Babies who grow up in a house with pet dogs and cats have less than half the chance of developing asthma than those who do not, according to research published yesterday.
Scientists in the United States found that having two or more pets in the home during the first year of life also greatly reduced hay fever and other common allergies.
Dr Dennis Ownby, from the Medical College of Georgia, who led the study, said the results had shocked researchers, who had been looking for evidence that pets increased allergies. "The data didn't look the way it was supposed to. As a matter of fact, it was very strongly the opposite of what we expected to find," he said.
"Allergists have been trained for generations that dogs and cats in the house were bad because they increased the risk of you becoming allergic to them. We know that before you become allergic to something, you have to be repeatedly exposed to it."
He said the findings, reported in the Journal of the American Medical Association, appeared to support the "dirty hypothesis" – the theory that early exposure to bacteria can prime the immune system and help prevent allergies.
"The bottom line is that maybe part of the reason we have so many children with allergies and asthma is we live too clean a life," said Dr Ownby. "What happens when kids play with cats and dogs? The animals lick them. The lick is transferring a lot of bacteria and that may be changing the child's immune system response in a way that helps to protect against allergies."
Recent studies have shown that children who live in cities have higher rates of allergy than children of farmers, perhaps because of their limited contacts with animals.
Doctors followed a group of 474 healthy babies in the Detroit area from birth to about the age of seven. Positive reactions to common allergens ranged from 33 per cent in children without exposure to dogs or cats, to 15 per cent in children who had been regularly exposed to two or more animals.