A cat and mouse game
October 28, 2008
This is fourth in a series of posts about the Science Writers 2008 conference. Here are my notes from Robert Sapolsky’s plenary lecture on “Stress, parasites, and human behavior.”
Haven’t we all heard of toxoplasmosis by now? (Well, I guess it hasn’t gotten any less interesting.)
When a parasite invades its host, its job is not over – it has to create the conditions in which it can reproduce. If a host doesn’t want you there, what can you do?
HIV avoids becoming a victim of the immune system by attacking that system first. Since the immune system targets antigens by their surface proteins, one organism [didn’t catch which one] takes on the host’s surface proteins as a disguise. Since it takes about 14 days to mount an attack on an intruder, trypanosomes change their surface proteins every 13 days or so.
Some parasites change their host’s behavior, including parasitic barnacles that prey on crabs. The barnacle somehow makes the crab more attractive to mates, and lays its eggs on the lover crab, when such a crab comes along. There is a mite that rides on an ant’s head, and knows how to stroke the ant’s mandible to make it regurgitate food. The mite is essentially fooling the ant into thinking it’s feeding its larvae.
Yet another barnacle parasite “feminizes” male crabs, making its host dig as if for a nest. But instead of laying crab eggs in the hole, the barnacle lays its own eggs there. This parasite also targets female crabs, destroying their gonads before triggering the digging behavior.
Rabies is a famous brain manipulator. Its particles are shed in saliva, so it changes the canine host’s behavior to make it more likely to bite other potential hosts. Neuroscientists are still struggling to understand the pathways in the brain that influence aggression – but a little virus called rabies has this system all figured out.
There is a bacterium that can only reproduce sexually in a cow’s digestive system. So it gets pooped out, eaten by an ant, and manipulates that ant into climbing to the top of a blade of grass. When it gets out of the lawn’s understory, the bacterium – which is photosensitive – paralyzes the ant’s mandible. The idea is that it freezes just as it’s biting into the blade of grass, and finds itself stuck there. Then a cow comes along, and eats it.
Toxoplasmosis has a similar problem – it only reproduces in the feline gut. Cat poop gets eaten by rodents, and the toxo has to find a way of getting its rodent host into a cat’s stomach.
So it seems the rodents seek out cats, rather than running in fear from them. When I first heard that idea, I thought it was unlikely – that the rodents probably are just careless around cats, or maybe they just can’t run away as effectively when they’re sick. The scientists thought of this too, and there is now a lot of experimental evidence supporting the idea that rats very specifically seek out the smell of cats.
Rodents normally avoid cat scent. Even born-and-bred lab animals that have never met a cat in their lives show an innate aversion to the scent. Except, that is, if they’re infected with toxo. There is even a dose-response curve like you would see with a drug.
And meanwhile, everything else about the rat is normal. Its sense of smell, its social behaviors, its ability to learn, and even its ability to learn to fear things. Its anxiety about other stimuli (everything dangerous except cats) is completely normal. The effect extends to the smell of both domestic cats and bobcats, but not to the scent of other animals they’ve tested, including dogs and humans.
By attaching a fluorescent tag to the toxo organism (which, by the way, is a protozoan), you can watch where in the rodent it localizes. It spreads throughout the body, causes various symptoms, and disappears from most areas of the body – except the brain.
Where in the brain? It seems it especially goes to the amygdala, which is known to have a major role in fear and anxiety. There, it causes dendrites to atrophy, essentially disconnecting the neurons from each other.
Here my notes are a little fuzzy, but the upshot is that, when a normal rat smells cat urine, fear-related areas light up. When a toxo-infected rat does the same, sex areas of the brain light up. Toxo makes cat urine smell sexy to male rats.
Don’t ask me why they’ve only tested male rats so far. Sapolsky doesn’t have data yet for female rats, but he thinks he will learn that, for toxo-infected females, cat urine smells like babies. We’ll see about that…
Blasting bits of toxo’s genome, you get hits for two of the enzymes involved in making the neurotransmitter dopamine. This seems to be unique to toxo; related protozoa, like plasmodium, have no such gene.
What’s the exact role of dopamine here? I’m not sure. This talk left me with many more questions than answers. Apparently, only a very small number of papers have been published on the mysterious behavioral effects of toxo. In humans, toxo is only well-studied in its effects on fetuses, which can include retardation, seizures, blindness, and death. If the kid survives, though, its post-toxo life isn’t studied. But toxo may have long-term behavioral effects in humans as well as in rats.
In a psych battery, toxo patients show slight differences from uninfected people in impulse control – for example, in a test where you have to switch from counting backwards to counting forwards, they have trouble making that switch on command. The effects are subtle in men, and even more subtle in women. But on a less subtle note, people infected with toxo are two to four times more likely to die in a high-speed car crash.
A popular hypothesis now is that toxo, as Sapolsky put it, “makes us enjoy making our body hurtle through space uncontrollably.” Anecdotally, transplant surgeons see toxo more often in organ donors who died in motorcycle accidents. The theory goes that toxo makes risky things seem somehow attractive. And toxo is very common in humans – 70% incidence in some tropical areas, and possibly 30% worldwide. There may be a connection between toxo and schizophrenia or psychopathy.
Sapolsky closed by saying the lesson here is one of phylogenetic humility – rabies and toxoplasmosis know more about how the brain works than our best neurologists.