Early exposure to other youngsters and their germs appears to protect children from developing asthma later on, according to a study that could reassure parents feeling guilty about putting their infants in day care.
The study found that children who attended day care in their first six months or had two or more older siblings were about half as likely to have asthma at age 13 as youngsters who had one or no older siblings and did not attend day care until they were older.
This echoes the hot new "hygiene theory" that says children who do not get outside and get dirty every now and then are not being exposed to enough germs to stimulate proper development of their immune systems.
"This paper reflects the growing belief that the more sterile the early environment, the more problems later in life," said Dr. Leonard Bielory, director of the Asthma and Allergy Research Center at the University of Medicine and Dentistry of New Jersey in Newark.
The findings come amid an unexplained explosion of asthma among children and worries that smaller families and households scrubbed with antibacterial cleansers may be weakening our immune systems.
Asthma cases jumped 158 percent from 1980 through 1998, with many of the new cases among children, according to the Centers for Disease Control and Prevention.
The new study, funded by the National Heart, Lung and Blood Institute, was published in Thursday's issue of the New England Journal of Medicine.
Researchers at the University of Arizona College of Medicine have been following about 1,000 children for 15 years, studying their respiratory health and allergens in their environment. After previously showing that children in day care get more respiratory infections than others, the researchers examined more closely the role of day care and family size.
They found that protection against asthma came from frequent exposure to other youngsters, but only if the contact took place in the first six months, a key period for a new immune system.
The theory is that if the immune system isn't stimulated early in life by germs, it overreacts later to allergy-inducing substances, said Anne L. Wright, a pediatrics research professor who led the study.
The children most exposed to other youngsters were about 40 percent more likely than the group with less contact to suffer from frequent wheezing in their first few years. But doctors believe that in toddlers, wheezing usually is due to their small airways or respiratory infections. And wheezing almost always disappears by age 6 unless the child has asthma.
Asthma, the most common chronic childhood disease, is an incurable condition in which allergic reactions to such things as pollen or dust mites trigger a narrowing of the airways, wheezing and trouble breathing.
Asthma afflicts some 17 million Americans, including at least 5 million under 18, and kills about 5,400 people annually. While it is partly inherited, less-understood factors also are at work.
Two recent German studies similarly found asthma less common in children who started day care early, and other research indictates that growing up in the country, on a farm with animals or in a home with a dog protects against asthma, noted Dr. Sandra C. Christiansen of Scripps Research Institute.
Dr. Marshall Plaut, chief of allergic mechanisms at the National Institute of Allergy and Infectious Diseases, said the Arizona group's study is the strongest evidence of this idea to date.
But he said more research is needed on how children's immune systems mature, how that affects whether they develop asthma and whether the apparent benefits of early infections outweigh the dangers.
On the Net: http://www.nejm.com
National Heart, Lung and Blood Institute site on asthma: http://www.nhlbi.nih.gov/health/public/lung/asthma/asthfs.pdf
American Academy of Asthma, Allergy and Immunology site: http://www.aaaai.org/public/default.stm
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