Children who grow up with dogs and cats in the home have a significantly reduced risk of developing common allergies- some by 50 percent or more - a surprising finding resulting from a study following hundreds of children from birth to nearly age 7.
"We simply started looking at our data to see if exposure to dogs and cats really increases the risk and 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," said Dr. Dennis R. Ownby, chief of the Medical College of Georgia Section of Allergy and Immunology and lead investigator on the study published in the Aug. 28 issue of the Journal of the American Medical Association.
"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."
But when doctors followed a group of 474 healthy babies in the Detroit area from birth to about age 7, comparing the 184 exposed during infancy to two or more dogs or cats to the 220 who were not exposed to these animals, they found that the children exposed to two or more indoor pets were half as likely to develop common allergies.
Also, fewer of the children who had early exposure to indoor pets had hyper-responsive and easily irritated airways, a risk factor for asthma. Reactivity was based on the airway's response to a chemical stimulant called methacholine. In fact, children raised with two or more dogs or cats had 45 percent less hyper-reactivity; rates went from 24.1 percent to 15.8 percent. The boys experienced an even greater reduction, from 25.5 percent for boys with no indoor pets to 5.1 percent for those with multiple pets. About 7 percent of the children developed asthma during the study, which is on target for national averages.
"This is exactly the opposite of what we would have predicted from the beginning and it's very significant," Dr. Ownby said. "This contributes to the mounting evidence that the things allergists have believed for years and parents have lived by are wrong."
The babies were born between 1987 and 1989 to largely white, middle-class parents who were members of a large, Midwestern health maintenance organization. Dr. Ownby began the studies, funded by the National Institutes of Health, while a senior staff physician at Henry Ford Hospital in Detroit.
Both girls and boys with pets had fewer positive skin tests than those without to common indoor allergens (dust mite, cat and dog) and outdoor allergens (grass, ragweed and Alternaria, a fungus found in air). The reduction remained significant even after adjusting for risk factors such as older siblings, parental history of asthma and parental smoking.
For example, 15.5 percent of the children without a dog or cat in the home were allergic to cats compared to 11.6 percent with one cat or dog and 7.7 percent with two or more pets in their home.
Similar trends were found in children's positive response to allergies to dogs; it went from 8.6 percent to 3.5 percent to 2.6 percent respectively. The same essentially held true for atopy, a positive skin test to any of several common allergens including cat, dog and grass; positive results went from 33.6 percent in all children without exposure to dogs or cats to 15.4 percent in children with regular exposure to two or more of the animals. The exception here was a slight increase in allergies - from 33.6 to 34.3 percent - for children exposed to only one dog or cat.
Dr. Ownby noted that several retrospective studies in this country and others have provided the first bits of evidence suggesting that exposure to animals may reduce a child's risk of allergies. For example, studies in southern Germany and Switzerland have shown that city dwellers' children have higher rates of allergies than children of farmers.
The original goal of this study was to detect at birth children at highest risk for becoming allergic, follow them through their early years to see what they were exposed to and use those data to change the environment of future at-risk kids.
The researchers think that exposure to dogs and cats leads to lower risks of allergies because children living with these animals are probably exposed to higher levels of endotoxins, the breakdown products of Gram-negative bacteria commonly found in the mouth of a cat or dog. "Exposure to endotoxins is thought to force the body's immune system to develop a different pattern of response that makes you less likely to become allergic," he said.
Immediate allergic reactions are caused when immunoglobulin E, a class of antibodies that causes allergic responses, become bound to mast cells, a type of white blood cell. This coupling is needed because antibodies recognize allergens but it's the mast cells that release histamines and other chemicals that cause allergic symptoms, the most rapid type of immune response. The result can be swelling, redness and itching within minutes
But there are checks and balances within the immune system, and allergic sensitivity can be down-regulated by other portions of the immune system. "We feel that early exposure to endotoxin activates a down-regulatory portion of the immune system, reducing the risk of allergies," Dr. Ownby said.
"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," he said. "What happens when kids play with cats or dogs? The animals lick them. How many cute pictures like that have you seen? The lick is transferring a lot of Gram-negative bacteria and that may be changing the way the child's immune system responds in a way that helps protect against allergies."
The reason for the identified differences in responses between boys and girls in the study is unclear, Dr. Ownby said. "We've known for a long time that, prior to puberty, boys are nearly twice as likely to have allergies and asthma as girls. Our data suggest that the effect of animal exposure may be highest on those more likely to develop allergies. By the time they reach young adulthood, women are more likely to have asthma than men. We have always assumed that had something to do with the hormonal changes that take place during puberty, but no one has ever proven that or come up with a good theory of why that would be."