“It is definitely not smart to intentionally infect yourself. I’ve already had people ask.”
A third of the world has been infected, though. Tiny cysts nested in one's brain and muscles attest. The parasite Toxoplasmosis gondii comes into us by undercooked meat, well-intentioned placentas, gardening soil, or, most infamously, cats. It is the reason that pregnant women are not supposed to empty litter boxes.
“If you’re young and healthy and have it already, it might provide some benefit, as we saw in our research,” Ann-Kathrin Stock, a cognitive neurophysiology researcher at the University of Dresden in Germany, told me. “But the adverse effects are potentially huge. If you ever really get sick it might be what kills you.”
Many people have what feels like a cold after they get infected with Toxo. The symptoms pass, and the person feels fine. But the Toxo lives on inside them, hidden dormant in little cysts, kept in check by constant pressure from the person's immune system. If our immune systems become weak, because of a serious illness later in life, though, the Toxo can break out and attack organs like the brain or retina.
“You might lose your ability to see, or lose your cognitive faculties,” Stock said.
Neuroscientist Joraslav Flegr, an eminent voice in Toxo research, told The Atlantic last year that, “Toxoplasma might even kill as many people as malaria, or at least a million people a year.”
What does it mean to learn that it can also have beneficial effects?
Toxo has been all over the news in recent years, since it became known that the parasite manipulates people’s behavior. Maybe most interestingly and notoriously, it seems to make men more introverted, suspicious, unattractive to women, and oblivious to the way others see them. Infected women, inversely, have been shown to be more outgoing, trusting, sexually adventurous, attractive to men, and image-conscious. Infected men tend to break more rules than their uninfected peers, and infected women tend to pay them more heed. Infected men and women are 2.5 times more likely to have traffic accidents, more likely to develop schizophrenia, and more likely to engage in self-directed violence.
As these stories made news, though, they lacked a logical explanation. At least 60 million Americans have it, if almost always unknown to them, so understanding this is beyond academic. Thinking about the potential scale of the parasite’s effect on civilization and history can be overwhelming, like imagining the twin brother you never had. If you do have a twin brother in real life, it’s like imagining your triplet. Already have a triplet? You get the idea.
Why, though? Why does Toxo affect human behavior? In The New York Times last year, Choire Sicha channeled cats in a memo (“From the desk of: Cats”) that read, “We have trained, by means of this gentle biological warfare, your women to let us into your homes, and your men to stay home and scratch us in our difficult places.”
Who are you going to believe? The cats?
Toxo manipulates its hosts indiscriminately, according to the most prevalent hypothesis. The parasite just wants to improve its odds of disseminating, living on. These fascinating human behavior changes are serendipitous, equal-opportunity mind control.
“It interferes with brain chemistry, but the parasite itself doesn’t ‘intend’ to harm someone,” Stock told me. “It always acts by the same mechanism, whether it’s in humans or rodents. It’s just that humans are very rarely prey to cats. That doesn’t help its goal. We’re a dead-end host.”
That means its effects in humans are various. Some might be good, and some are bad, but none are intended to incapacitate us. We are collateral damage.
Stock is the lead author on a study published this week in the journal Brain, Behavior, and Immunity that found an apparent contradiction in the traditional understanding that the parasitic infection is essentially negative for humans.
“We had people sit in front of a monitor,” Stock explains, “and they were required to press a button with one hand if a ‘go’ signal appeared. When a ‘stop’ signal instead appeared, they were required to shift and press a button with their other hand. In this part of the experiment, the people who were infected were consistently, significantly faster to respond.” When the task was simpler, all ‘go’ and no ‘stop,’ the infected subjects performed the same as the uninfected.
They describe this as an improvement in action control; the first study to demonstrate this. It is somewhat at odds with research by Flegr that demonstrated delayed reaction times in infected people (a leading explanation for the traffic accidents). Flegr’s experiments involved different tasks, though, which might involve different, brain pathways.
Joanne Webster’s research group at Imperial College London has lately championed an idea that reconciles these and other findings: the effects of Toxo cysts in our brains are due to an increase in an enzyme called tyrosine hydroxylase. This is the basis of the dopamine hypothesis, which Stock believes her research supports.
Tyrosine hydroxylase is involved in production of the normally-occurring neurotransmitter dopamine. More of the enzyme means more dopamine. This changes behavior of mice, and Webster and Stock extrapolate, people.
That explanation means that Toxo infection increases dopamine in our brains. It’s different, though, from the kind of dopamine boost we usually hear about in pop neuroscience likened to a runners' high.
“When you’re doing something rewarding—drinking a cup of coffee, talking to a friend, having sex, whatever—you have a boost of dopamine specifically in the limbic regions of your brain,” she said. “But Toxoplasmosis spreads all over your brain, infecting dopamine-producing neurons in many pathways. Given that the dopamine-based system is complex and influences many aspects of cognition and behavior, there is a plethora of effects that might be observed.”
“Organisms do the simplest thing that gets them where they want to be. This parasite obviously found a way to produce tyrosine hydroxylase [the same enzyme used in human brain cells], and by providing that extra enzyme to the cell, it can produce more dopamine.”
For rodents, this can mean losing their fear of cats. They will even approach them with audacity, putting themselves at great risk of being eaten. Toxo wants them to be eaten, because it needs to get into the cat. Cats are the only place that the parasite can reproduce sexually.
Another possible way that Toxo could be affecting our brains is by not actually directly affecting them. Flegr has focused on the role of cytokines and the immune system in the human reaction to Toxoplasma. In this theory, Toxo just triggers our immune system, which, in trying to eradicate the little armored cysts, ends up damaging the adjacent nerve tissue.
“Alternatively,” Stock offers, “the cyst may be advantageous, helping the brain tissue around it because it wants to thrive itself. There is not proof of this, though.”
“We also haven’t yet seen sufficient data on the elderly and the role of the overall duration of infection,” she adds, “which might be an important factor modulating potential benefits from Toxo.” Does 50 years of carrying the cysts manifest differently than five?
The idea of infections changing a host’s behavior in order to survive, Kathleen McAuliffe noted in The Atlantic last year, has also been seen in people with the flu. Afflicted people have been noted to be more social — possibly at the behest of the virus, so they can cough all over people and spread it. Patients in the later stages of AIDS and syphilis have, analogously, reported increased libido.
The parasitic mind control of Toxo, though, seems to be incidental and even potentially of some benefit. Still, again, don’t intentionally get it. I’m not even sure how one might go about trying. It is at least good to know the cats aren’t trying to control us. Unless they are, and Stock is working for the cats, and this is all part of their plan.
We want to hear what you think about this article. Submit a letter to the editor or write to firstname.lastname@example.org.