Better Hygiene May Increase Alzheimer's Risk

People living in industrialized countries may be more likely to develop Alzheimer's disease (AD) due to greatly reduced contact with bacteria, viruses, and other microorganisms, according to a new study.

Researchers at the University of Cambridge (United Kingdom), McMaster University (Hamilton, ON, Canada), and other institutions use regression models to test whether pathogen prevalence—as a proxy for microbial diversity—across 192 countries could explain a significant amount of the variation in age-standardized AD disability-adjusted life-year (DALY) rates. The researchers also reviewed and assessed the relationship between pathogen prevalence and AD rates in different world populations.

The results showed that after adjusting for differences in population age structures, countries with higher levels of sanitation had higher rates of AD. For example, countries where all people have access to clean drinking water, such as the United Kingdom and France, have 9% higher AD rates than countries where less than half have access, such as Kenya and Cambodia. Countries that have much lower rates of infectious disease, such as Switzerland and Iceland, have 12% higher rates of AD compared with countries with high rates of infectious disease, such as China and Ghana.

More urbanized countries exhibited higher rates of AD, irrespective of life expectancy. Countries where more than three-quarters of the population are located in urban areas, such as the UK and Australia, exhibit 10% higher rates of AD compared to countries where less than one-tenth of people inhabit urban areas, such as Bangladesh and Nepal. In all, differences in levels of sanitation, infectious disease, and urbanization accounted respectively for 33%, 36%, and 28% of the discrepancy in AD rates between countries. The study was published online on August 11, 2013, in Evolution, Medicine, and Public Health.

“The 'hygiene hypothesis', which suggests a relationship between cleaner environments and a higher risk of certain allergies and autoimmune diseases, is well established. We believe we can now add Alzheimer's to this list of diseases,” said lead author Molly Fox, PhD, of the Cambridge biological anthropology division. “There are important implications for forecasting future global disease burden, especially in developing countries as they increase in sanitation.”

“The increase in adult life expectancy and Alzheimer's prevalence in developing countries is perhaps one of the greatest challenges of our time. Today, more than 50% of people with Alzheimer's live in the developing world, and by 2025 it is expected that this figure will rise to more than 70%,” added Dr. Fox. “An awareness of this byproduct of increasing wealth and development could encourage the innovation of new strategies to protect vulnerable populations from Alzheimer's.”

Since increasing global urbanization, beginning at the turn of the 19th century, the populations of many of the world's wealthier nations have very little exposure to so-called 'friendly' microbes, which stimulate the immune system, due to diminishing contact with animals, feces, and soil. Other aspects of modern life, such as antibiotics, sanitation, clean drinking water, paved roads, and so on, also lead to lower rates of exposure to these microorganisms.
This lack of microbe and bacterial contact can lead to insufficient development of the white blood cells that defend the body against infection, particularly T-cells. Deficiency of anti-inflammatory T-cells has links to the types of inflammation commonly found in the brain of those suffering with AD, and could therefore be linked to the general hygiene levels of a nation's population.

Related Links:
University of Cambridge
McMaster University