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

gondii?

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.

Neospora caninum and Neosporosis

The sign on this public footpath in the UK reminds dog
owners to pick up after their pets. Cattle in the area have
aborted due to Neospora caninum.  Image by Peter Barr
 CC BY-SA 2.0

Neospora caninum, a parasite of dogs, causes abortion in cattle

Neospora caninum is a coccidian parasite of domestic dogs but it causes big problems for the cattle industry. We’re still learning about N. caninum and neosporosis.

In August 2007, dog owners in Somerset, UK were asked to be especially vigilant about picking up after their dogs. The reason was a spike in the number of cattle aborting fetuses in the area—some of the cattle tested positive for a parasite, Neosporum caninum, carried by domestic dogs.

The parasitic disease called neosporosis was first recognized in domestic dogs in Norway in the 1980s. It’s now known that the parasite is present in dogs, cattle, and other animals worldwide. A coccidian, it’s related to Toxoplasma gondii and Cryptosporidium, well known causes of parasitic disease in humans.

Life Cycle of Neospora caninum

The life cycle of N. caninum in dogs is very similar to that of T. gondii in cats. An infected dog has parasites multiplying sexually in the intestine and the infective oocyst is passed in the dog’s feces. Meanwhile, parasites are also multiplying asexually in other tissues.

Harbouring the sexual form of N. caninum in the intestine makes domestic dogs the definitive host of the parasite. It’s not known whether other animals are capable of serving as a definitive host, but wild dogs, such as foxes, wolves, coyotes etc. may do so.

Animals other than dogs that ingest either oocysts in dog feces or animal tissue in which the parasite is present become intermediate hosts – they have only the asexual stage, multiplying in the tissue. Thus, dogs pass on the parasite in their feces, in their tissue if they are eaten by other animals, or to an unborn fetus. No animals other than dogs have been known to spread the parasite in feces.

Neosporosis in Dogs

Neospora caninum infects domestic dogs worldwide with varying prevalence. Studies testing dogs for antibodies to the parasite suggest that more than 30% of dogs are infected in some areas, with the highest numbers in South American countries and in rural dogs, especially those living on cattle farms.

Most infected dogs have no symptoms. When symptoms occur, neosporosis is most severe in newborn puppies, infected during gestation when the parasites move from the bitch’s tissues to the fetus. Puppies suffer paralysis, particularly of the hind legs, and often do not survive. Adult dogs may suffer from an illness similar to toxoplasmosis in cats, or they may develop dermatitis.

Neosporisis in Cattle

Like dogs, cattle everywhere harbour N. caninum, and most show no signs of it. In some herds, close to 90% of cattle are infected and the parasite is thought to account for more than 40% of abortions – a significant cause of economic loss for cattle farmers. Many infected fetuses and calves appear normal, however, and it is still unclear what factors cause or prevent disease symptoms.

In cattle, N. caninum is transmitted only from a pregnant cow to her fetus—the parasite does not pass between cows in a herd. Some cows, then, must acquire the parasite from dogs, consuming oocysts while grazing where dogs have defecated. It’s easy to imagine how farm dogs and livestock (sheep, goats, and horses can also be infected) may have increased the prevalence of the parasite, with dogs eating the remains of aborted young, becoming infected, and then passing infective oocysts in feces deposited where livestock graze. Calves born without symptoms, meanwhile, pass the parasite on to their own young.

Whether the abortions occurring in Somerset in England resulted from infected dogs defecating on cattle farms, or whether they came from silent infections already present in the cattle remains unknown; however, picking up after your dog is always good practice, and will lessen the risk of spreading not only neosporosis, but other diseases as well.

Sources:

Foundations of Parasitology 8th Ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2009.

Review of Neospora caninum and Neosporosis in Animals. Dubey, J. P. The Korean Journal of Parasitology 41:1 Mar 2003, 1-16.