Prenatal exposure to the Hong Kong Flu, which claimed more than 700,000 deaths worldwide in the late 1960s, linked to reduced intelligence in adulthood, says study.
Writing about the study, Dr. Willy Eriksen of the Norwegian Institute of Public Health has revealed that it involved records of more than 180,000 men born between 1967 and 1973 who served in the military.
Military service is compulsory for young men in Norway, who are evaluated medically and psychologically before they enter the service, according to background information in the study report.
Eriksen further reveals that the intelligence test data used in the study consisted of a composite score from arithmetic, word similarity, and figures tests similar to those commonly used in intelligence tests.
It was observed that the mean intelligence score increased in every birth year from 1967 to 1973, except for a downturn in 1970, writes Eriksen.
The intelligence scores of men born in July through October of that year, six to nine months after the main outbreak of the Hong Kong flu in Norway, were lower than the mean values for those born in the same months during the preceding and following years.
The mean intelligence score of men born during those months was also lower than the mean score of men born in any other month in 1970, and this trend was not seen in the other years.
Given that the flu outbreak took place during the winter months, the exposure during the first three to four months of pregnancy seems to have had the strongest impact on intelligence scores.
"This is the first report of a possible association between prenatal exposure to an influenza virus epidemic and the mean level of intelligence in the general population," says Eriksen, co-author of the study.
According to the study's authors, several possible explanations can be given for the results. They say that exposure to the influenza virus might have interfered with the cerebral development of the fetus, as has been observe in lab experiments on animals.
Another possible explanation could be that the influenza virus would have crossed the placental barrier, causing some fetuses to suffer a cerebral infection.
One more possibility they suggest is that a maternal infection during pregnancy might have had an effect on the fetal brain through maternal immune response or high body temperature, or through medication used to treat infections.
The authors suggest that if 20 percent of the men born between July and October 1970 were exposed to the flu virus, and assuming they were all affected neurologically, prenatal exposure to such a virulent virus may reduce intelligence scores by three to seven points on a standard IQ scale.
"If cerebral complications occurred in only a small group of those who were exposed, however, the effects on the intelligence of the susceptible individuals may have been considerably larger," says Eriksen.
A research article on the study has been published in the journal Annals of Neurology.