ACAAI 2011: Can Cleanliness Cause Illness?

BOSTON—Early exposure to animals and infections could decrease the tendency to become allergic, according to Thomas A.E. Platts-Mills, MD, PHD, FACAAI, Asthma and Allergic Disease Center, University of Virginia Medical School, Charlottesville, VA. Platts-Mills spoke at the American College of Allergy, Asthma & Immunology’s annual meeting this morning.

Research has found that children who entered day care at six months of age had decreased incidence of asthma at six years; children exposed to farm animals in the first year of life have decreased allergies; and having an animal in the house decreases asthma. Platts-Mills said that hay fever incidence increased long before the start of the increase in asthma in 1960 in western countries. And, asthma has increased over a period when use of daycare was also increasing.

"For some time, the 'hygiene hypothesis' was an explanation for the increase in allergic sensitization and asthma,” he said. That is, improvements in personal cleanliness and migration from farms to urban living have weakened our immune system. Several changes documented over the past few decades and attributed to the hygiene hypothesis include: decreased helminth infection, reduced physical proximity to farm animals, decreased exposure to mycobacteria found in the soil, changes in bacterial colonization of the gut, and decreased Hepatitis A infection. These all may have contributed to increased sensitization, “but, none can explain the time course, the scale or the consistency of the increase in asthma,” Platts-Mills said.

That begs the question, he said--are parasites part of the signal(s) that prevent the development of modern allergic disease and can they be used to treat or prevent allergy?

Many lifestyle changes impact the increase in asthma and allergies, Platts-Mills said. The effect “has been enormous.” Increased exposure to perennial allergens changes immune responsiveness. A change from Th1 to Th2 results in increased allergy. Changes in diet and a decline in exercise changed inflammatory response, he said.

Another lifestyle change Platts-Mills noticed is that mothers of newborns often bathe them daily while just a couple of decades ago, newborns were bathed weekly. All that washing strips lipids from the skin and makes the skin more permeable, he said. “Lipid layers in the skin are thought to control access of proteins through the skin.” Researchers have noted that the Amish typically don’t use running water and have one of the lowest levels of allergies in a population.

We’ve seen a progressive increase in vaccinations and antibiotics and decline in enteric infections, said Platts-Mills. These lifestyle changes, including diet, physical activity and housing, have been continuous since 1950, he said. Attempts to understand the causes of the increase in allergic disease have been met with an unequal attempt to keep up with real decreases in lifestyle.