Friday, 26 June 2015

Crazy – Cats, Toxoplasma, and Your Mental Health

Is your cat making you crazy? If you’ve been reading the many recent articles about a theoretical link between Toxoplasma gondii infection and schizophrenia (and other mental health issues) you may think that your cat is a significant threat to your mental health.

Infected rodents pass Toxoplasma gondii
to wild and domestic cats. Yintan, CC BY-SA 3.0

But to say that your cat could literally make you mentally ill is a bit disingenuous. It’s not the cat itself, it’s the parasite Toxoplasma gondii that’s linked to mental illness. And while it’s true that T. gondii is a parasite of cats, it’s equally true that it infects more than 300 other species of animals, and there are a number of ways to catch it without a cat being anywhere in sight. Don’t be too hard on your pampered kitty which, in all likelihood, is not capable of infecting you with T. gondii.

Cats and Toxoplasma gondii

 Cats typically catch T. gondii by eating infected rodents. In the cat, T. gondii multiplies in the intestine and millions of microscopic oocysts (you might think of them as eggs, or germs) are released into the outside world in the cat’s feces. But an infected cat will usually only shed these oocysts for one to two weeks of its entire life - about 0.2 percent of its life if it lives to be sixteen. Cats that don’t hunt, don’t eat raw meat, and particularly cats that don’t go outside, are unlikely to be infected, and extremely unlikely to infect you with T. gondii.

Toxoplasma gondii in the Environment

There’s a bigger picture of course. Domestic cats have been instrumental in spreading T. gondii around the world and contaminating the environment everywhere with oocysts. But this parasite can survive without cats for a very long time, if not indefinitely. This is because the parasite multiplies asexually (clonally) in the tissues of any warm blooded vertebrate (and even fish can be infected). It can multiply in human tissues, in chickens, sheep, dogs, rats, mice, horses, and 300+ other animals. If a pig eats an infected rat, the pig will catch toxoplasmosis. If a wild or feral dog eats the pig, the dog will catch toxoplasmosis. So blaming it all on the cats at this point is like closing the barn door after the horses have left.

How Do Humans Get Toxoplasma

The cat is the only animal in which Toxoplasma gondii
can reproduce sexually, but it is widespread in the
tissues of other species. CDC image.

Humans become infected with Toxoplasma gondii in various ways:
•    Eating undercooked meat
•    Consuming food or water contaminated with oocysts
•    Exposure to oocysts in soil (gardening etc.)
•    Congenitally, from mother to fetus
•    Through blood transfusion or organ donation
•    Direct contact with an acutely affected cat
•    Sexual transmission (theoretical)

In the end, it’s clear that while it is possible to catch T. gondii from contact with your beloved domestic cat, if the cat has an acute case of toxoplasmosis, this is an unlikely source of infection. And while cats certainly bear responsibility for spreading this parasite far and wide, it’s the parasite that’s linked to mental illness, not the cat.

Further reading

Centers for Disease Control and Prevention. “Toxoplasmosis (Toxoplasma infection).”

Flegr, Jaroslav (2015). “Schizophrenia and Toxoplasma gondii: an undervalued association?”
Expert Review of Anti-infective Therapy 13:7 , 817-820

Flegr, J., Klapilová, K., & Kaňková, Š. (2014). “Toxoplasmosis can be a sexually transmitted infection with serious clinical consequences. Not all routes of infection are created equal.” Medical Hypotheses, 83(3), 286-289.

McAuliffa, Kathleen. (2012). “How Your Cat is Making You Crazy.The Atlantic

Zhang, M., Yang, Z., Wang, S., Tao, L., Xu, L., Yan, R., ... & Li, X. (2014). “Detection of Toxoplasma gondii in shellfish and fish in parts of China.” Veterinary Parasitology, 200(1), 85-89.