Children who tested positive for dog allergies were less likely to develop eczema by age 4 if they had lived with a dog before their first birthday, according to a study released Thursday by the University of Cincinnati and Cincinnati Children's Hospital Medical Center.

On the other hand, kids with dog allergies who didn't have dogs while they were infants were four times more likely to develop eczema, a chronic skin condition marked by extreme dryness and irritation.

It's possible dogs might act as four-legged allergy shots to protect allergic children, said Tolly Epstein, a UC allergist and corresponding author of the study, published in the "Journal of Pediatrics."

"Dog ownership seemed to have a protective effect," she said. "It's one hypothesis that it's a kind of natural immunotherapy for these children."

She hopes parents can use the information to help choose a family pet.

Cat ownership didn't offer any protection against eczema, the researchers found.

In fact, children with cat allergies who lived with cats before their first birthday were 13 times more likely to develop eczema by age 4 than allergic children who didn't have cats.

Cats in general are considered to cause more allergy symptoms than dogs.

The current study only looked at eczema, she said, but previously published research has shown children who lived with dogs were less likely to develop wheezing later in life. But that research didn't distinguish between children with and without dog allergies, she said.

Epstein and her colleagues reviewed data from the Cincinnati Childhood Allergy and Air Pollution Study, a long-term study looking at the effects of environmental particulates on childhood respiratory health and allergy development.

The study is following 636 children considered to be at high risk for developing allergies because their parents had allergies.

Epstein said 14 percent of the children in the study had eczema symptoms.

The results are based on skin tests, as well as questionnaires completed by parents.

Doctors are seeing an increase in the number of children with allergic eczema, but they don't know why, she said.

"It's becoming very common," she said, adding 10 percent to 30 percent of children experience it. Estimates of adults with allergic eczema range from 2 percent to 10 percent.

For the ongoing allergy and air pollution study, children were tested annually for 17 different allergies - including foods, airborne allergens like pollen and mold and environmental exposures like diesel particulates.