Christopher Eppig and colleagues at the University of New Mexico, Albuquerque recently reported that "a country's disease burden is strongly linked to the average IQ of its population". You can read the article here at NewScientist: http://www.newscientist.com/article/mg20727670.301-link-found-between-infectious-disease-and-iq.html) Now, being a researcher, I am always suspicious of articles (especially clinical or translational medicine articles) that make such bold statements while only showing a small table or graph that seems to support the hypothesis, so I decided to take a look at the original article. Sure enough, Christopher Eppig is pubmed-able and I was able to read his paper (PMID: 20591860).
Basically, Eppig et al. state that cognitive ability or intelligence is related to the intensity of infectious diseases on the basis that a developing human brain requires a certain metabolic cost and fighting an infection decreases the ability of the body to provide sufficient nutrition. In human newborns, the metabolic demand of the developing central nervous system is great, approximately 90% of the body metabolic demand is supplied to the brain (Holliday 1986) and nutrition is vital to mental development, and malnutrition can lead to decreased brain mass, head size, and lower psychometric intelligence (Lynn 1990, 1993). It is well realized that deficient metabolic supply will diminish the ability of the brain to develop properly, especially at a developmental timepoint, where the brain is undergoing a high level of structural and functional assembly. Eppig et al. offers the parasite-stress hypothesis where parasitic infections can energetically affect the body in four ways: i) loss of tissue that must be replaced at a biological cost to the host, ii) malabsorption of nutrients through the gastrointestinal tract, iii) hijacking of host cellular machinery at high biological cost to the host, and iv) activation of the immune system to fight the infection. Now this doesn't seem like a very far-fetched idea: energetic costs associated with fighting an infection, especially a chronic infection, would severly diminish the level of metabolic supply for other aspects, such as brain development during early age.
However, this hypothesis fails into taking account that the brain is a very plastic organ. Assembly and reassembly on a molecular and cellular level of the brain (and central nervous system for that matter) is continous and does not just end in preadolescence. In fact, it is well known that the brain continues its development well through adolescence and into adulthood (Cunningham et al. 2002; Sowell et al. 1999; Casey et al. 2005). Furthermore, Gage 2002 shows that neurogenesis, the process by which neurons are generated. is evident even in the adult brain. Also, most infections persist only for a short while and not for years, so long as the body is able to fight and clear the infection. Its seems unlikely that the metabolic demand to fight infection would be diverted away from brain development for such an extended period of time to have devastating effects on cognitive ability. And in children with severe infections, where the pathogen persists and the disease is chronic, have more to worry about than just retarded brain development.
Of course, I am ignoring the inconsistency of using IQ tests as an assessment of intelligence and cognitive ability. And on a social standpoint, in my opinion, the lack of access to education and the hardships associated with living in resource-poor countries provides more of an association for a lower IQ score. Futhermore, what about children born in infectious "high risk" countries who then migrate to "developed" countries during adolescence or adulthood? How do their IQ scores compare to control populations?
In conclusion, it seems that although parasitic infections are a harsh reality for children living in resource-poor countries, its association with lower intelligence and cognitive ability is unclear, and is likely not a major contributor.
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