My friend Mick alerted me that Carl Zimmer was featured in a recent episode of RadioLab dedicated to the subject of parasites (Abumrad, Jad and Robert Krulwich, “Parasites,” 7 September 2009). Despite being somewhat annoying in format and low-density in it’s information presentation, the show contains a number of points interesting to my project.
In the second part of the second segment (starting at 31:25), they deal with the symbiosis between hookworms and the human immune system. The segment consists of a profile of Jasper Lawrence, a man who had severe allergies and — having chased down a certain direction of research — decided to travel to Cameroon to infect himself with hookworms. The research in question is that of the hygiene hypothesis: the notion that many developed world afflictions, including allergies, result in part from the excessively sterile human environment. Asthma is 50 percent less likely in a person who has had a hookworm and in Africa allergies are almost entirely unknown. It is theorized that similar to the dependence of digestion upon a symbiotic relationship with non-human microflora of the digestive tract, the immune system is dependent on certain microorganisms for regulation and calibration of the immunoresponse. The complex chain of events that is the immunoresponse evolved in the constant presence of parasites, evolved around parasites; they have co-evolved to the point where their presence became necessary. “We function like rainforests; we’re ecosystems,” Mr. Lawrence says. This is the hypersea washing through humanity.
The final segment is on Toxoplasma Gondii (starting at 47:55), a parasite that lives in cats and makes their feces dangerous to pregnant women. Like many parasites, it has a multi-phase lifecycle that takes place in a multiple hosts. It only reproduces in members of the Felidae family (cats), but can live the remainder of its life in any warm-blooded creature. T. gondii is expelled by cats when they shit and the cat shit is ingested by other creatures (consumption of unwashed vegetables, inhaled while digging close to the ground — which is why pregnant women are advised against gardening). T. gondii needs to get back into the digestive tract of a cat to reproduce, so it wriggles its way to the amygdale, the part of the brain responsible for emotional reaction, and causes the host to become attracted to cats, thus, in the case of small mammals or birds, becoming easy prey for cats (it is Carl Zimmer’s argument in his book, Parasite Rex that in this way parasites are like ecological catalysts, spinning food webs ever more tightly together).
But then there is the question of humans. It is one thing to say that T. gondii might make a bird or a rat suicidal. But T. gondii infects humans too. What then?
The scientific interviewee for the segment is Robert Sapolsky, a professor of neurology at Stanford University. On the question of T. gondii altering human behavior, he declares it highly plausible:
Sapolsky: Pure speculation, but people who think about this stuff view it as not purely speculative. The notion that toxo can produce some sort of attraction to cats in humans: they don’t think it’s all that crazy.
That’s right: crazy cat lady is that way because she’s been body-snatched by toxoplasma gondii.
Less controversial than the idea that T. gondii might be making crazy cat people out of us is the idea that it can make people more prone to engage in risky behavior. Dr. Sapolsky mentions two independent studies that show that people infected with T. gondii are two to six times more likely to get in a car crash than those not infected. With this information in hand, host and guest make the larger point:
Ellen Horne: It might be possible — might be possible — that toxo is guiding our emotions, changing who we are in some basic way. And if you consider that toxo might just be one of thousands of tiny little parasites inside us, pulling our strings from the inside, well that thought is pretty creepy.
Sapolsky: Even if the entire lesson with toxo is that a small subset of infected people now have one half of one percent more likelihood of wanting to drive really recklessly, even lurking in that one half of one percent are some serious implications for thinking about free will. We haven’t a clue the biology lurking in the background that makes free will seem a little bit suspect.
I’m less concerned with that old philosophical saw of free will versus determinism, than with extending an idea from segment on the hookworms. Mr. Lawrence says, “We function like rainforests; we’re ecosystems.” Presumably he is referring to our bodies. But the implication of toxoplasma gondii is that we are ecosystems in out minds as well. To the naïve sort of homunculus, Herman’s Head notion of consciousness, we must now add a few animal spirits.
Update, 23 October 2009: For instance this woman must have a pretty severe infection of T. gondii.
Update no. 2, 5 June 2010: Parasitogenic felinophilia (Toxoplasma gondii) may be treatable with haloperidol, an antipsychotic (“A Game of Cat and Mouse,” The Economist, 3 June 2010). Repost from my twitter feed.