A parasite which makes for fearless mice

A new paper about the effects of the toxoplasma parasite.

Wendy Marie Ingram, Leeanne M. Goodrich, Ellen A. Robey, Michael B. Eisen. Mice Infected with Low-Virulence Strains of Toxoplasma gondii Lose Their Innate Aversion to Cat Urine, Even after Extensive Parasite ClearancePLoS ONE, 2013; 8 (9): e75246 DOI:10.1371/journal.pone.0075246

Abstract: Toxoplasma gondii chronic infection in rodent secondary hosts has been reported to lead to a loss of innate, hard-wired fear toward cats, its primary host. However the generality of this response across T. gondii strains and the underlying mechanism for this pathogen-mediated behavioral change remain unknown. To begin exploring these questions, we evaluated the effects of infection with two previously uninvestigated isolates from the three major North American clonal lineages of T. gondii, Type III and an attenuated strain of Type I. Using an hour-long open field activity assay optimized for this purpose, we measured mouse aversion toward predator and non-predator urines. We show that loss of innate aversion of cat urine is a general trait caused by infection with any of the three major clonal lineages of parasite. Surprisingly, we found that infection with the attenuated Type I parasite results in sustained loss of aversion at times post infection when neither parasite nor ongoing brain inflammation were detectable. This suggests that T. gondii-mediated interruption of mouse innate aversion toward cat urine may occur during early acute infection in a permanent manner, not requiring persistence of parasite cysts or continuing brain inflammation.

Science Daily writes:

Researchers found that mice’s lack of fear of cats persisted even when inflammation markers or cysts could not be detected in mice. (Credit: Wendy Ingram and Adrienne Greene)

The Toxoplasma parasite can be deadly, causing spontaneous abortion in pregnant women or killing immune-compromised patients, but it has even stranger effects in mice. 

Infected mice lose their fear of cats, which is good for both cats and the parasite, because the cat gets an easy meal and the parasite gets into the cat’s intestinal track, the only place it can sexually reproduce and continue its cycle of infection.

New research by graduate student Wendy Ingram at the University of California, Berkeley, reveals a scary twist to this scenario: the parasite’s effect seem to be permanent. The fearless behavior in mice persists long after the mouse recovers from the flu-like symptoms of toxoplasmosis, and for months after the parasitic infection is cleared from the body. ….

“Even when the parasite is cleared and it’s no longer in the brains of the animals, some kind of permanent long-term behavior change has occurred, even though we don’t know what the actual mechanism is,” Ingram said. She speculated that the parasite could damage the smell center of the brain so that the odor of cat urine can’t be detected. The parasite could also directly alter neurons involved in memory and learning, or it could trigger a damaging host response, as in many human autoimmune diseases. …..

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