Once a Toxoplasma Parasite Infects Mice, They Never Fear Cats Again

Toxoplasma gondii, a protozoan parasite that can only sexually reproduce within cat guts, regularly infects warm-blooded mammals. In healthy humans, it usually does not cause adverse affects, though it can seriously tamper with other species’ behaviors. Infected mice, for example, are known to approach their arch-enemy, the cat, without a shred of fear.

Toxoplasma changes the mice’s innate, natural fear of cats, though researchers don’t really understand how the parasite pulls this off. Some speculated that inflammation or parasite eggs in the brain might account for the mice’s inexplicable feline love. Now, it seems, that is not the case. According to new research, that rewiring persists even after the mice have been purged of their parasite load.

Scientists placed ten previously infected and ten never infected mice into enclosures containing traces of either rabbit or bobcat urine. They monitored the mice’s movements and repeated the experiment two weeks, two months and four months after the infected group had first been cured. While the never infected group cowered and avoided the bobcat urine as mice are expected to do, the previously infected rodents were unfazed by the bobcat’s traces. ”It is remarkable that even after the infection has been largely or completely cleared, a profound behavioral change persists,” the authors said in a statement. “Simply having a transient infection resulting in what is potentially a permanent change in host biology may have huge implications for infectious disease medicine.”

Toxoplasma is estimated to infect nearly one-third of humans worldwide, but what, if anything, these results mean for humans remains to be seen. At the very least, we can speculate that even if infected people were to rid themselves of their benign, cat-derived parasites, they’d probably still love their kitty just as intensely.

More from Smithsonian.com:

The Parasite that Makes a Rat Love a Cat 
Top 10 Real Life Body Snatchers 

Rachel Nuwer | | READ MORE

Rachel Nuwer is a freelance science writer based in Brooklyn.