The beloved classic cartoon “Tom and Jerry” brings amusement and laughter to the young and old alike. As the house cat Tom attempts to capture the mouse Jerry, we also learned our first lesson on predator-prey behavior: mice fear cats.
But then nature pulled a fast one on us.
Behavioral scientists noticed that some mice, which had been afraid of cats, no longer showed any aversion to their natural predator. Some mice were even fatally attracted to them! How did this innate, genetically wired fear of the predator (cat) suddenly vanish out of the prey’s (mouse’s) brain?
It turns out that the fearless mice had been infected with a neurotropic parasite, Toxoplasma gondii (T. gondii). The culprit to this twist in the cat-and-mouse story has a two-part life cycle: part one is exclusively in the cat’s gut, where the parasite reproduces; and part two occurs in a host mammal’s body, which are usually rodents. The life cycle of the parasite can only be completed when both parts are put together (i.e. when the cat preys on the infected-mouse). To ensure that this occurs, mice infected with the parasite lose their fear of cats. This fetches the cat an easy meal, while also enabling the parasite to now reach the cat’s gut and continue its life cycle. Yep, sometimes nature can be sneaky!
T. gondii causes cysts in the brains of infected mice, which may explain their fear behavior changes. Because T. gondii can also infect humans, its ability to modulate behavior and its known association with schizophrenia is an important concern that is being actively investigated by research teams. One such study by scientists at UC Berkeley tested the feline aversion of mice infected by two strains of the parasite, one residing in the brain longer than the other.
To evaluate the feline-aversion in infected mice, the team developed a behavioral assay. The mice were enclosed in a rectangular chamber with a small plastic dish affixed to one end. This dish contained either cat urine or rabbit urine. A normal mouse that fears cats would immediately flee from the smell of cat urine and spend most of its time in the far end of the chamber, but an infected mouse would show no such place preference. As expected, the mice infected with T. gondii were unalarmed and unthreatened by the smell of cat urine. Remarkably, the aversion to cat urine persisted even in mice that no longer had any brain-resident parasites! This is an interesting finding because it was once believed that the physical presence of the parasite changed fear behavior, but this evidence suggests that even a short infection could cause prolonged changes in brain structure.
This study has important implications for the treatment options given to people infected by Toxoplasma gondii. Medication that kills the parasite might not necessarily help restore behavioral deficits. Instead, therapeutic strategies targeting the underlying brain dysfunction might be more promising.
Ingram W.M., Goodrich L.M., Robey E.A., Eisen M.B. & Wilson E.H. (2013). Mice Infected with Low-Virulence Strains of Toxoplasma gondii Lose Their Innate Aversion to Cat Urine, Even after Extensive Parasite Clearance, PLoS ONE, 8 (9) e75246. DOI: 10.1371/journal.pone.0075246.g003
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