Asthma is on the rise worldwide. Virtually unknown at the dawn of the 20th century, the wheezing respiratory disease caused 255,000 deaths globally in 2005, according to the World Health Organization. These recurring attacks of breathlessness from the swelling of bronchial tubes in the lungs is now the most common chronic disease among children—and no one knows why. Theories range from an increase in smog—an attack trigger—to a rise in the use of antibiotics. Now a new study of 13,116 children from birth to age seven reveals that use of certain antibiotics during infancy increases the risk of developing childhood asthma.

Drug epidemiologist Anita Kozyrskyj of the University of Manitoba in Winnipeg and her colleagues used health care and prescription databases to find that children who had received antibiotics during their first year of life had a significantly greater risk of developing asthma by age seven. "Children who had received more than four courses of antibiotics were almost twice as likely to have asthma develop," the researchers wrote in the journal Chest.

Previous studies that found similar results have been challenged on the grounds that many antibiotic treatments in infancy are for respiratory infections, which may also lead to asthma. To rule that out, the researchers isolated 148 children who had received antibiotics for skin or urinary tract infections. They found that these kids had an even greater chance of developing asthma than children with respiratory infections who had been treated with antibiotics. "The risk of asthma was twofold higher in antibiotic use for nonrespiratory tract infections," Kozyrskyj says. And "the antibiotic effect did not go away when the analysis was adjusted for upper respiratory tract infections, including ear infections and colds, meaning there wasn't a separate effect."

The researchers also found that the presence of a dog in the home lowered asthma risk (perhaps because they carry germs that engage children's immune systems), whereas treatment with so-called broad-spectrum antibiotics that wipe out many different kinds of microbes (as opposed to those targeting specific bacteria) increased the risk. In addition, rural kids had less of a chance of developing asthma than urban children, overall. The researchers speculate that this may have to do with the differing microbes that colonize the guts of city and rural children as well as country kids' wider exposure to a variety of microorganisms.

"Evidence for this hypothesis comes from epidemiologic studies, which link variations in gastrointestinal microflora and probiotic administration with less allergy and asthma," Kozyrskyj says. "Perhaps the gut microflora in urban children has already undergone change and the addition of an antibiotic makes no difference." The authors note that more research is needed. In the meantime, they suggest carefully weighing the risks and benefits of antibiotic use before age one.