65 years of age.

Within that group were 396 people with schizophrenia and 381 with bipolar disorder, all of whom were recruited from Sheppard Pratt Health System inpatient, day hospital, and rehabilitation programs. There were also 594 control group members from the Baltimore area who had been screened to rule out any current or past psychiatric disorders.

All the people in the study were asked if they had a cat or dog (or both) at home during their first 12 years of life. Those who reported that a pet cat or dog was in their house when they were born were considered to be exposed to that animal since birth.

Exposure to a Dog Early in Life May Make You Much Less Likely to Develop Schizophrenia

While the researchers found no links between exposure to dogs and bipolar disorder, or between cats and either disorder (more about this later), they discovered a surprisingly significant decrease in the risk of developing schizophrenia in people exposed to a dog early in life.

Their findings suggest that people exposed to a family dog before their 13th birthday are significantly less likely — as much as 24% — to be diagnosed with schizophrenia later in life. And the largest protective effect seems to be with kids who either had a dog in the home when they were born, or who were first exposed after birth but before age 3.

There are 3.5 million people diagnosed with schizophrenia in the U.S. According to Yolken, if the measure the research team used is an accurate reflection of relative risk of developing the disorder, it suggests that about 840,000 cases or 24% might be prevented by childhood exposure to a dog or to other factors that come along with exposure to a dog in the home.

"There are several plausible explanations for this possible 'protective' effect from contact with dogs — perhaps something in the canine microbiome that gets passed to humans and bolsters the immune system against or subdues a genetic predisposition to schizophrenia," Yolken says.

Needless to say, this study is just the tip of the iceberg. Per ScienceDaily:

"The researchers caution that more studies are needed to confirm these findings, to search for the factors behind any strongly supported links, and to more precisely define the actual risks of developing psychiatric disorders from exposing infants and children under age 13 to pet cats and dogs."3

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Can Exposure to a Cat in Early Life Increase the Risk for Schizophrenia?

As I noted earlier, this study found no links between exposure to dogs and bipolar disorder, nor links between cats and either disorder — except the researchers did find a slight increase in risk for both disorders in people who had their first exposure to cats between 9 and 12 years of age. According to Yolken, this finding suggests that the age at exposure may be "critical" as to whether or not it alters the risk.

For the record, Yolken co-authored a 2003 review paper that suggested evidence from several epidemiological studies conducted over the previous 50 years showed a connection between exposure to the Toxoplasma gondii parasite (cats are the primary hosts of this parasite, which causes toxoplasmosis) and an increased risk for schizophrenia.4

According to Yolken, a large number of people in those studies diagnosed with serious psychiatric disorders, including schizophrenia, also had levels of antibodies to the toxoplasmosis parasite.

However, according to Duke University geneticist Karen Sugden, testing the hypothesis that toxoplasmosis causes mental illness in humans is extremely difficult. 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."5

With that said, Sugden 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 (in which Yolken also participated) looked at 80,000 blood donors.6 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.

Yolken 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."7

Do Families With Cats Need To Be Concerned About Schizophrenia?

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.

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 that you probably aren't even aware of (e.g., living in a city).8

If someone in your family suffers from schizophrenia or another mental illness, research actually suggests pet ownership can be tremendously beneficial. To play it very safe — especially if you're expecting a child — it's best to avoid potential exposure to the parasite. This is easy to do by simply following a few commonsense precautions:

  1. 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
  2. Wear disposable gloves to clean the litterbox, as well as a face mask if you're immunosuppressed
  3. 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
  4. If you also have a dog, make sure he doesn't snack on cat poop
  5. 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
  6. 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
  7. 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
  8. Cover outdoor sandboxes when not in use to keep kitties from using them as litterboxes
  9. 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
  10. 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

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