Laughing in the face of danger: Parasitic infection causes hard-wired fear to disappear in mice

I've been watching The Strain on FX, a vampire/virus show that is directed by Guillermo del Toro and Chuck Hogan (based off their novel series) and the show is starting to pick up. Set in New York, an ancient vampiric disease spreads throughout New York, while an unlikely team set off to try to find the cure and save the people from becoming infected. Infected individuals have little parasitic worms that travel through their body, latch onto the brainstem and slowly take over the host. Eventually, the infected humans shed all their humanistic qualities, only to becoming vampiric monsters whose main mission is to infect those they care for most. 

What sets this show apart from the other vampiric shows out there is the parallel references with likening a viral outbreak with vampires (one of the main characters in the show is an epidemiologist who is trying to find a cure for the vampire outbreak). It's entertaining to watch, the suspense builds up well, and it's starting to pick up, thankfully (previous episodes drew too much on unnecessary drama).

Watching this show, seeing how resilient this "virus" is to infecting as much individuals as possible, as well as the smart way the virus continues to infect hosts, reminds me of the amazing strategies nature uses to survive... such as how the parasite toxoplasma gondii, or T. gondii, infects its host. T. gondii's host is the cat and disease is often transmitted through contact with feline feces, or contaminated by-products. Recent work has identified a unique way the parasite ensures a sure-fire way to get into its host - by creating fearless mice.

T. gondii fluorescent image (photo courtesy of AJ Cann)

In a paper published back in 2013, researchers studying T. gondii found that mice infected with different types of the parasite exhibited decreased fear levels compared to their control counterparts, even showing a slight attraction to the cat scent (think Tom and Jerry, only this time Jerry running circles around Tom, baiting Tom to eat him!). These experiments placed mice in an enclosed chamber with a petri dish of either rabbit urine (non-predatory) or cat urine (predatory), and measured the level of exploration of the mice. Infected mice lost all their fears, often found exploring the cat scented urine, laughing in the face of danger! When the infection was cleared, mice STILL showed a lack of fear when presented with predatory smells. The parasite has identified a smart strategy to infect mice and make them lose their natural fear of predators like cats. This allows cats to more readily eat the mice, making it easy as can be for the parasite to infect its host. One thing that would be interesting is to see if the parasitic infection causes mice to all around be fearless of any predator, or just felines (their host).

Either way, T. gondii has adapted such a fail-proof way to infect its host. The infected mice seem to have permanently altered brain function (not quite zombie-like, but fearless risk-takers!), despite treatment against the parasite. Scrappy and resilent!

Eye of the tiger, right? (photo courtesy of Rama)

Until next time! I don't want to say happy eating because this post isn't meant to entice anyone to eat... hopefully.



References:

Ingram WM, Goodrich LM, Robey EA, Eisen MB. Mice infected with low-virulence strains of Toxoplasma gondii lose their innate aversion to cat urine, even after extensive parasite clearace. 2013. PLoS One.