NEW YORK - The use of commonly prescribed "broad-spectrum" antibiotics, which destroy a wide range of bacteria, during infancy may raise the risk of asthma later in childhood, according to a Canadian report in the journal Chest.

"Antibiotics are prescribed mostly for respiratory tract infections, yet respiratory symptoms can be a sign of future asthma. This may make it difficult to attribute antibiotic use to asthma development," lead author Dr. Anita Kozyrskyj said in a statement. "Our study reported on antibiotic use in children being treated for nonrespiratory tract infections, which distinguishes the effect of the antibiotic," she explained.

Kozyrskyj, from the University of Manitoba in Winnipeg, and colleagues, analyzed data for 13,116 children who were born in 1995. Antibiotic use during the first year of life was compared with the development of asthma by age 7 years.

Antibiotic use during infancy was a risk factor for asthma in later childhood. Moreover, the risk rose as the number of antibiotic courses increased; with four or more courses, the likelihood of asthma increased by 46 percent compared with no antibiotic use.

The link between antibiotic use and asthma was particularly strong for children in rural areas and for those who had mothers without a history of asthma and without a dog. Previous reports have suggested that by "priming" the immune response, such as having regular exposure to a dog early in life, may reduce the risk of asthma, the authors note.

Further research is needed to better understand the interplay between early antibiotic usage and asthma, the investigators say. "In the interim, it would be prudent to avoid the use of broad spectrum antibiotics in the first year of life when other antibiotics are available."