Early exposure to the germs in dust and grime may strengthen children's immune systems
Slobs rejoice. Well, maybe not, but parents who don't keep their homes spotless can at least breathe a sigh of relief. Last week came the strongest evidence yet for the "hygiene hypothesis," which blames squeaky-clean modern habits and habitats for a rise in asthma and allergies.
Exposure to dirt and germs in infancy, the theory holds, helps a baby's immature immune system develop properly. A century ago, infants had to battle typhoid, diphtheria, polio, and a host of other virulent bugs. But indoor plumbing, vaccinations, and antibiotics helped control those plagues. Infant mortality plunged, from more than 15 percent in 1900 to less than 1 percent now. But scientists have been puzzled as to why today's children are having many more problems with asthma and allergies. One in five Americans now suffers the drippy nose and teary eyes of allergies. The prevalence of asthma rose 75 percent from 1980 to 1994 and now affects 7 percent of the population, causing 5,000 deaths a year.
The hygiene hypothesis, first proposed in 1989, may explain why immune systems are now more likely to react inappropriately to innocuous substances like pollen and cat dander, triggering hives, wheezing, even deadly anaphylactic shock. Scientists are applying the theory in developing new allergy vaccines that act like artificial dirt, nudging the immune system toward a healthier response. The vaccines may also prove useful in treating autoimmune diseases such as rheumatoid arthritis and inflammatory bowel disease.
The latest evidence, reported in last week's New England Journal of Medicine, indicated that European children who live on farms are much less likely to have allergies and asthma than their nonfarm counterparts. Only 4.1 percent of the farm children had hay fever, while 10.5 percent of those who didn't live on farms suffered from allergies. Farm animals are a major source of bacteria. The research team, led by Charlotte Braun-Fahrlander of the Institute of Social and Preventive Medicine in Basel, Switzerland, found that the higher the levels of bacterial components on a child's mattress, the less likely it was that the child would have allergies and asthma.
More means less. Earlier studies have found that children with many older siblings, and ones who spend time in day care, are less likely to develop asthma. In both cases, more children means more germs. But it remains unclear whether having pets in the home helps or hurts. That may well be because the protective effect is activated primarily in the first months of infancy, while the immune system is learning to combat a dirty world. "Timing is everything," says Donald Leung, head of pediatric allergy and immunology at the National Jewish Medical and Research Center in Denver. "After asthma is established, exposure probably makes it worse." So parents have no license to let dust pile up--or bring in livestock. Rather, the hygiene hypothesis offers a chance to come up with safer, better treatments.
Researchers at Johns Hopkins University in Baltimore have used an experimental vaccine created from bits of synthetic bacterial DNA to reduce symptoms of ragweed allergy in humans. Patients who received six shots of the vaccine over six weeks reported many fewer allergy symptoms and much less need for antihistamines and decongestants. The vaccine is substantially safer than conventional allergy shots.
Earlier this year, researchers at the University of California-San Diego showed that a bacterial DNA vaccine could reduce the effects of inflammatory bowel disease in mice. Eyal Raz, a UCSD associate professor of medicine who has also used DNA vaccines to treat asthma and allergies in mice, says the treatment might be used to keep asthma from becoming a chronic disease.
Indeed, over the past decade researchers have learned that the airway damage caused by asthma is almost impossible to reverse. What's needed, says Leung, is a way to test children at birth for genetic susceptibility to asthma and to treat them so they'll never know what it's like to gasp for breath.
Asthma affects 7 percent of Americans. One in five suffers from allergies.
This story appears in the September 30, 2002 print edition of U.S. News & World Report.