Toxoplasma gondii

Many parasites have complicated lifecycles requiring specific hosts at specific stages.  Toxoplasma gondii, the parasite responsible for toxoplasmosis, is no exception.  It can only reproduce sexually while within the intestines of a cat.  Unfortunately for the parasite, it periodically itself excreted from its preferred home, and thence into a less desirable host, such as a rat.  It then falls to T. gondii to persuade the rat to get it back into a cat. Patrick House and Robert Sapolsky of Stanford are one step closer to understanding how this is accomplished.

The researchers found that while exposure to cat urine triggers fear and avoidance pathways in unaffected rat brains, male rats infected with T. gondii are attracted to cat urine in the same way that they are attracted to female rats.  This was not true for other types of predatory urine.  Apparently, T. gondii has no interest in getting rats into dogs or owls. 

Exactly how this mind control is exerted is not yet known.  However, T. gondii has been found in neurons, as shown below.  T. gondii has also been known to raise dopamine levels by up to 15%.  Taken together, it’s clear that the parasite is able to exert its influence directly on the brain.

Caption: Individual toxoplasma parasites (green) are shown invading neurons (red) grown in a petri dish in the lab. The blue areas are fluorescently tagged cell nuclei.

Credit: I-Ping Lee

By the way, toxoplasmosis is extremely common in humans.  Although most people with the disease display few symptoms, there is a growing body of work suggesting that infected people display differences from the uninfected population in terms of their degree of aggression, neuroticism and love of cats.