Dust Mites, Cats Doubted as Cause of Childhood Asthma

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Oct. 20, 2000 -- The number of children with asthma has increased dramatically in recent years, and the underlying causes for this increase are puzzling. For years researchers believed that early exposure to allergens around the house such as dust mites or cats might not only cause allergic reactions but also could lead to childhood asthma. New findings from a German study fail to support that theory.

In the study, published in the Oct. 21 issue of The Lancet, researchers from Humboldt University in Berlin and Ludwig Maximilians University in Munich, studied almost 1,000 children from birth to age seven. They checked to see how many allergens were present in the home, whether the children had developed allergies, and whether they had developed asthma. At age seven, about 10% of the children had experienced wheezing in the past year, and 6% had been diagnosed with asthma. However, there was no direct relationship between early exposure to indoor allergens and the prevalence of asthma or wheezing.

"This study seems to put to rest the idea that the onset of asthma is caused by exposure to one or two allergens in early life," writes Roni Grad, MD, in an accompanying editorial. Grad is associate professor of pediatrics at the University of Alabama School of Medicine in Birmingham.

But although allergens don't seem to be the underlying cause of asthma, it is still very important to control them, because once someone has developed asthma, allergens can definitely trigger an asthma attack. Grad advises parents of children with asthma to do everything they can to keep their home clean.

And for asthmatic parents, who don't yet know whether their children have the same condition, "there is a genetic factor in asthma. Parents with this condition are more likely to have children who also develop asthma, so they, too, should emphasize a clean environment within the home," Grad says."

Keeping a home clean and allergen-free is especially difficult with pets around. Cats are a problem for those with allergies and asthma because of a particular protein found in their saliva, Anthony Montanaro, MD, tells WebMD. The cats lick themselves and when their saliva dries, this protein floats into the air. As a result, families with asthma probably should not keep cats as pets. Montanaro is professor and chief of the division of allergy and immunology at the Oregon Health Sciences University in Portland.

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Dogs are a problem, too, because of their pelt and fur, he says. If you do have dogs, keep them out of the bedroom, and keep them in rooms with hard-surfaced floors.

Montanaro has further advice for people living with asthma. "Your lungs are exposed to bedroom air more than any other environment, so pay special attention to creating a clean environment there," he says. "Cover pillows and mattresses with allergen encasements. That means a cover specifically designed to keep allergens out of air you and your children are breathing."

Wall-to-wall carpets are another problem, since they are a major reservoir for dust mites and animal danders. Ordinary vacuuming just moves the dust around. It's better to have a hard floor with area rugs that can be removed and thoroughly cleaned.

So if the environmental allergens aren't causing the spike in childhood asthma, what is? The Lancet study authors list several proposals, such as lack of severe or repeated infections, obesity and lack of physical exercise, decreased family size, or changing dietary habits.

In any case, though, the causes of the increasing prevalence of asthma remain a mystery, Montanaro says. "It is probably due to a complex interaction between [genetics] and the environment."

Grad points to a study from Southeast Asia. Children in different localities had similar rates of allergies, but widely different rates of asthma. "In addition to learning about the factors that make people become allergic, we also need to learn why some of those people become asthmatic and the rest don't," he tells WebMD. "There may be environmental factors related to living in those communities. It may be related to lifestyle or diet. Additional research on these issues is needed."

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