Some parasites seem to cast a spell on their hosts. Even a seemlingly lowly protozoan can do all sorts of mischief including making rats fall in love with their worst enemy - cats. Such adaptations are great for the parasites, but the story usually doesn't end well for the host.
This story in The Scientist takes off on a topic I covered in a column last fall called Evolutionary Invasion of the Body Snatchers.
The Scientist story goes into gory details on the mind-controlling effect of toxoplasma, an infection carried by mice, cats and their human companions.
Cats and rodents are a classic predator/prey system, popularized in familiar cartoons and demonstrated every day by household cats around the world. Naturally, mice and rats have many defense strategies to avoid their mortal enemies, including an innate fear of the smell of cat urine. That is, until the animals become infected with a protozoan called Toxoplasma gondii. Then, rodents’ deathly terror of cats turns into a fatal attraction.
Toxoplasmamust find its way to a cat’s intestines in order to reproduce sexually. The oocysts formed there are shed in the cat’s feces, and use rats as a vehicle to travel to their next cat host. When the protozoa form cysts in the rat’s brain, the animal not only loses its fear of cat urine, but actually seems to take a liking to it. In 2007, Robert Sapolsky of Stanford University and his colleagues showed that the cysts tended to localize in the amygdala, a brain region that responds to both predatory and sexual stimuli, and that mediates innate approach and avoidance behaviors.3Curious as to whether the rats really were becoming attracted to the smell of cat urine, as it appeared, or just becoming distinctly less afraid of it, Sapolsky and his PhD student Patrick House decided to take a closer look at the neural activity of infected animals….they found that infected rats exposed to cat urine showed elevated activity in the posterodorsal medial amygdala (MEApd), a brain region involved in reproductive behaviors.4In fact, infected rats showed a similar level of MEApd activity in response to cat urine as uninfected rats showed upon encountering an estrous female.
That’s a long way of saying toxoplasmosis addles the rat’s brain, making it fall in lust with cats, allowing the cat to get an easy meal. The cat is happy, and the parasite gets to where it needs to be to replicate. The big loser in this scenario is the rat.
That leaves open the question of what the parasite might do to people, since many of us apparently can pick it up from our furry friends. One of the scientists quoted in the article wondered the same thing.
One of the most intriguing things about this particular system is that Toxoplasma is one of the few parasites that can cross the blood-brain barrier in mammals—including humans. Although Toxoplasmais not considered a major health problem—it mainly causes serious health consequences for severely immunocompromised patients—the parasite infects some two billion people worldwide. “If you can show these things are secreting neuroactive compounds into a rodent brain, and you know that 20 to 40 percent of the world’s population have these things in their brain, you have to ask yourself, what effect is that having on human behavior?” Adamo says.
Indeed, more than three dozen studies have found a positive link between neurological disorders such as schizophrenia and Toxoplasmainfection. “If you have schizophrenia, you’re more likely to have the parasite than the average population,” House says. The data, however, are merely correlational, and it could be that exposure to and infection by Toxoplasmais a consequence, rather than a cause, of the neurological disorder, he adds. “But it’s a compelling link given the hypothesis that maybe this parasite is actually working in the rodent to increase dopamine,” as the neurotransmitter has been linked to schizophrenia.”