August 14, 2013

Improvements in hygiene could partly explain increased rates of Alzheimer's disease seen in many developed countries, according to research into the link between infections and the condition. Researchers studied the prevalence of the neurodegenerative disease across 192 countries and compared it with the diversity of microbes in those places. Taking into account differences in birth rate, life expectancy, and age structure in their study, the scientists found that levels of sanitation, infectious disease, and urbanisation accounted for 33%, 36% and 28% respectively of the discrepancies seen in Alzheimer's rates between countries. In their report which was published in the journal Evolution, Medicine and Public Health, the researchers concluded that hygiene was positively associated with risk of Alzheimer's disease. Countries with a greater degree of sanitation and lower prevalence of pathogens had a higher burden from the disorder. Countries with a greater degree of urbanisation and wealth also had higher Alzheimer's burdens. Whether hygiene causes the pattern is not yet clear – cleanliness or infectious disease might be associated with some other factor – but the team does have a speculative hypothesis for how the two factors might be linked: exposure to micro-organisms – good and bad – is important for the body to develop proper immune responses. The researchers' "hygiene hypothesis" suggests that as societies have become cleaner, the reduced level of contact with bacteria and other kinds of infectious agent might stall the proper development of important elements of the body's immune system such as white blood cells. The team suggests that developing Alzheimer's might be linked to autoimmune disease, in which the body's immune system attacks itself.