While we tend to think of the concept of a cat-and-mouse chase to be a one-way affair, this isn’t always the case with the animals that originated the phrase: a parasite, called Toxoplasma gondii, can create a suicidal aggressiveness in mice, specifically directed toward cats, in order to get the targeted feline to eat the infected rodent. Needless to say, there’s a point to such self-destructive behavior, as the Toxoplasma parasite can only reproduce inside the intestines of a cat.

After the mouse ingests the parasite, it enters the bloodstream through the intestines, and takes up residence in the rodent’s immune cells, and from there it tricks the mouse into trying to get itself eaten by a cat. Dr. Antonio Barragan, a professor in the Department of Molecular Biosciences at Stockholm University in Sweden, explains that "The parasite does this in a very clever way. It enters the cells, lives inside in a little cocoon inside the cell. And from there, it starts to communicate with the cell." Dr. Barragan recently published a study that discovered how Toxoplasma gondii hijacks the rodent’s immune cells, as part of its own reproductive cycle.

"The parasite makes the cell produce also [a molecule called] GABA, which is a neurotransmitter, and that triggers an activation — a whole system in the cell." The GABA neurotransmitter interacts with the cell’s calcium ion channels, prompting the cell to migrate into the mouse’s brain. From there, it starts altering the mouse’s behavior, shutting off the creature’s aversion to feline odors and causing it to become fearless — this results in reckless behavior that almost appears to taunt the cats that the rodent would normally try to avoid.

Once eaten by the cat, the parasite can reproduce freely in the feline’s intestine — for some unexplained reason, Toxoplasma gondii can only sexually reproduce in the intestines of cats, and not in other species. After forming protective oocysts (a protective egg-like structure) around the offspring, the new parasites are passed in the cat’s stool, and can remain dormant for months, waiting to be eaten by another potential host.

Thankfully, Toxoplasma gondii isn’t typically harmful to humans, and is estimated to currently infect between 30 and 50 percent of the world’s population. However, it can produce a condition called toxoplasmosis in individuals with compromised or underdeveloped immune systems — this is why pregnant women are discouraged from cleaning cat litter boxes — and some studies suggest that there is a correlation between Toxoplasma and some neurological disorders.

Dr. Barragan’s research is aimed at preventing Toxoplasma infections in humans, and is currently testing a blood pressure medication that affects the body’s calcium channels, the parasite’s means of controlling infected immune cells.

"Indeed, that medicine had an effect on the spread of the parasite, explains Dr. Barragan. "But I should add that we don’t claim this medicine is a way to cure toxoplasmosis, but we could prove at least that stopping the message from arriving from the parasite could impede the spread of the parasite." 

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