Monday, October 27, 2014

Real Zombie-Making Parasites Among Us

The mummified cat and the rat in the crypt of Christ Church in Dublin.
Photo by Adrian Grycuk at Wikimedia Commons.
The Happening, M. Night Shyamalan’s worst panned movie of all time, is a science fiction thriller about people going into a mysterious trance and committing suicide as a result of other mind-hacking species. One of the leading criticisms raised against this movie is the ridiculousness of the premise. One species can’t cause another to willingly commit suicide! …Or can they?

Toxoplasma gondii (we’ll call it T. gondii) is a protozoan parasite that has developed just such mind-hacking abilities! As far as we can tell, T. gondii only reproduces in the digestive tract of cat species, where it lays fertile eggs that are pooped out into the environment. From there, T. gondii eggs can contaminate any number of things that are consumed by other animals, such as rodents, birds, or even humans. When cats eat prey animals that are infected with T. gondii, another generation of parasites is now positioned to reproduce and the cycle continues.

However, prey animals can be pretty good at avoiding cats, in part by avoiding the smell of cats. This is a problem for the reproductive plans of T. gondii. The tiny protozoan has responded to this problem with remarkable biological sophistication: They alter the behavior of their rodent hosts so that the infected rodents find the smell of cat urine so irresistible that they run straight towards their predators! Now, researchers have found that T. gondii-infected rats don’t only like the smell of cat urine, but they even prefer the smell of wild cat urine over the smell of urine of weaker domesticated cats.

A rat checks out odor-soaked papers
in a Y-shaped apparatus. Image from
Kaushik, et al. (2014) in
Integrative and Comparative Biology.
Maya Kaushik, Sarah Knowles and Joanne Webster at the School of Public Heath at the Imperial College of London compared the responses of rats that were either infected with T. gondii or not to urine produced by domestic cats or wild cats. To do this, they put infected or uninfected rats into a Y-shaped apparatus. For each trial, tissue paper soaked in domestic cat urine or wild cat (cheetah or puma) urine was placed in two of the three arms and nothing was placed in the third arm. The researchers then measured how much time the rats spent in each of the three arms and how much they moved.

As expected, the T. gondii-infected rats avoided the cat-urine-soaked arms less than the uninfected rats did. Furthermore, when presented with a choice between arms with wild cat urine versus domestic cat urine, the infected rats (but not the uninfected rats) preferred the smell of the predatory wild cats over the domestic cats! Infected rats also moved more slowly around the wild cat urine compared to domestic cat urine, as if just begging any wild cats that may be around to eat them. It appears that T. gondii have developed a mechanism to turn rats into mindless zombies that practically run into the mouths of the nearest, most vicious cat they can find.

These mind-hacked rat-zombies may not be the only victims of T. gondii. People (particularly those that change their kitties’ litter boxes) can also become infected with the parasite. Some estimates suggest that nearly one-third of all people are already infected! Furthermore, people that test positive for T. gondii infection find the smell of cat urine more attractive than people who test negative! Although we are not likely to run to be eaten by our house-bound kitties, we may be more likely to change the litter box (or get more cats and become a crazy cat lady). So it looks like many of us are mind-hacked zombies too!

Want to know more? Check this out:

Kaushik, M., Knowles, S., & Webster, J. (2014). What Makes a Feline Fatal in Toxoplasma gondii's Fatal Feline Attraction? Infected Rats Choose Wild Cats Integrative and Comparative Biology, 54 (2), 118-128 DOI: 10.1093/icb/icu060


  1. The thought that a parasite can influence a mind so powerfully that it can make a organism seek out it's natural predator is a bit scary. It makes me wonder if a different parasite could effect humans strongly enough to make us harm are our selves like these rats do. -Stephanie Kirk

  2. I love this - I read a fantastic paper about this very parasite at Uni last year - and it got me to thinking. This could well explain why humans will often blindly run into the arms of the least appropriate breeding stock!! I'd have to agree, in my observations of human behaviour - a LOT of people's brains have been infected with T. gondii. Darn it would be great if someone could get permission to test that during autopsies! What are we doing to continue that parasites lifecycle too?