If you’re a regular reader of the Loom, you’re no doubt familiar with the parasite Toxoplasma gondii. If you’re not, now is the perfect time to meet this sinister creature which may very well be residing in your brain. It seems like every year or two, it gets more remarkable, and today it’s taken another step into awesomeness.
Here’s a quick Toxoplasma primer. It’s a single-celled protozoan that reproduces inside the digestive tract of cats. The cats poop out egg-like Toxoplasma cells into kitty litter and dirt. Other animals take up the parasite, which makes its way into their tissues, especially the brain. There it forms cysts that can linger for years or decades. Only if that animal gets eaten by a cat can Toxoplasma complete its life cycle.
This life cycle opens up opportunities for Toxoplasma to evolve. For example, natural selection should favor mildness in the parasite in its hosts, because cats do not like to eat corpses. And, indeed, Toxoplasma is fairly harmless, only causing trouble to people with suppressed immune systems. (Hence the rule that pregnant women should not handle kitty litter. If they get infected by Toxoplasma for the first time, the parasite runs amok in the fetus.) On the other hand, if there’s any way for the parasite to increase the odds that it can get from prey to cat, natural selection may favor genes for that strategy too.
And it turns out that Toxoplasma does have that very ability. In studies on rats, scientists have found that infected rodents lose their fear of the scent of cats. In fact–and please remember, I am a science writer, not a Hollywood script doctor–the rats may even become sexually aroused by the smell of cats. They embrace their doom, and the parasite benefits.
These findings have lots of interesting implications for humans, because perhaps a quarter of all people on Earth carry these parasites in their heads, where they no doubt secrete their mind-altering compounds. There’s some preliminary work that suggests some changes to the personality of infected people, but nothing definitive.
That would be enough for Toxoplasma to earn its place in the Parasite Hall of Fame. But, no, it needed to go one better.
It turns out that rats and other non-cat hosts can spread Toxoplasma to each other through sex. The first reports have only just emerged from studies on dogs and sheep. Recently Ajai Vyas, a neuroscientst at Nanyang Technological University in Singapore, decided to see whether rats can spread Toxoplasma the same way. In the journal PLoS One, he and his colleagues describe how they mated infected males with uninfected females. They found Toxoplasma in the male rats’ semen, and, after mating, in the female rats’ vaginas. And later, they found signs of Toxoplasma in the female rat brains.
These are Toxoplasma cysts moving from rat to rat, so this exchange is kind of like a side track on the parasite’s life cycle. But it still benefits Toxoplasma, because it means it can infect even more potential prey that may get eaten by cats. And so the logic applies once more: if Toxoplasma can raise the odds of getting from infected males to uninfected females, it may have more reproductive success.
You know where this is going–it’s turning into a David Cronenberg horror movie with an all-rodent cast. Vyas wondered if there’s any difference in how female rats mate with infected and uninfected males. So he and his colleagues put a male rat with Toxoplasma at one end of a two-armed maze, and an uninfected male in the other arm. Females then got to choose which rat to approach. Vyans found that they preferred the infected males, spending more time with them and mating more often.
In other words, Toxoplasma makes its host sexy, in order to get into other hosts through sex.
As I wrote in Parasite Rex, many parasites have evolved the ability to manipulate hosts. But I was disappointed to find no good examples of parasites that  manipulate the sexual behavior of their hosts. In fact, female rats have actually evolved to steer clear of male rats infected with some other parasites. They can detect these infections even when the male rats look healthy, and they avoid these males to avoid getting sick. Now Vyas’s research suggests that there is at least one parasite that manipulates sex. Toxoplasma may be exquisitely unusual among parasites. But it’s also possible that there are other sex-hijacking creatures lurking out there. As for what this means for humans, I should point out there’s zero evidence of it moving from person to person, nor is there any evidence of it affecting the sexual behavior of humans. Then again, nobody has looked. For now, you can just let your inner Cronenberg take matters from here….
Originally published November 2, 2011. Copyright 2011 Carl Zimmer.