A fair amount of research has taken place on Toxoplasma gondii, the bizarre parasite that makes mice unafraid of cats, and the latest chapter is a strange one.
A new study shows that even a brief infection with a weakened form of the protozoan caused mice to permanently lose their innate fear of cats.
The protozoan is known to cause this change in mice after a lingering infection and after it produces cysts in the mouse brain, according to the study, published online Sept. 18 in the journal PLOS ONE. But until now scientists didn't know this apparently long-lasting change could occur after only a short infection, and without development of cysts and brain inflammation. The study also showed the change occurred with weakened forms of all three major variants of the protozoa found in North America. [The 10 Most Diabolical & Disgusting Parasites]
"It is remarkable that even after the infection has been largely or completely cleared, a profound behavioral change persists," said Wendy Ingram, a study author and researcher at the University of California, Berkeley, in a statement from PLOS ONE. "Simply having a transient infection resulting in what is potentially a permanent change in host biology may have huge implications for infectious disease medicine."
Ingram isn't sure the mechanism involved in the long-term behavior change, though she speculates the parasite may disrupt the smell region of a mouse's brain, preventing the rodent from detecting cat odor that would trigger the fear. Another possibility is that the parasite directly modifies mouse brain cells that are linked to memory and learning.
Toxoplasma gondii is found throughout the world and infects a large number of mammals, including humans. However, the protozoan can only reproduce within the bodies of cats, and in mice, the mind-controlling parasite has evidently evolved to make mice unafraid of felines and even, according to some research, sexually attracted to the odor of cat urine; this makes it more likely infected mice will be eaten, and the parasite will make it back into a cat to spawn.
The parasite is found in as many as one-third to one-half of humans, and its presence in the brain has been linked with suicide attempts. It may affect other areas of mental health: One study suggested that people with the parasite scored higher on tests of neuroticism, an emotional or mental trait characterized by high levels of anxiety or insecurity.