can cats cause mental illness?

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Posted by Dr. Kim Smyth on Jul 01 2015


A recent study linked childhood cat ownership with mental illness later in life, and the researchers think it might be because of exposure to the toxoplasma organism, which causes the infectious disease toxoplasmosis.

Now, I’m not saying that toxoplasmosis doesn’t cause mental illness in humans. Maybe it does. But I CAN say that if you have toxoplasmosis, you probably got it somewhere other than your cat. Once you understand the how the life cycle of toxoplasma organisms work, and how cats spread the disease, you’ll feel better about it, too.

Toxoplasmosis is a disease spread by the protozoan parasite Toxoplasma gondii. This particular organism has a very complex life cycle, but we can simplify it a bit by saying that cats are the definitive host. That means that of all the creatures, the only one in which toxoplasma can reproduce is the cat.

The organism can infect other species, however, they mostly wind up as infective cysts within the tissues. So, our feline friends become infected when they eat infected animals. Cats can also (rarely) become infected when they ingest infective eggs in the soil or water. But mostly it’s from eating infected prey.

Now, here’s the part you’ll want to pay attention to. Infected cats will only shed toxoplasma eggs in their stool for the first three to 20 days after they are infected. That’s it! And they generally develop immunity after their first infection, so they only shed eggs once in their lifetime. (Of course, there is an exception—cats who are immunosuppressed may shed eggs again later in life.)

So, during the course of your cat’s nine lives, if he becomes infected with toxoplasma organisms, he will only be infective to you or your family for about two weeks. Two weeks out of his entire life! What’s more, the eggs aren’t even infective in the stool for the first 24 hours (sometimes longer!), so if you’re scooping the litter box daily, you aren’t even exposed to infection even in those 17 days that your cat could potentially be shedding the organism!

You are much, much more likely to contract toxoplasmosis from eating undercooked meat (the same way your cat can get it) or gardening in soil used by a cat who was shedding the eggs of the organism, not by changing your house cat’s litter box.

Yes, it’s a mad, mad world. But it’s not your cat’s fault.