FRIDAY, April 30, 2004 (HealthDayNews) -- A household full of hairy pets and siblings can be a protective environment for allergy-prone children.
But that's not the case not if those children suffer an infection -- such as a cold -- during the first six months of life, a new Danish study says.
The study, which looked at more than 24,000 mother-and-child pairs prone to allergies, contradicts just one premise of the "hygiene hypothesis." This theory, promulgated since the 1980s, holds that exposure to allergens in the environment early in life reduces the risk of developing allergies by boosting immune system activity.
The one part of the hypothesis that did not stand up maintains that having an infection -- a cold, ear ache, pneumonia or other infectious disease -- early in life does good by strengthening immune responses, said Dr. Christine Stabell Benn. She is a research fellow at the Danish Epidemiology Science Centre, and lead author of the research that appears in the April 30 issue of the British Medical Journal.
"We show that infectious diseases early in life do not have a protective value," Benn said.
The study looked at the incidence of atopic dermatitis -- a skin allergy that keeps people scratching -- in the allergy-prone children.
It found the incidence of the condition decreased 21 percent for children with three or more brothers and sisters, and for those who owned pets, lived on a farm or attended day care -- all of which increase early life exposure to allergy-causing environmental factors.
But the study also found the risk of dermatitis increased with each infectious disease contracted before 6 months of age.
"That finding goes against the common wisdom which says that infections transferred from older siblings help to mature the immune system," Benn said. "We find that infectious diseases do not have a protective effect."
Benn's advice is "to stop following advice," because "the advice we have been giving for infants may not be on solid ground."
Dr. Clifford W. Bassett, an allergist in private practice in New York City and a spokesman for the American Academy of Allergy, Asthma & Immunology, opts for a family-by-family approach.
The Danish study, he said, "supports the hygiene hypothesis that things that are bad for you may be good for you." But the "common wisdom" that advises an allergy-prone family to have a pet for its protective value may be good or bad, depending on individual circumstances, Bassett said.
Overall, the Danish statistics are solid, he said. A child with one allergic parent has a 25 percent chance of developing an allergy; two allergic parents raise the risk to 50 percent. Environment and genetics both contribute to the risk, he added.
"But we don't want to make a single recommendation for everyone," Bassett said. "It is so difficult to tell someone in a family with allergy and asthma to go out and get a pet. We need to individualize when we make recommendations."
"Our job is to find out which patients and which families benefit, and I don't think we're there yet," he said.
A full explanation of the hygiene hypothesis can be found at the American Academy of Allergy, Asthma & Immunology. For more on pets and how they may protect children from allergies, visit the National Institute of Allergy and Infectious Diseases.