Toxoplasma gondii is a brain parasite that makes mice lose their instinctive fear of cats and may lead to schizophrenia in humans. Scientists think that the parasite works by inflaming the brain or by creating cysts. But it now appears that Toxoplasma is doing something far more insidious — and permanent — to the brains of its hosts.

Toxo is a common and highly successful brain parasite that infects mammals — including humans — and birds. However, this neurotropic protozoan can only reproduce sexually in the stomachs of cats, thus making them the parasite’s primary host.

When the parasite infects an intermediate host, like a mouse, it infiltrates the central nervous system and forms slow-growing cysts inside neurons where it can persist for the entire life of the infected animal. But in order to complete its reproductive life cycle, it must be ingested by a cat. The parasite is able to achieve this by altering the behavior of the intermediate host. In the case of mice, it alters their psychology such that it loses its innate, hard-wired aversion to the smell of cat urine.


Just how the parasite accomplishes this feat of neural rewiring, however, has largely remained a mystery.

To learn more, Wendy Ingram from the University of California at Berkeley and colleagues removed various strains of Toxo in mice to see if it would have any effect. To their surprise, they found that the mice still retained their lack of fear after removal of the infection; they remained unafraid of cat urine odor even after inflammation markers or cysts could no longer be detected.

And based on experiments performed with various strains, the researchers learned that the brains of mice are permanently altered after just three weeks following an infection.


Ingram speculates that Toxo works by injecting certain effector proteins into the host cells — and without active parasite replication.

“Thus T. gondii may interact with and manipulate its intermediate hosts without the requirement of cyst formation or parasite persistence,” note the authors in the study, which now appears in PLoS. “In light of these findings and our data reported here, we believe that a new noncyst-centric model of T. gondii-mediated behavior manipulation of the mouse intermediate host is warranted.”

This news is actually quite disturbing on account of the prevalence of Toxo in humans; it’s believed that up to one-third to half of the world’s population is infected by the parasite. It’s typically transmitted by handling infected cat feces, but the greater risk of infection comes through the ingestion of infected meat or the handling of unwashed vegetables and fruit.

And like mice, our brains are not immune to the effects of an infection. In fact, studies show that the parasite could be responsible for increased rates of schizophrenia and suicide.

Read the entire study at PLoS: “Mice Infected with Low-Virulence Strains of Toxoplasma gondii Lose Their Innate Aversion to Cat Urine, Even after Extensive Parasite Clearance.”

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