Number of kids in daycare may affect asthma risk

NEW YORK (Reuters Health) - The more other children toddlers are exposed to at day care, up to a certain point, the lower their risk of developing asthma, new research shows.

The findings provide more evidence to support the "hygiene hypothesis," or the idea that early exposure to immune system stimuli like germs and animals -- and other kids --can help ward off asthma, the study's lead author, Dr. Matthew Gurka of the University of Virginia in Charlottesville, told Reuters Health.

But right now, he added, the findings shouldn't be used to guide parents' decisions on whether or not to put their children in group day care. "Parents shouldn't stress about this," he said.

Gurka and his team looked into whether early child care experience might influence asthma risk by reviewing long term data from the National Institute of Child Health and Development's Study of Early Child Care and Youth Development, which followed more than 1,000 children born in 1991 up to age 15.

The current analysis included 939 children and their families. Three percent of the children had asthma persistently since age 3 years, while another 16 percent had developed asthma by age 15. About 1 in 5 of the children were cared for at home by their parents up to age 3, while another 1 in 5 started center-based child care between 16 months and 3 years of age. Fewer than 10 percent started at a day care center as infants, or before 15 months of age.

The number of other kids the study participants were exposed to during infancy had no influence on their risk of persistent or late-onset asthma. But it did matter during toddlerhood (age 16 to 36 months).

"The fewer the children exposed to as toddlers, the higher the probability of persistent or late-onset asthma by age 15," the researchers report in the latest issue of The Journal of Pediatrics.

For every additional other child in his or her primary care setting, a child's risk of asthma decreased, up until age 9. Asthma risk began to increase again for toddlers in day care with 10 or more other kids, suggesting that there may be a "threshold for this protective effect."

When Gurka and his team took the number of respiratory infections a child had into account, the effect remained, suggesting that there was something else about being in day care with several other children that was protective against asthma. Just what this might be is unknown, the researcher said in an interview, but possibilities include gastrointestinal infections or even just germs in general.

What's new about the findings, Gurka noted, is that they point toward toddlerhood as being the time when exposure to immune-stimulating factors such as germs, pets and other children is most protective against asthma. "No one's really been able to pinpoint an exact time period when it really matters," he said.

In a commentary on the study, Dr. John T. McBride of the Northeastern Ohio Universities College of Medicine in Rootstown notes that while the symptoms of asthma are uniform, it's becoming clear that the causes can vary widely, with genetics being most important, and environmental factors less so.

"Although it is useful to have positive data to share with parents who have no choice but to depend on child care," he adds, "I am hesitant to make strong recommendations about child care for individual children."

Gurka agreed, and said his findings will be most important in helping guide further investigations. "This is just a great springboard for future research to hone in on differences in child care environments" and asthma risk, he concluded.

SOURCE: Journal of Pediatrics, December 2009.