BABIES given antibiotics during their first six months are far more likely to develop asthma, according to a US study. The reason is not clear, but the team says antibiotics might be partly responsible for the steady rise in asthma cases.
In many western countries the incidence of asthma has nearly doubled in the past two decades, with up to 7 per cent of children suffering. There is no shortage of theories: everything from ozone to the gas produced when people pee in chlorinated pools has been blamed.
A handful of studies blame antibiotics, but most are suspect because they relied on the memories of parents years after events. Instead, Christine Johnson鈥檚 team at the Henry Ford Health System in Detroit followed 448 children from birth to age seven, regularly checking on their health.
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Nearly half of the children were given antibiotics in their first six months, a quarter had two courses and a fifth had three or more. At the end of the study, tests revealed that 21 of the children had developed allergic asthma, in which attacks are triggered by environmental factors.
Overall, children given antibiotics in their first half-year were 2.6 times as likely to develop allergic asthma, the team will tell a meeting of the European Respiratory Society this week. With broad-spectrum antibiotics, which kill a wide range of bacteria, the risk was far higher: children were 8.9 times as likely to suffer from asthma. The team also found that taking antibiotics in the first half-year of life increased the risk of allergies to things such as grass, cats and dogs by 50 per cent.
Known risk factors for asthma, such as having a mother with asthma or having fewer than two household pets, seemed to amplify the effect. A child who was given broad-spectrum antibiotics and whose family had no pets faced 11.5 times the risk of allergic asthma.
But even though Johnson鈥檚 study is better designed than previous ones, not everyone is persuaded. Since so many of the children being treated with antibiotics were ill with respiratory tract infections, it might be that the infections, not the antibiotics, triggered the asthma, cautions Wilfried Karmaus at Michigan State University in East Lansing. Or it may just be that children prone to asthma are more prone to respiratory infections. In his own unpublished study of 600 children from birth to age three, Karmaus also found a link between asthma and antibiotics 鈥� but it disappeared when these factors were allowed for.
But Johnson stands by her findings. Although the numbers get small, she claims the effect still holds true when all the children with respiratory tract infections are excluded. Even children treated for non-respiratory illnesses such as kidney infections had a higher risk of developing allergic asthma.
Johnson speculates that the drugs disrupt the developing immune system because they alter the bacterial communities in the gut. This might make it more difficult for a baby鈥檚 immune system to learn which bacteria are good and which are bad. 鈥淲e think it has to do with bacteria in the intestinal tract. Antibiotics tend to knock out some good guys,鈥� she says.
The findings fit in with a large body of evidence on the origin of childhood asthma, known as the hygiene hypothesis, says Thomas Kovesi at the Children鈥檚 Hospital of Eastern Ontario, Ottawa. 鈥淭he cleaner you make things, the greater the risk of allergy,鈥� he says. 鈥淭he immune system gets bored.鈥�