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January 20, 2014 at 10:06 AMComments: 2 Faves: 1

Study: Cat Bites May Cause Depression

By Erin Froehlich More Blogs by This AuthorFrom the Feline 101 Blog Series

This isn’t the first we’re hearing about cats and the odd behavior of the toxoplasma gondii parasite.  About a year ago, researchers revealed a list of absolutely bizarre sci-fiesque side effects they were causing. As it turned out, this common feline parasite actually rewires fear and pleasure areas of the brain - an extreme example of survival adaptation that amounted to mind control. Rats infected by t. gondii not only lost their natural fear of cats, they were actually drawn to them. Even people showed strange personality changes. Now, new studies suggest t. gondii may also be causing depression in people bitten by cats.  Could t. gondii explain the tendency toward neurotism observed in “cat people?” Could this finding be the key to a new method of depression treatment? Should cat owners be worried? Here’s what the experts are saying.

The Rat Studies

While t. gondii is relatively harmless to most people, we knew well before these findings – all the way back to the 1920’s, in fact - that it could cause mental defects and even death in infants or people with weakened immunity.  (That’s the reason doctors advise pregnant women to hand over litter duties!) As it turned out though, there was more going on than anyone had suspected.  When lead researcher Jaroslav Flegr discovered that rats infected with t. gondii had completely lost their in-born fear of cats, he knew something significant was going on. And when he found that the parasite had actually rewired both the fear and pleasure areas of the rat’s brains, he became even more curious. What else could this thing be doing?!

On further research Flegr found that besides their loss of feline fear, the rats brain’s altered fear center caused infected rats to become much more active (and thus more attractive to cats) and their brain’s altered pleasure center caused them to be drawn to, and even sexually aroused by, the smell of cat pee. T. gondii even made them more desirable to other rats.  “It’s a very strong effect…” said neurobiologist Ajai Vyas, who worked alongside Flegr. Seventy-five percent of the females would rather spend time with the infected male!”

In short, the parasite had developed an ingenious adaptation to ensure its reproduction.  By making rats more attractive to both rats and cats, toxoplasma promoted its spread both through reproduction and ingestion. Yeah. Creepy. But as shocking as those finding were, there was more - rats weren’t the only creatures being affected by t. gondii. Toxoplasma has been causing a list of subtle, yet bizarre changes in the healthy people we previously thought it had no effect on.

T. Gondii in Humans

Of course, after observing these effects in rats, researchers were eager to find out what sort of effects toxoplasma might be having on people.  The results were similarly strange. Human studies in the Czech Republic (where 30 to 40% of the population were already t. gondii carriers) showed consistent, gender-specific personality changes.

  • Infected men were found to be more introverted, suspicious, unselfconscious, sloppily dressed, likely to disobey rules, and had fewer friends than uninfected men.
  • Infected women were exactly the opposite – more extroverted, trusting, self-conscious, well-dressed, popular and rule-abiding than uninfected women!

And though more research will be needed to confirm this, preliminary results indicated sexual side-effect similar to the ones identified in rats as well. It’s believed the parasite may cause a spike in testosterone levels in men, promoting more masculine features and making infected men more attractive to women.  Also, though no study participants admitted to being “turned on” by the smell of cat pee, they were less bothered by it than uninfected people.

At this point you may be wondering how on earth it could be that we didn’t notice this already. A personality-changing parasite seems like it would pretty hard to miss! However, as Flegr explains, these changes, while unnerving, are minor. Though toxoplasma can make a woman more extroverted than she would have been, it won’t turn an introvert into a party animal. And at the risk of sending a swarm of insecure men wading through cat litter boxes, though t. gondii might make a guy a look a little more masculine,  the minor effects definitely aren’t worth seeking out a parasite infection.

Especially considering the increasing evidence linking t. gondii to more serious mental disorders.

Toxoplasma and Depression?

Back to the results of the new Lund University study - apparently a staggering 40% of people bitten by cats go on to develop depression. Coincidence? Could it simply be that depressed people are more likely to own cats, and thus, more likely to be bitten by them?  Maybe.  Statistics do show as many as 28% of dog bite victims also go on to develop depression. But the incidence with cats bites is still 12% higher than those of dogs, and when you consider the national incidence of depression in just 8.8% of the total popular, 40% is, at the very least, compelling.  Especially in light of previous studies linking t. gondii to mental disorders.

Besides the odd, yet minor personality changes t. gondii can cause, research suggests there is some risk of more serious mental side effects as well. Studies have found that people infected with toxoplasma are 7 times more likely to commit suicide than uninfected people and are even between 2 and 3 times more likely to develop schizophrenia.  In fact, some researchers even claim a direct link between the increase of cat ownership and the rising incidence of schizophrenia in the 18th century.

“Textbooks today still make silly statements that schizophrenia has always been around… [But] the epidemiology literature contradicts that completely…” psychiatrist E. Fuller Torrey told The Atlantic. He says it was the cat craze that began with the “poets and left-wing avant-garde Greenwich Village types” in Paris and London which triggered the jump in schizophrenia, and to his credit, recent genome studies on schizophrenia are in keeping with that assertion.*

 “I don’t want to cause any panic… In the vast majority of people, there will be no ill effects, and those who are affected will mostly demonstrate subtle shifts of behavior. But in a small number of cases, [Toxo infection] may be linked to schizophrenia and other disturbances associated with altered dopamine levels—for example, obsessive-compulsive disorder, attention-deficit hyperactivity disorder, and mood disorders. The rat may live two or three years, while humans can be infected for many decades, which is why we may be seeing these severe side effects in people. We should be cautious of dismissing such a prevalent parasite.” said Joanne Webster, a parasitologist at Imperial College London.

T. Gondii Prevention and Treatment

So given all this, aren’t cats just too dangerous to keep in our homes as pets? Surprisingly, even Mr. Flegr says no. While we should definitely be cautious, the risks are avoidable. Only outdoors cats pose any danger (indoor-only kitties don’t carry the parasite) and even they shed the parasite for just three weeks after becoming infected - usually when they are young and have just began hunting. Flegr himself has two school-aged kids and two indoor/outdoor cats! 

Infection prevention is relatively simple he points out:

  • Keep Cats Indoors: The simplest way to prevent your kitty from picking up and spreading the parasite.
  • Make Eating Surfaces No-Kitty Zones:  And be sure to clean them anyways, just in case a your kitties have taken advantage while you’re away.
  • Scrub or Peel Produce Before Eating: You never know if they grew in a place where cats live.
  • Cook Meat Thoroughly: Or, if you must go rare, freeze before cooking to kill any cysts.  Also, wash your hands and anything that has touched raw meat thoroughly after use.
  • Wash Hands After Cleaning Litter: No matter how careful you are to avoid touching anything.
  • Feed Cats Only Prepared or Cooked Foods: Raw meat is as risky for them as it is for us when it comes to t. gondii.
  • Cover Sandboxes If You Have Them: And have children wash their hands after playing outdoors.

What About Cat Bites?

While t. gondii infection rates are high among people bitten by cats, and cat bites are very serious injuries you need to seek medical attention for, t. gondii can not actually be transmitted this way!

If you’ve been feeling depressed however – whether or not you’ve been bitten– it’s a good idea to seek support.  Depression can become a serious and debilitating disorder if efforts are not made to manage it and there’s no reason to suffer alone! NAMI offers free resources for depression sufferers on their website.

As for suspicions that you may already be infected, while there are blood tests your doctor can administer, there is currently no cure for toxoplasmosis infection.  There are treatments that can be make t. gondii less active and improve immune system functioning in symptomatic or at-risk people (pregnant women, infants, people with immune deficiencies), but that’s as much as they can do. Unfortunately, once you have t. gondii, you’ll always be a carrier. Fortunately, side effects are typically so mild most people will never even know they have it and any cat-loving side effects only makes them cooler.

*Interestingly, the same antipsychotic medications prescribed to control schizophrenia have been shown to disrupt the growth of t. gondii in Petri dishes and to prevent the infamous feline attraction side effect when given to infected rats.


Daily Mail: Why A Cat Bite Could Trigger Depression

The Atlantic: How Your Cat is Making You Crazy

Medical News Today: How Cat Litter Parasite Toxoplasma Gondii Influences the Brain

Photo Credits



leanne surfleet@flickr

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