Popular Science: Cytomegalovirus infection affects intelligence in humans
Cytomegalovirus from the herpesvirus group is estimated to be widespread among more than half of human population. In otherwise healthy people, the infection is usually deemed asymptomatic, but it can cause severe complications in immunosuppressed patients. In pregnancy, it can impair fetus development, especially the nervous system. However, a number of studies suggest that even a chronic infection can impair cognitive function, including intelligence. Scientists from the Laboratory of Evolutionary Biology (Faculty of Science, Charles University), led by Professor Jaroslav Flegr, tested this possibility on a sample of the Czech Republic’s population.
Approx. three hundred participants, students of the faculty, completed a standardized three-hour general intelligence test and donated a blood sample. Their specific antibody levels were used to assess whether they are infected. However, the antibody level in long-term chronic infections can fall below the detection threshold. That was why the authors also ran permutation tests to determine the likely percentage of infected people assessed as not infected – false negatives.
In total, 52% of the participants were assessed as infected based on their antibody levels. Statistical analysis taking into account false negatives showed that infected students had on average lower intelligence than non-infected ones. With more time from the infection (estimated on the basis of antibody levels), intelligence lowered in infected people even after controlling for their age. However, without taking into account the false negatives, it would appear that infected people were in general more intelligent. This apparent paradox is explained precisely by the existence of false negatives, who have sub-threshold antibody levels and at the same time on average lower intelligence. Already five percent of false negatives in the tested population could be responsible for the “positive” effect of the infection.
The study has revealed as of yet unknown negative influence of widespread viral infection on human intelligence. However, it’s also relevant for any studies testing the influence of pathogens on physical or mental health. The longest infected people may have so low antibody levels that regular laboratory tests will mark them as uninfected. This means that to study how chronic infections affect health, endurance or other traits, we need to use special approaches that enable us to eliminate the larger than previously expected effect of false negatives.
Reference: Chvátalová, V., Šebánková, B., Hrbáčková, H., Tureček, P., & Flegr, J. (2018). Differences in cognitive functions between cytomegalovirus-infected and cytomegalovirus-free university students: a case control study. Scientific reports, 8(1), 5322.