Around 6.8 million children in the US have asthma. The exact cause of the condition is unknown, but new research published in The Lancet Respiratory Medicine finds that children who receive antibiotics before the age of 1 year appear to have an increased risk of asthma later in life.

Infant antibiotic use has been linked to increased asthma risk before. A 2010 study published in the American Journal of Epidemiology found that when babies are given antibiotics, their risk of developing asthma before the age of 6 years increases by 50%.

This and similar studies suggest that antibiotics interfere with infants' beneficial gut bacteria, which cause a child to have an undeveloped immune system and become more susceptible to asthma.

But this latest study, conducted by researchers from the University of Manchester in the UK, is believed to be the first to show that it is not the antibiotics themselves that increase asthma risk for children, but impaired viral immunity and certain genetic variants at an early age.

The research team, led by Prof. Adnan Custovic, analyzed data from the Manchester Asthma and Allergy Study, which followed more than 1,000 children from birth until the age of 11.

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Researchers discovered two genes near the chromosome 17 region that were linked to increased risk of antibiotic prescription in early life.

The researchers looked at participants' medical records to determine their use of antibiotics, episodes of wheezing and flare-ups of asthma. At ages 3, 5, 8 and 11 years, the children underwent skin reaction tests to show whether they were hypersensitive to allergens.

At age 11, the researchers collected blood samples from children who had received at least one course of antibiotics or no antibiotics in their first year of life. Their immune system responses to rhinovirus (responsible for the common cold) and respiratory syncytial virus (the cause of respiratory tract infections) were compared, as were responses to the bacteria Haemophilus influenzae and Streptococcus pneumoniae.

In addition, the children were subject to genetic testing to see if there were any links between genetic variants and early life antibiotic prescription.

Two genes identified that increase need for antibiotics in early life

The team found that infants who had wheezing and were treated with antibiotics before their first birthday were more than twice as likely to develop asthma flare-ups or severe wheezing, and had lower introduction of cytokines (immune cells that help fight infection), compared with children who were not treated with antibiotics before the age of 1.

The researchers note that they found no association between early antibiotic prescription and increased risk of allergic reactions.

From genetic testing, however, the team discovered two genes in the chromosome 17 region - known as 17q21 - that were linked to increased risk of antibiotic prescription in early life.

Explaining what this finding means, Prof. Custovic says:

"We speculate that hidden factors which increase the likelihood of both antibiotic prescription in early life and subsequent asthma are an increased susceptibility to viral infections due to impaired antiviral immunity and genetic variants on 17q21.

However, further studies will be needed to confirm that the impaired immunity was present at the time of the early childhood respiratory symptoms and predated antibiotic prescribing, rather than as a consequence of the antibiotics."

In an editorial linked to the study, Prof. Julie Crane and Dr. Kriistin Wickens from Otago University in New Zealand, agree that further investigation is needed to confirm these findings. But they question whether a randomized trial of antibiotics would be "feasible."

"This trial would need to be large and would be ethically difficult, but perhaps not impossible, at least in terms of restricted versus usual antibiotic prescriptions," they write.

"In view of the concerns over the rapidly waning efficacy of antibiotics, partly from overprescription, the fact that many are prescribed for disorders that they cannot benefit and the disquiet many parents express about overmedicating their children, the proposal for a randomized control trial is perhaps worthy of some consideration."

Elsewhere on Medical News Today, as part of National Asthma and Allergy Awareness Month, we ask whether asthma is seen as a "minor" condition, look at how the condition can be kept under control, and ask what is contributing to increasing asthma prevalence and death rates.