Review: Link between infections and cognitive disorders

Review: Link between infections and cognitive disorders

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The high susceptibility of humans to infectious diseases can have unintended and sweeping consequences. Note that a body of research has focus its attention on the link between infections and cognitive impairment. To be specific, studies have suggested a correlation between infections and poor intelligence quotient and other forms cognitive impairments.

Infections and the development of poor IQ and cognitive disorders

A large-scale, national cohort study conducted by M. E. Benros et al. involving 161,696 Danish men revealed that infections could impair cognitive ability as determined by intelligence quotient or IQ scores. Results specifically indicated that those with a history of infection had significantly lower cognitive ability. Furthermore, the number of infections or severity of infections correlated with decreased cognitive ability.

Infections and activated immune responses could affect the brain through several pathways that might affect cognition according to the study. The researchers explained that experiments on animals have previously demonstrated that the immune system could affect cognitive ability. In addition, minor studies in humans have also suggested similar conclusions.

Note that Benros et al. associated different infectious diseases with cognitive impairment. It is possible that the immune system is the actual culprit behind the decline in cognitive function rather than the involved infectious agents.

Other studies explored the direct link between infection and cognitive impairment or cognitive disorders. For example, the study of A. E. Ezeamama et al. investigated the specific link between helminth infection and cognitive impairment among Filipino children.

As a backgrounder, helminth infection is caused by different types of parasitic worms or helminths such as flatworms, tapeworms, nematodes, and trematodes, among others. These worms are among the most common infectious agents in developing countries.

The study of Ezeamama et al. established a correlation between helminth infection and lower performance in three of the four cognitive domains examined. These domains were learning, memory, and verbal fluency.

Another study by M. Katan et al. found an association between infection caused by herpes simplex type 1 virus and cognitive problems, especially memory impairment. Note that this virus causes cold sores or oral herpes infection.

The study specifically involved testing thinking and memory functions among 1,625 subjects with an average of 69. Blood samples were also drawn from these participants to determine their exposures to different infections and infectious agents.

Results revealed that people who had higher levels of infections are 25 percent more at risk to score poorly on a common cognitive test called the Mini-Mental State Examination. Thus, the study concluded that people exposed to the herpes simplex type 1 virus might be at greater risk for developing cognitive problems, particularly memory problems.

The national cohort study of Benros et al. also mentioned previous studies that investigated particular types of infections and their link with cognitive disorders. Some of the conclusions from these reviewed studies included a correlation between hepatitis or HIV infection and poor cognitive abilities. Furthermore, patients who recovered from systemic infections such as sepsis and encephalitis later demonstrated cognitive impairment and neurodegeneration. In elderlies, particularly those with dementia, infections are the main triggers of delirium.

Conclusion: The link between infections and cognitive disorders

The referenced studies represent an established research interest centered on exploring and understanding the link between infections and cognitive impairment or cognitive disorders. From these studies nonetheless, it appears that the distress of an individual from an infection does not end upon treatment and recovery. Furthermore, this also means that infections serve as a factor that triggers the onset of decline in cognitive abilities.

An exact and universal reason as to how infections lead to the development of cognitive impairment remains unknown. However, it is worth mentioning that there are other studies focusing on linking infections and the development of mental disorders. The referenced studies above and other related research seem to suggest that the cognitive impairment due to infection can be due to the direct harmful effects of pathogen or infectious agents.

The response of the immune system against a particular infectious agent might also be the main culprit behind the link between infections and cognitive disorders. Studies suggest that the immune system might generate an autoimmune response that can affect the brain and/or nervous system. Thus, like any other autoimmune response, antibodies might attack neurons or areas of the brain, or interfere with normal brain functioning.

FURTHER READINGS AND REFERENCES

  • Benros, M. E., Sørensen, H. G., Nielsen, P. R., Nordentoft, M., Mortensen, P.B., & Petersen, L. 2015. The association between infections and general cognitive ability in young men—A nationwide study. PLoS One. DOI: 10.1371/journal.pone.0124005
  • Ezeamama, A. E., Friedman, J. F., Acosta, L. P., Bellinger, D. C., Landon, G. C., Manalo, D. L., Olveda, R. M., Kurtis, J. D., & McGarvey, S. T. 2005. Helminth infection and cognitive impairment among Filipino children. The American Journal of Tropical Medicine and Hygiene. 72(5): 540-548. PMCID: PMC1382476
  • Katan, M., Park Moon, Y., Paik, M. C., Sacco, R. L., Wright, C. B., & Elkin, M. S. V. 2013. Infectious burden and cognitive function. Neurology. 80(13): 1209-1215. DOI: 10.​1212/​WNL.​0b013e3182896e79