Hygiene and the world distribution of Alzheimer's disease: Epidemiological evidence for a relationship between microbial environment and age-adjusted disease burden.
OBJECTIVE
Alzheimer's disease (AD) shares certain etiological features with autoimmunity. Prevalence of autoimmunity varies between populations in accordance with variation in environmental microbial diversity. Exposure to microorganisms may improve individuals' immunoregulation in ways that protect against autoimmunity, and we suggest that this may also be the case for AD. Here, we investigate whether differences in microbial diversity can explain patterns of age-adjusted AD rates between countries.


METHODS
We use regression models to test whether pathogen prevalence, as a proxy for microbial diversity, across 192 countries can explain a significant amount of the variation in age-standardized AD disability-adjusted life-year (DALY) rates. We also review and assess the relationship between pathogen prevalence and AD rates in different world populations.


RESULTS
Based on our analyses, it appears that hygiene is positively associated with AD risk. Countries with greater degree of sanitation and lower degree of pathogen prevalence have higher age-adjusted AD DALY rates. Countries with greater degree of urbanization and wealth exhibit higher age-adjusted AD DALY rates.


CONCLUSIONS
Variation in hygiene may partly explain global patterns in AD rates. Microorganism exposure may be inversely related to AD risk. These results may help predict AD burden in developing countries where microbial diversity is rapidly diminishing. Epidemiological forecasting is important for preparing for future healthcare needs and research prioritization.
Division of Biological Anthropology, Department of Anthropology and Archaeology, University of Cambridge, Pembroke Street, Cambridge CB2 3QY, UK, Department of Anthropology, University of Utah, 270 S 1400 E, Salt Lake City, UT 84112, USA, Department of Psychology, Neuroscience & Behaviour, McMaster University, 1280 Main Street W, Hamilton, ON L8S 4K1, Canada and Institute of Neuroscience and Psychology, University of Glasgow, 58 Hillhead Street, Glasgow G12 8QB, UK.
Molly
M
Fox
Leslie A
LA
Knapp
Paul W
PW
Andrews
Corey L
CL
Fincher
Alzheimer’s disease; Darwinian medicine; dementia; hygiene hypothesis; inflammation; pathogen prevalence