Sept. 9 (HealthDay News) -- Early day care, so the story goes, exposes kids to more germs and illnesses at an early age, thereby protecting them from asthma and allergies later on.

Scientists call it the ''hygiene hypothesis," but a new study casts doubt on its existence and finds early infections have no effect on asthma and allergy rates at age 8.

An unexplained increase in asthma among children in both industrialized countries and developing nations has scientists searching for ways to reduce the disease. In the United States, asthma affects nearly 5 million children and is the most common serious chronic childhood disease, according to the American Academy of Allergy, Asthma & Immunology.

Dutch researchers followed more than 3,600 children from birth through 8 years, noting their daycare use and health problems, such as wheezing. Those who started daycare early were twice as likely to experience wheezing in their first year as those who didn't attend daycare.

By age 5, however, the early daycare kids -- who started from birth to age 2 -- had less wheezing than the kids who didn't go, the researchers found.

But three years later, the discrepancy disappeared. "We found no evidence for any protection for asthma, allergy and airway hyper-responsiveness at 8 years," said Dr. Johan C. de Jongste, professor of pediatric respiratory medicine at Erasmus University Medical Center in the Netherlands, who led the study.

The researchers examined annual parental reports that asked about symptoms. Other assessments included blood tests to measure IgE, antibodies typically elevated in a person with allergies. They tested for lung function and hyper-responsiveness of the airways, and they controlled for such factors as a mother's allergies.

Would the results for Dutch children hold in the United States? "The study population was a reasonable reflection of the Dutch general population, including rural and urban areas, so white children with a western European lifestyle," de Jongste wrote in an e-mail interview. "This should not be very different from U.S. children who live under comparable conditions."

The bottom line? Early exposure to germs and other organisms does cause more symptoms early in a child's life, but without a counterbalancing health benefit later on, as was previously believed. The children had an increase in airway symptoms until age 4, an overall decrease from age 4 to 8, but no protection at age 8 compared to those who weren't in daycare.

The study is published in the Sept. 15 issue of the American Journal of Respiratory and Critical Care Medicine.

The study is considered a "landmark investigation" by Dr. John Heffner, past president of the American Thoracic Society. In a prepared statement, Heffner said that the investigation "is the first to follow children prospectively from birth to 8 years and assess the effect of daycare enrollment on both asthma symptoms and immunologic evidence of allergic disease."

Heffner added: "This study provides strong evidence that enrollment in daycare provides no protection against asthma and cannot be promoted as a public health strategy to decrease asthma prevalence."

De Jongste said parents should consider daycare for many other reasons, however. "Daycare may serve many purposes, including social contacts and enabling both parents to work," he said.

SOURCES: Johan C. de Jongste, M.D., Ph.D., professor, pediatric respiratory medicine, Erasmus University Medical Center-Sophia Children's Hospital, Rotterdam, The Netherlands; John Heffner, M.D., Portland, Ore., past president, American Thoracic Society; Sept. 15, 2009, American Journal of Respiratory and Critical Care Medicine