Dear Doctor Column, September 23, 2002

Question:

I heard on the news recently that children who grow up with pets in the home have fewer allergies. What can you tell me about this? Is this true?

Answer:

A study published in the August 28, 2002, issue of the Journal of the American Medical Association found that 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 compared with children raised without pets. The study was supported by the National Institute of Allergy and Infectious Diseases and the National Institute of Environmental Health Sciences.

Specifically, the study showed that children exposed to two or more cats or dogs in the first year of life were about 75% less likely to have allergic sensitization (positive skin test reactions) at age 6 than children with no pets. These results give weight to earlier studies that have shown that children who grow up on farms with animals are less likely to develop allergies than children growing up in cities.

The researchers followed 474 children from birth to 6 or 7 years of age. 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 2 years old, researchers measured the level of dust mite allergen in their bedrooms. When the children were 6 or 7 years old, the researchers tested them for allergic antibodies to common allergens by two approaches — a skin prick test and a blood measurement.

After adjusting for factors such as dust mite allergen levels, parental smoking, and current dog or cat ownership, the researchers found that children exposed to two or more dogs or cats during the first year of life were on average 66% to 77% less likely to have any allergic antibodies to common allergens compared with children exposed to only one or no pets during their first year.

One of the most striking findings of the study 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.

To add another twist to the pet versus allergy story, the results of another new study, supported by the National Institutes of Health, sheds further insight into the complex relationship between pet exposure and the risk of developing asthma.

The study, published in the September 7, 2002, issue of The Lancet, confirms the protective effect of cat exposure for at-risk children in all but one situation: when the child's mother has asthma. If the mother has asthma, then a cat in the home actually triples the risk that a child will develop persistent wheezing — an initial indication of asthma — by age 5.

The study followed 448 children with a family history of allergic diseases from birth to age 5. In the group of children with nonasthmatic mothers, those exposed to a cat were 40% less likely to experience persistent wheezing compared with those with no cat exposure. This risk reduction remained consistent in each of the 5 years.

In the group of children with asthmatic mothers, however, those exposed to a cat were significantly more likely to wheeze compared with those with no cat exposure. In fact, the risk of wheezing increased in each of the 5 years of the child's life. By the third year, the risk of wheezing doubled, and by the fifth year, it more than tripled. This finding suggests that the children of asthmatic mothers become more readily sensitized to cat allergen and wheeze when exposed to it.

The finding may be explained by genetic influences passed on from the mother, or it may result from environmental influences shared by mother and child. The researchers plan to continue following this group of children and test them for allergies to cats, as well as other potential asthma triggers, such as cockroaches and dust mites.

Stay tuned.