Couple of articles in The Economist caught my eye recently. The first on the effects of toxoplasmosis and human behavior, the second on the link between disease and intelligence.
Toxoplasma gondii is a parasite whose lifecycle alternates between rodents and cats. When it infects rats and mice it lodges itself in their brains and causes them to behave in an erratic, risk tolerant, manner. It may even make them attracted to the smell of cats. Once the infected rodent is eaten by a cat, the parasite eventually ends up in its feces, to be ingested by a rat. Repeat ad infinitum...
It turns out toxoplasma has this effect by producing dopamine which then acts on their hosts' nervous systems. What then is its impact on humans? Some studies show a correlation between toxoplasmosis and schizophrenia. Others, a higher level of road accidents in infected drivers (there's that increase in risk tolerance again). But "some researchers go further and propose that entire societies are being altered by Toxoplasma".
"In 2006 Kevin Lafferty of the University of California, Santa Barbara, published a paper noting a correlation between levels of neuroticism established by national surveys in various countries and the level of Toxoplasma infection recorded in pregnant women (a group who are tested routinely). The places he looked at ranged from phlegmatic Britain, with a neuroticism score of -0.8 and a Toxoplasma infection rate of 6.6%, to hot-blooded France, which scored 1.8 and had an infection rate of 45%. […]To repeat, correlation is not causation, and a lot more work would need to be done to prove the point. But it is just possible that a parasite’s desire to get eaten by a cat is shaping the cultures of the world."
The second article reviews a study comparing national IQ and a country's disease burden, i.e. the "disability-adjusted life years lost caused by 28 infectious diseases". They found a 67% correlation between the two and though they tried to find other causes, they kept coming back to the impact of disease on IQ.
The article is worth reading in its entirety. As with the case of toxoplasmosis, correlation is not causation, but if true, it's a key finding.
"If [the researchers] are right, it suggests that the control of such diseases is crucial to a country’s development in a way that had not been appreciated before. Places that harbour a lot of parasites and pathogens not only suffer the debilitating effects of disease on their workforces, but also have their human capital eroded, child by child, from birth."
So if we have evidence of diseases' deleterious effect on humans, couldn't other diseases make you smarter, stronger, or healthier? Wouldn't this give them a better chance at long term survival?
In May of this year, scientists presented evidence of just such a effect: a bacteria linked to increases in learning behavior.
The researchers found that that mice fed live Mycobacterium vaccae "navigated the maze twice as fast and with less demonstrated anxiety behaviors as control mice" and speculated that "that creating learning environments in schools that include time in the outdoors where M. vaccae is present may decrease anxiety and improve the ability to learn new tasks."
All this makes me wonder how prevalent such effects are in our lives. Could it be that these little symbionts have shaped our evolution unbeknownst to us? And how would we know?
Here's one way: let's see if our collective IQ decreases as we all use increasing amounts of anti-bacterial soap! :-)