Researchers say that people living in wealthier countries may be at higher risk of developing Alzheimer's disease, according to a study published in the journal Evolution, Medicine, and Public Health.
Lead study author Dr. Molly Fox and fellow researchers at the University of Cambridge, say that people living in industrialized countries have significantly reduced contact with bacteria and viruses, meaning they may have problems developing immunity and be at higher risk of dementia.
For their research, the team wanted to determine whether this "hygiene hypothesis" could explain why there are significant differences in rates of Alzheimer's disease over 192 countries.
They used data from the World Health Organization's (WHO) Global Burden of Disease (GBD) report published in 2009, which provides data for 2004. The data was adjusted for differences in age population structures.
This report included information regarding the prevalence of Alzheimer's disease and other dementias, although the researchers note that Alzheimer's accounts for 60-80% of all dementia cases.
Wealthy countries have higher rates of Alzheimer's
Results of the analysis revealed that countries with higher levels of sanitation also had higher rates of Alzheimer's disease.
Countries where populations have access to clean drinking water, such as France and the UK, showed a 9% higher Alzheimer's rate compared with countries that had less than half the relative access to clean water.
Countries with significantly lower rates of infectious disease, such as Iceland and Switzerland, had a 12% higher Alzheimer's rate compared with countries such as Ghana and China, who have high rates of infectious disease.
Countries in which more than 75% of the population resides in urban areas, such as the UK and Australia, displayed Alzheimer's rates 10% higher than in countries where fewer than one-tenth of people live in urban areas, such as Nepal and Bangladesh.
Additionally, the researchers say that the overall differences in levels of sanitation accounted for 33% of the discrepancy in Alzheimer's rates between countries, while infectious disease accounted for 36% and urbanization accounted for 28%.
Dr. Molly Fox says:
"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."
"There are important implications for forecasting future global disease burden, especially in developing countries as they increase in sanitation," she adds.
Hygiene hypothesis 'helps explain Alzheimer's rates'
The study authors say that exposure to microorgnisms is critical in order for the immune system to be regulated. Lack of microbe and bacterial contact may lead to insufficient development of the white blood cells - the cells that are responsible for protecting the body against infection.
They add that this is particularly important for T cells, a type of white blood cell that combats infection in the blood stream.
The researchers say that previous studies have suggested stimulation of the immune system may increase the risk of Alzheimer's disease through T cell or microglial action - cells that reside in macrophages of the brain cell and spinal cord.
"However, our results indicate that some immune stimulation may protect against AD risk," they add.
"The traditional hygiene hypothesis has effectively explained prevalence rates of autoimmunity and atopy in many populations. We have found that the hygiene hypothesis can help explain some patterns in rates of AD."
Challenges of Alzheimer's in developing world
Dr. Fox adds that the increase in adult life expectancy and the prevalence of Alzheimer's disease in developing countries is "perhaps one of the greatest challenges of our time."
She says that more than 50% of people with Alzheimer's live in the developing world, and by 2025 this figure is expected to rise to more than 70%.
"A better understanding of how environmental sanitation influences Alzheimer's risk could open up avenues for both lifestyle and pharmaceutical strategies to limit Alzheimer's prevalence," she adds.
"An awareness of this by-product of increasing wealth and development could encourage the innovation of new strategies to protect vulnerable populations from Alzheimer's."
Dr. Fox told Medical News Today that the next step in testing the relationship between environmental microbial diversity and Alzheimer's etiology should be to directly investigate patients' immune activity throughout life and Alzheimer's risk, "preliminarily in a cross-sectional study utilizing medical records, serum analysis, or reliable interview techniques."