Studies have suggested that exposure to high levels of cat allergen during infancy can actually protect children against developing asthma. The new study supported by the National Institutes of Health (NIH) adds another twist to the developing story.

The new study confirmed 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 five.

"Asthma is a complex disease in which there appear to be many factors, both genetic and environmental," says Marshall Plaut, chief of the allergic mechanisms section at the National Institute of Allergy and Infectious Diseases (NIAID). "To better understand asthma, we need studies like this one that sort out these factors and define the relationships among them."

The study followed 448 children with a family history of allergic diseases from birth to age five. Through periodic telephone interviews with caretakers, the researchers gathered information on the children's exposure to pets and on how often the children experienced episodes of wheezing. In addition, the researchers tested dust samples from each child's home for levels of cat allergen. The researchers then analyzed the data they collected, adjusting for factors such as sex, household income, and day-care attendance.

In the group of children with non-asthmatic mothers, those exposed to a cat were 40 percent less likely to experience persistent wheezing as compared to those with no cat exposure. This risk reduction remained consistent in each of the five years.

In the group of children with asthmatic mothers, however, those exposed to a cat were significantly more likely to wheeze as compared to those with no cat exposure. In fact, the risk of wheezing increased in each of the five 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 - the first step in developing asthma - and wheeze when exposed to it.

The researchers plan to continue following this group of children and to test them for allergies to cats as well as other potential asthma triggers such as cockroaches and dust mites.