But more and more evidence is proving that the opposite is true. Children raised in a house with two or more dogs or cats during the first year of life may be less likely to develop allergic diseases as compared with children raised without pets, according to a study in the latest issue of the Journal of the American Medical Association.
Dr. Dennis Ownby, the chief investigator of the study printed in JAMA, explains on The Early Show that allergists were trained for generations that dogs and cats in the house were bad because they increased the risk of kids becoming allergic to them. The views were based on the knowledge that before you become allergic to something, you have to be repeatedly exposed to it.
The latest study, he says, finds that the reverse seems to be true. Children who grow up with dogs and cats in the home have a significantly reduced risk of developing pet allergies — plus common ones, like allergies to pollens and molds — by as much as 50 percent or more.
This study followed hundreds of children from birth to nearly age 7. Ownby says that he and his researchers simply started looking at their data to see if exposure to dogs and cats really increased the risk of allergies. The data generated surprised researchers because it concluded that pets gave the opposite reaction. However, the findings only applied to infants exposed during their first year of life, when the immune system is still developing.
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 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.
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 also be regulated by other portions of the immune system.
Ownby theorizes that early exposure to endotoxin activates a down-regulatory portion of the immune system, reducing the risk of allergies. He notes that several 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 Germany and Switzerland have shown that city dwellers' children have higher rates of allergies than children of farmers.
The Opposite Reaction
The children in the study were born between 1987 and 1989 to largely white, middle-class parents who were members of a large, Midwestern health maintenance organization. The study was funded by the National Institutes of Health. 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.
When the children were one year old, the researchers contacted parents by telephone to find out how many pets were in the home. When the children were two years old, researchers measured the level of dust mite allergen in their bedrooms. When the children were six or seven, the researchers tested them for allergic antibodies to common allergens by two approaches — a skin prick test and a blood measurement. Allergies usually do not develop until children are older.
They found that the children exposed to two or more indoor pets were half as likely to develop common allergies. Children exposed to two or more dogs or cats during the first year of life were on average 66 to 77 percent less likely to have any allergic antibodies to common allergens, as compared with children exposed to only one or no pets during their first year.
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. 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. Children raised with two or more dogs or cats had 45 percent less hyper-reactivity. The boys experienced an even greater reduction of asthmatic symptoms than girls. About 7 percent of the children developed asthma during the study, which is on par with national averages.
The results were exactly the opposite of what Ownby and his team would have predicted from the beginning, and the study is a significant contribution to the mounting evidence that the things allergists have believed for years and parents have lived by are wrong. The striking finding here is that high pet exposure early in life appears to protect against not only pet allergy but also other types of common allergies, such as allergy to dust mites, ragweed, and grass. Other studies have suggested a protective effect of pet exposure on allergy and asthma symptoms but generally have looked only at whether pet exposure reduced pet allergy. This new finding changes the way scientists think about pet exposure.
Scientists must now figure out how pet exposure causes a general shift of the immune system away from an allergic response.
Some of the Study Findings
- Cat allergies: 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.
- Dog allergies: 8.6 percent of the children without a dog or cat in the home were allergic to dogs, compared to 3.5 percent with one cat or dog and 2.6 percent with two or more pets in their home.
- Atopy (positive test to any of several common allergens including cat, dog and grass): 33.6 percent of children were allergic without exposure to dogs or cats, compared to 15.4 percent allergic 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.
Society Too Sterile?
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. When kids play with cats or dogs and the animals lick them, the transfer of bacteria may be changing the way the child's immune system responds in a way that helps protect against allergies. Parents should not be concerned about having pets in the home with a new baby but the findings do not go far enough in allergy prevention to warrant the purchase of pets.
Perhaps one day, scientists might be able to develop a new allergy therapy based on future research on pets and their bacteria.