Study links asthma rate to birth month
- |
Babies born four months before the peak of cold and flu season - in other words, roughly now - are 30 percent more likely to develop childhood asthma than children born at any other time of year, new research suggests.
The findings speak to doctors' longstanding question of whether respiratory tract viral infections during infancy can cause asthma. This study indicates they do.
Scientists at the Center for Asthma Research at Vanderbilt University studied birth and medical records of more than 95,000 children and their mothers in Tennessee. They found that babies who reached 4 months old at the peak of the winter virus season were far more likely to develop bronchiolitis, a common lung infection that - when severe enough to require medical attention - is associated with a four- to fivefold increased risk of childhood asthma.
About 300 million people have asthma worldwide, and 255,000 die from it.
"We know viruses are triggers or can exacerbate asthma; the question is how important are they in causing asthma itself," said Dr. Stuart Abramson, an associate professor of pediatrics and immunology at Baylor College of Medicine who was not involved with the study. "But this study was a very comprehensive analysis."
The study's documented 30 percent asthma risk increase for autumn babies is "population-wise . . . a pretty substantial number," he said.
Asthma isn't the only disease that has been linked with the season of birth. It is widely believed that there is an interaction between genetic predisposition to diseases and environmental exposures, including some that vary seasonally, such as circulating viruses.
Various studies have shown patients with bipolar disorder and schizophrenia are slightly more likely to have been born in winter or early spring when compared with the general population. The same is true of Alzheimer's patients, adults with brain tumors, and children with congenital valvular heart disease.
So does it make sense for parents to plan pregnancies with such information in mind?
No, at least not in the case of winter viruses and asthma, says Dr. Tina Hartert, director of the Center for Asthma Research at Vanderbilt University, the principal investigator in the study.
"Timing of conception, I guess, is not a very practical thing. Everyone's asked me, but I think the last thing you need to give these poor parents in this day and age is one more thing to worry about," said Hartert. "We're not going there. It's too premature to do that."
And even if parents did plan births with the asthma risk in mind, it would not guarantee their child remained free of the disease, given that family background as well as exposure to allergens such as dust mites also play a role in determining which children develop asthma.
"The risk of progressing from bronchiolitis to asthma is almost certainly influenced by genetic factors," Hartert wrote. "The decades-old debate has been, is bronchiolitis just a marker of the child who has all the genetic machinery to go on and develop asthma? Or is it causal? Or both? We think it's probably both."
The study would seem to contradict the so-called hygiene hypothesis behind asthma, which has shown that early exposure in the first six months of life to some bacteria, pets, and other children in day care may provide immunological protection against the disease.
However, Abramson says, severe respiratory virus infections early in life can physically damage the lungs, leading to childhood asthma.
Unlike other organs, babies' lungs are not fully developed at birth. Lungs continue to develop until children are 2 years old - leaving the organ particularly susceptible to damage.
Compounding the danger is the fact that a baby loses its mother's antibodies, transferred to the baby through the placenta, after three months of age and a child doesn't have much of his or her own until six months of age. Babies born in the fall are likely to reach that window of vulnerability just as cold and flu season peaks.
Unfortunately, protecting babies against winter virus infection is nearly impossible. A mother's antibodies are passed through breast milk, but not in sufficient amounts to prevent infections. Seventy percent of infants are infected with the highly contagious respiratory syncytial virus, or RSV, in their first 12 months of life. It can cause everything from a mild cold to pneumonia. What's more, the peak of winter virus season is hard to predict; it varies by up to 10 weeks from year to year.
Hartert is studying a medication already available to prevent RSV in premature babies. That medication, Synagis, made by
At about $1,000 per Synagis injection, a course of treatment can cost as much as $6,000. Meanwhile, several vaccines are in development.
Hartert said her next step is to see whether preventing RSV infections prevents asthma. She is studying babies born in autumn who received RSV prophylaxis to see whether the drug prevents the onset of childhood asthma by age 5 or 6.
"I've heard this kind of theme before. It makes sense," said Dr. Wanda Phipatanakul, an allergy and asthma specialist at Children's Hospital in Boston who was not involved in the study. "It's a lot of great work on a very important disease."
Childhood asthma deaths are rare, but the disease can hinder normal child activity. It is one of the leading causes of school absenteeism, and the third-leading cause of hospitalization among children.
The study appears in the first December issue of the American Journal of Respiratory and Critical Care Medicine, a publication of the American Thoracic Society.
Leigh Hopper Oberholzer can be reached at leighhop@gmail.com.
