Published by the Students of Johns Hopkins since 1896
February 21, 2024

Parasite erases fear in the minds of mice

By JOSH SCARALIA | October 3, 2013

What if I were to tell you that a simple infection could cure you of your insurmountable fear of that Orgo Exam?

Sorry, but that’s not going to happen so just keep trucking.

However, when mice become infected with Toxoplasma gondii, they lose their innate fear of cats. This has been known for a while, but until now its long term effects have not been known. Research conducted by Wendy Ingram at the University of California, Berkeley has shown that the parasite’s effect is permanent.

Scientists first placed the mice in a 15 by 7 inch dark enclosure and put a dish at one side with either bobcat or rabbit urine.  Upon introduction to the enclosure, infected mice  did not shy away from the bobcat urine like uninfected mice did. To test whether the effects were long lasting, researchers also did the same test on previously infected mice. The data collected showed a similar trend; previously infected mice did not have a preference for any one side of the enclosure.

In order to refute the idea that T. gondii simply affects the olfactory system, researchers conducted a second test which they called the “Hidden Cookie Test.”  During this test the researchers placed a cookie at one end of the darkened enclosure and tracked the movement of the mice.  They saw that all mice, whether infected or uninfected, preferred the side with the cookie, finding it in approximately 2 minutes.

To track the movement of the mice, researchers used a Motor Monitor Smart Frames System by Kinder Scientific.  This system set up a gridline of beams much like the ones you see in all of the spy movies.  Whenever a mouse “tripped” a beam, a data point for the mouse’s location was created.  Unlike the movies, the mice were not given access to smoke and did not have the aerobatics necessary to avoid the beams.

T. gondii is a parasite that infects the central nervous system of warm blooded mammals and creates cysts in neurons.  These cysts grow slowly, exist for life, but are mainly dormant and are kept in check by the immune system.  However, if the parasite infects a person with a compromised immune system, such as someone with AIDS or perhaps even pregnant women, the cysts can awaken and lead to death, schizophrenia or suicidal behavior.  It is unnervingly estimated that one third of the world’s population is or has been infected with T. gondii.

Luckily, humans are not the parasite’s primary host. These parasites tend to prefer cats.  Furthermore, they are only able to reproduce in one place on earth. You guessed it, the feline intestinal tract.

Researchers hypothesize that the specific reason infection by the parasite causes a loss of fear for cats in mice is that it increases the parasite’s probability of getting back into the feline intestinal track.

Can this hypothesis be true for humans?  Is this parasite the reason why so many humans have an intense love for cats?


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