Cigarette smoke worsens respiratory infections in infants

Studying Respiratory Syncytial Virus (RSV) to learn what puts children at risk for the most severe infections, Washington University researchers at St. Louis Children's Hospital found that a child's age at the time of infection and whether that child lives with a smoker could mean the difference between the sniffles and the intensive care unit.

RSV infection is common in children but sometimes can be hard for parents to recognize. By two years of age, 95 percent of all children have been exposed to RSV, but the symptoms can range from a mild cold all the way to serious pneumonia, which requires a child to be hospitalized and placed on a ventilator. The Washington University researchers have been trying to learn why some kids have a relatively easy time while others get so very sick.

The research team has been following 206 children since they were hospitalized or treated in the Emergency Department at St. Louis Children's Hospital. The children are part of a project called the RSV Bronchiolitis in Early Life (RBEL) study. The infants all were under 12 months of age when they were enrolled in the RBEL study and had a wide range in the severity of their RSV infection, according to Mario Castro, M.D., associate professor of medicine and pediatrics at Washington University School of Medicine and principal investigator of the RBEL study.

After the children were treated at the hospital and recruited into the study, Castro and colleagues looked at medical records and measured the children's oxygen saturation levels during RSV infection. The lower the oxygen saturation, the more serious the infection. Later, they went to the children's homes and vacuumed up dust and allergens to see whether those products might have played a role in the severity of RSV infections.

Researchers asked the babies' mothers detailed questions about their pregnancies, including whether or not they smoked cigarettes either before or after giving birth. And they asked mothers about their own problems with allergies and asthma. The idea was to learn about several potential risk factors to determine why RSV affects some children so severely.

"What we're trying to study isn't the likelihood of getting the disease because it seems just about every kid gets RSV," Castro says. "We wanted to learn what factors are driving these severe, life-threatening infections."

Not surprisingly, one of the most important risk factors for severe infection was exposure to cigarette smoke.

"Smoking in the household is really detrimental to these kids," Castro says. "The kids who lived with mothers or others who smoked developed more severe RSV infections than the kids who were not exposed to cigarette smoke."

The study found that 28 percent of the children in the study lived with a mother who smoked. A total of 40 percent lived in a house where at least one person smoked. And Castro says those children had more severe infections. But somewhat surprisingly, smoking during pregnancy didn't seem to increase the risk of severe infection.

"It's a great thing not to smoke during pregnancy," Castro says, "but it's equally important not to smoke after pregnancy. Kids spend a lot of time with their mothers during the first few months of life, and if the mother is smoking, it's going to be detrimental to the child's health."

Age is another risk factor for serious RSV infection. The younger the child, the more serious the infection tended to be. Castro blames that increase in risk on lung and airway development. The tiny windpipes of small children can become clogged with mucous and other secretions more easily than the bigger, more developed airways of older children.

Surprisingly, the RBEL study also found that African-American children tended to have less serious infections than Caucasian children. Because RSV infection is linked to childhood asthma and because African-Americans are more likely to suffer from asthma and to be hospitalized because of it, Castro and colleagues hypothesized that African-American children would be at increased risk for serious RSV infection. But the study showed just the opposite.

Another surprise was that children who came from homes with the highest levels of allergens had no effect on the severity of RSV infection. The researchers measured levels of dog and cat allergen, two types of dust mites and cockroach allergen, all of which have been linked to allergy and asthma problems in older children and adults.

"We were thinking that the kids who came from homes that had high allergen levels would already be compromised because they would have a reaction to those allergens and develop inflammation in their windpipes, so they would tend to do worse with RSV," Castro says. "But we found that even in the kids who came from the homes with the highest allergen levels, there was no increased risk."

Having a mother with allergies or asthma also tended to have a protective effect. Those children whose mothers had allergies or asthma tended to have less serious RSV infections.

That's somewhat surprising because the virus has been linked to the risk for asthma later on in life. As the children in the RBEL study get older, they will be followed to see if they develop asthma problems.

"We believe the severity of that initial RSV infection has a role in increasing the risk for asthma later in childhood," Castro says. But he won't know for sure until the children get older.