One might be tempted to dismiss the bulk of Flegr’s work as hokum—the fanciful imaginings of a lone, eccentric scholar—were it not for the pioneering research of Joanne Webster, a parasitologist at Imperial College London. Just as Flegr was embarking on his human trials, Webster, then a freshly minted Ph.D., was launching studies of Toxo-infected rodents, reasoning, just as Flegr did, that as hosts of the parasite, they would be likely targets for behavioral manipulation.
She quickly confirmed, as previous researchers had shown, that infected rats were more active and less cautious in areas where predators lurk. But then, in a simple, elegant experiment, she and her colleagues demonstrated that the parasite did something much more remarkable. They treated one corner of each rat’s enclosure with the animal’s own odor, a second with water, a third with cat urine, and the last corner with the urine of a rabbit, a creature that does not prey on rodents. “We thought the parasite might reduce the rats’ aversion to cat odor,” she told me. “Not only did it do that, but it actually increased their attraction. They spent more time in the cat-treated areas.” She and other scientists repeated the experiment with the urine of dogs and minks, which also prey on rodents. The effect was so specific to cat urine, she says, that “we call it ‘fatal feline attraction.’”
She began tagging the parasite with fluorescent markers and tracking its progress in the rats’ bodies. Given the surgically precise way the microbe alters behavior, Webster anticipated that it would end up in localized regions of the brain. But the results defied expectations. “We were quite surprised to find the cysts—the parasite’s dormant form—all over the brain in what otherwise appeared to be a happy, healthy rat,” she says. Nonetheless, the cysts were most abundant in a part of the brain that deals with pleasure (in human terms, we’re talking sex, drugs, and rock and roll) and in another area that’s involved in fear and anxiety (post-traumatic stress disorder affects this region of the brain). Perhaps, she thought, T. gondii uses a scattershot approach, disseminating cysts far and wide, enabling a few of them to zero in on the right targets.
To gain more clarity on the matter, she sought the aid of the parasitologist Glenn McConkey, whose team at the University of Leeds was probing the protozoan’s genome for signs of what it might be doing. The approach brought to light a striking talent of the parasite: it has two genes that allow it to crank up production of the neurotransmitter dopamine in the host brain. “We never cease to be amazed by the sophistication of these parasites,” Webster says.
Their findings, reported last summer, created immediate buzz. Dopamine is a critical signaling molecule involved in fear, pleasure, and attention. Furthermore, the neurotransmitter is known to be jacked up in people with schizophrenia—another one of those strange observations about the disease, like its tendency to erode gray matter, that have long puzzled medical researchers. Antipsychotic medicine designed to quell schizophrenic delusions apparently blocks the action of dopamine, which had suggested to Webster that what it might really be doing is thwarting the parasite. Scientists had already shown that adding the medicine to a petri dish where T. gondii is happily dividing will stunt the organism’s growth. So Webster decided to feed the antipsychotic drug to newly infected rats to see how they reacted. Lo and behold, they didn’t develop fatal feline attraction. Suddenly, attributing behavioral changes to the microbe seemed much more plausible.
