Alzheimer's: could good hygiene and less contact with bacteria pose 'greater risk'? Study looked at link between cleanliness and Alzheimer's in 192 countries Countries with clean drinking water have 9% higher Alzheimer's rates Countries with less infectious disease have 12 per cent more Alzheimer's Too much clean living may be contributing to a surge in cases of Alzheimer's disease in developed countries, experts believe. Scientists have linked the 'hygiene hypothesis; - the idea that lack of exposure to germs, viruses and parasites harms the immune system - to rising rates of Alzheimer's in richer nations. Evidence shows that countries where the risk of infection is relatively low have more people suffering from Alzheimer's. Likewise, better sanitation and the expansion of cities go hand in hand with higher incidence of the disease, the most common form of dementia. The study found a 'very significant' relationship between a nation's wealth and hygiene and number of Alzheimer's patients within a country's population Taken together, infection levels, sanitation and urbanisation account for 42.5% of the variation in rates of Alzheimer's between different countries. Cambridge University scientists looked at the link between hygiene and Alzheimer's rates in 192 rich and poor countries. According to the researchers, clean environments might cause the immune system to develop poorly and could expose the brain to the inflammation associated with Alzheimer's disease. The "hygiene hypothesis," which suggests a relationship between cleaner environments and a higher risk of certain allergies and autoimmune diseases, is well- established," said Dr Molly Fox, lead author of the study. We believe we can now add Alzheimer's to this list of diseases. Clean environments in developed nations result in far less exposure to a diverse range of bacteria (pictured). This might cause the immune system to develop poorly and could expose the brain to the inflammation associated with Alzheimer's disease There are important implications for forecasting future global disease burden, especially in developing countries as they increase in sanitation. The study found that countries where people have access to clean drinking water, such as the UK and France, have 9 per cent higher Alzheimer's 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 per cent higher rates of Alzheimer's compared with countries with high rates of infectious disease, such as China and Ghana. More urbanised countries exhibited higher rates of Alzheimer's, irrespective of life expectancy. Countries where more than three-quarters of the population are located in urban areas, such as the UK and Australia, had 10 per cent higher rates of Alzheimer's compared to countries, such as such as Bangladesh and Nepal, where less than one-tenth of people inhabit urban areas. VIDEO: Dr Molly Fox discusses her research on the link between hygiene and Alzheimer's Overall, differences in levels of sanitation, infectious disease and urbanisation accounted for 33 per cent, 36 per cent and 28 per cent of the variation in Alzheimer's rate between countries. Previous research has shown that Alzheimer's affects fewer people in Latin America, China and India than it does in Europe. Even within those regions, prevalence is lower in urban than in rural areas, according to the new findings. The hygiene hypothesis is based on the assumption that lack of contact with 'dirt' in the form of bacteria and other infectious agents upsets the development of white blood cells, key elements of the immune system. In particular, T-cells are said to be affected. T-cells have a variety of functions, including attacking and destroying foreign invaders and marshalling other parts of the immune system. Some, known as 'regulatory' T-cells, reign in the immune system when it starts to get out of control. Dysfunctional regulatory T-cells can lead to inflammation and autoimmune disorders. Regulatory T-cell deficiency is linked to the type of inflammation commonly found in the brains of people with Alzheimer's disease. Today, more than 50 per cent 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 per cent "Exposure to microorganisms is critical for the regulation of the immune system," wrote the researchers. They added that, since increasing global urbanisation beginning at the turn of the 19th century, the populations of many of the world's wealthier nations have increasingly very little exposure to the so-called "friendly" microbes due to "diminishing contact with animals, faeces and soil." "The increase in adult life expectancy and Alzheimer's prevalence in developing countries is perhaps one of the greatest challenges of our time," said Dr Fox. Today, more than 50 per cent 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 per cent. 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. The hygiene hypothesis is normally thought to be most relevant in childhood, when the immune system is still developing. But in the case of Alzheimer's, exposure to microbes across a person's lifetime might be important, say the scientists. This is because regulatory T-cell numbers peak at various points in life, for example at adolescence and middle age. The results of the study are published by the journal Evolution, Medicine and Public Health.