Asthma, a respiratory disease whose prevalence skyrocketed in the latter part of the 20th century, is  believed to have a genetic component, and environmental triggers—including pollution and pests— are also blamed.  Now a common infant virus—respiratory syncytial virus (RSV)—is being fingered as a possible cause.

Babies born in autumn, about four months before RSV's peak season, are 30 percent more likely to develop asthma by the time they're five than kids who are older and have stronger immune systems when RSV is in high circulation, according to research in this week's American Journal of Respiratory and Critical Care Medicine.

RSV causes everything from colds to croup, and nearly all children get it by the time they're two. But RSV is more likely to cause a severe infection of the lower airways called bronchiolitis if kids contract it in the window between when they lose most of their mothers' antibodies (at about three months) and develop their own (at about six months), according to study co-author Tina Hartert, a pulmonologist who directs the Center for Asthma Research at Vanderbilt University Medical Center.

It's already known that 30 to 40 percent of infants who are hospitalized for bronchiolitis develop asthma. The question has been whether catching RSV represents a genetic susceptibility to asthma, or if the virus itself causes the disease. By showing that increased asthma risk is associated with a baby’s age at the time of RSV season peak, the new research bolsters the link between the virus and the disease, Hartert says.

That's significant, because "you can't change a child's genes, but you can change things in the environment," says Hartert, who tracked more than 95,000 babies born over a 10-year period in Tennessee's Medicaid program. One option would be to give antiviral drugs to babies who are most at risk of asthma, based on how old they are when the RSV season is at its peak, writes Renato Stein of Pontificia Universidade Catolica in Brazil in an editorial accompanying the study. Stein has been a consultant to drugmaker Abbott.

The study didn’t address whether breastfed babies who continue to receive their moms’ antibodies are better protected against RSV – or asthma. Some research suggests that breastfeeding may reduce a child’s chances of developing the disease, but other studies indicate it may not, especially if their mother is asthmatic.

An estimated 20 million Americans have asthma, including nearly seven million children.

Asthma "moves in time with the peak of winter virus season, which is very strong evidence of a causal relationship between these viruses and the development of childhood asthma," Hartert says. "What you next to prove is that preventing respiratory viral infections actually prevents asthma."

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