[Zhihu question] Can cats really cause schizophrenia?
Can keeping cats trigger schizophrenia? At first glance, this idea sounds completely bonkers. There is still a lack of systematic evidence, but there are indeed a few intriguing pieces of circumstantial evidence.
“Cat craze” and the history of schizophrenia
The idea that “cats may cause schizophrenia” was first proposed by E. Fuller Torrey, a psychiatrist at the Stanley Medical Research Institute in the United States, in his book The Invisible Plague[i].
After investigating epidemiological data in historical records, Torrey discovered that schizophrenia was not a mental illness that had existed since ancient times. In fact, schizophrenia suddenly began appearing in large numbers in European medical records in the late 18th century. Its symptoms are striking—confused thinking, auditory hallucinations, visual hallucinations, and so on. Yet before descriptions of schizophrenia appeared simultaneously in Britain and France at the end of the 18th century, medical literature had hardly mentioned these symptoms at all. That is extremely strange.
So what happened in Europe around that time? One little thing: in the late 18th century, as the Romantic movement was gathering steam, keeping cats became fashionable among the poets, avant-garde artists, and left-wing intellectuals of Paris and London. A “cat craze” swept across Europe. Before this, almost nobody anywhere in the world kept cats as pets, apart from the ancient Egyptians. And during that very same period, the incidence of schizophrenia rose sharply.
But what makes us think the outbreak of schizophrenia was connected to keeping cats, rather than to some other event from the same period?
—Toxoplasma.
Toxoplasma: the mind-controlling parasite
Toxoplasma is a single-celled protozoan, a parasite whose primary host is the cat. Infected cats excrete Toxoplasma in their feces. When foraging, rats may come into contact with cat feces on the ground and become infected. If an infected rat is eaten by a healthy cat, Toxoplasma reproduces in the cat’s intestines and is then excreted again. With the help of the cat-and-rat food chain, Toxoplasma spreads from host to host, round after round.
What astonishes scientists most about this chain of transmission is that Toxoplasma infection can actually change a rat’s mind, making it behave in ways that make it more likely to be eaten by a cat.
Normally, rats are highly alert to the smell of cat urine and deliberately avoid areas that smell of it. But studies have found that rats infected with Toxoplasma actually become fond of the smell of cat urine. They do not merely fail to hide; they deliberately move toward places that smell of it[ii]. This greatly increases the odds that the rat will be eaten by a cat—and that Toxoplasma will infect a new host.
A team led by the renowned biologist Robert Sapolsky found[iii] that Toxoplasma can alter rats’ scent preferences because, on the one hand, it invades the rat brain and accumulates in the pleasure circuit, affecting dopamine secretion and regulating the rat’s sense of pleasure; on the other hand, other Toxoplasma parasites live in the rat’s reproductive system and regulate the secretion of sex hormones. Put the two together, and Toxoplasma has basically created “sexually deviant” rats—rats that find the smell of cat urine “sexy” and therefore approach areas frequented by cats. Talk about a truly lethal attraction!
Although many details still need to be studied, this part of the mechanism alone is already so ingenious that it has left scientists completely flabbergasted.
So what does any of this have to do with schizophrenia?
The connection is that Toxoplasma can also live in the human brain.
Toxoplasma and schizophrenia
Humans are not Toxoplasma’s primary host, but we can still become infected. The main routes of infection include:
1. Contact with cat feces.
2. Eating beef, mutton, and other meat infected with Toxoplasma (mainly raw meat), etc.
A growing body of research has found that humans infected with Toxoplasma also show various psychological and behavioral abnormalities.
Let’s talk about schizophrenia specifically:
As early as the 1950s, Czech parasitologist Otto Jirovec reported that people with schizophrenia were more likely to carry Toxoplasma[iv]. Later studies found that people with schizophrenia were two to three times more likely to produce antibodies against Toxoplasma than people without schizophrenia[v].
Another piece of evidence hinting at a connection between schizophrenia and Toxoplasma is dopamine. A notable feature of the brains of people with schizophrenia is their high dopamine levels. A 2011 study found that neuron samples carrying Toxoplasma produced 3.5 times as much dopamine as they did under normal conditions[vi].
It seems that there really is an association between Toxoplasma and schizophrenia that is difficult to ignore.
Isn’t schizophrenia hereditary?
But this hypothesis has one major problem. Schizophrenia is strongly heritable. People with a family history of schizophrenia have a much higher risk of developing it. Scientists have also identified a number of genes associated with schizophrenia. So how could schizophrenia be both a parasitic disease and a hereditary disease?
Torrey, the psychiatrist and author of The Invisible Plague, believes there is actually no contradiction here. In fact, the genes most closely linked to schizophrenia are precisely those related to the immune system’s ability to fight infection. So what may be passed down through generations in families with high rates of schizophrenia is a failed immune response to Toxoplasma. In other words, schizophrenia is strongly heritable because the people in those families all lack immunity to Toxoplasma, making each generation especially vulnerable to infection.
Do we need to be more wary of Toxoplasma, then? If loving cats could possibly make someone go “mad,” can we still keep them?
Although the evidence above puts Toxoplasma on the list of suspects as a possible cause of schizophrenia, scientists still have not found strong causal evidence that Toxoplasma causes schizophrenia. Besides, a large body of research shows that keeping cats can bring their owners many psychological benefits. So there is no need to worry excessively about Toxoplasma. You can keep right on loving your cat—just be careful when changing the litter, and avoid contact with cat feces. (As I mentioned earlier, pork, beef, and mutton may also contain Toxoplasma, so cook meat as thoroughly as possible before eating it.)
For more information about Toxoplasma, see Chapter 4 of The Uninvited Guest in My Brain: How Parasitic Organisms Manipulate Humans and Society.
That’s all.
Written by: Zhichao Wei
[i] Torrey, E. F., & Miller, J. (2001). The invisible plague: The rise of mental illness from 1750 to the present. Rutgers University Press.
[ii] Berdoy, M., Webster, J. P., & Macdonald, D. W. (2000). Fatal attraction in Toxoplasma-infected rats: a case of parasite manipulation of its mammalian host. Proc. R. Soc. B, 267, 267.
[iii] Sapolsky, R. (2003). Bugs in the Brain. Scientific American, 288(3), 94-97.
[iv] Jirovec, O. (1956). Die Toxoplasmose-Forschung in der Tschechoslowakei. Z. Tropenmed. Parasit., 7, 277-277.
[v] Torrey, E. F., & Miller, J. (2001). The invisible plague: The rise of mental illness from 1750 to the present. Rutgers University Press.
[vi] Prandovszky, E., Gaskell, E., Martin, H., Dubey, J. P., Webster, J. P., & McConkey, G. A. (2011). The neurotropic parasite Toxoplasma gondii increases dopamine metabolism. PloS one, 6(9), e23866.
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