29 May 2015. When you get a fever, you feel tired, and high fever can cause delirium or coma. But is it the fever or the infection or the immune response that make us feel exhausted? And in children, are there any long-term effects? In other words, can infections cause brain drain? This question has now studied in a large data base in Denmark. The researchers relied on a cognitive test administered to 161,696 military conscripts at about age 19 and then linked to hospital records of admissions for infectious disease.
Indeed, the 35% of the young men who had previously been hospitalized for infection had a lower cognitive score corresponding to about 1.1 IQ points. More importantly, staying in the hospital for more than a week showed greater deficits, and more than a month in the sickbed resulted in a loss of as much as 3.5 points after adjustment for other factors that could have influenced the score. Further, repeated hospitalizations showed a step-wise decrease in the IQ score, with a loss of more than 5 points for five or more hospital admissions.
These associations strongly suggest that serious infectious disease during childhood and adolescence can lead to a slower or incomplete cognitive development. However, there are also some confusing findings. In particular, hospitalizations that occurred more more than 5 years ago were associated with a loss of less than a single IQ point, while conscripts with a more recent hospitalization showed deficits of about 2 points. This finding runs contrary to the notion that the younger brain is more vulnerable to adverse effects. Could it be that compensation had occurred since younger-age infections?
Another intriguing observation is that a deficit as expected was clearly associated with meningitis or encephalitis, but other infections, whether skin, urinary system, or gastrointestinal disease contributed almost as much to IQ declines. Clearly, invasion of the brain by infectious agents does not appear to be a requisite for the deficit to occur, unless infections in general result in brain involvement in some way. Rather than bacteria from a gastroenteritis being capable of invading the brain, perhaps the fever and the immune response could mediate adverse systemic reactions that can involve in the brain. This notion is supported by poorer functions in 2-year-old children who had suffered a serious infection during their first year of life. In fact, inflammation by itself, e.g., from HIV infection, may affect transmitter systems in the brain, even if the infection mainly occurs elsewhere in the body.
Did the young men do less well on the IQ test because they had missed schooling? While the registry data revealed the duration of hospitalization, most likely the hospitalized children had to stay home for some time to fully recover before getting back to school. That’s hard to know, though missing school would not seem to explain all of the associations found.
Other possibilities also need consideration. Could it be that those who were more vulnerable to infection at the same time had inferior or delayed cognition? It could be that the hospitalized children had been exposed to chemical brain drainers that also weaken the immune system. For example, early-life exposure to dioxin or PCBs can lead to brain deficits, but these substances may also weaken the immune system and thereby predispose to serious infections. Likewise, the perfluorinated compounds are immunotoxic and are also suspected of being neurotoxic.
Whatever the mechanism, the link between serious infections in the past and adult intelligence once again demonstrates the vulnerability of the human brain.