Thursday October 2 2008
Infections were recorded until the children were six
The common cold virus “could increase child’s risk of asthma ten-fold”, reports The Daily Telegraph. When a group of children (who had parents with asthma or other allergies) were followed from birth to six years, it was found that those “close to the age of three who develop wheezing with the virus have a 30-fold risk of becoming asthmatic by the time they turn six”, the newspaper says.
Although this study does demonstrate an association between wheezing during childhood colds and later asthma, it does not mean that the cold is the cause of asthma. It may simply be that people who are more likely to have asthma in later life are also more likely to have wheezing when they have a cold or other viral illness during early childhood. Asthma is a difficult condition to diagnose in children, and although wheeze is the most well-known symptom, it can present in many different ways. Many children who develop asthma do not go on to have asthma as adults. This study should not be taken to mean that common colds – which are unavoidable – cause asthma.
Where did the story come from?
Daniel Jackson and colleagues from the University of Wisconsin-Madison and Wisconsin State Laboratory of Hygiene carried out this research. The study was funded by the National Institute of Health. The study was published in the peer-reviewed medical journal, American Respiratory Critical Care Medicine.
What kind of scientific study was this?
This was a cohort study where the researchers aimed to investigate the relationship between specific childhood illnesses and early development of asthma.
A group of 259 children (born between 1998 and 2000) were recruited from birth and followed up at the ages of one, three and six as part of the Childhood Origins of Asthma (COAST) study. All had at least one parent who suffered from a respiratory allergy (determined using a positive allergen skin test) and/or had medically diagnosed asthma.
At regular clinic visits during the first year of life, samples of mucus from the nose and throat were taken, and these were analysed for a number of common childhood viruses. Samples were also taken during periods of respiratory illness (these were identified by parents who contacted a study coordinator). When the children were one and three years of age, the researchers measured the levels of a particular antibody (IgE) that is known to be associated with allergic reactions. At five years, skin-prick testing was performed for a number of common environmental and household allergens.
Episodes of ‘viral infection’ were defined as when a virus was detected in a mucus sample. If the child was suffering from symptoms, this was referred to as ‘viral illness’. In order to be considered a ‘wheezing respiratory illness’ during the first three years of life, one or more criteria had to be fulfilled:
- Wheeze diagnosed by a doctor.
- Prescription of bronchodilator medication.
- Specific diagnosis given of asthma (or exacerbation of), wheezing illness, bronchiolitis or reactive airways disease.
At the end of the sixth year ‘current asthma’ was diagnosed based on the documentation of one or more of the following over the previous year:
- Asthma diagnosed by a doctor.
- Use of a (physician-prescribed) bronchodilator for cough or wheeze.
- Use of daily inhaled steroids or other asthma control medication.
- Step-up plan of bronchodilator and inhaled corticosteroids during illness.
- Use of oral steroids during illness.
The researchers examined the relationship between asthma at six years and the cause of wheezing illness during the first three years of life, taking into account other confounding factors including parental asthma, exposure to passive smoking, animals in the house, etc.
What were the results of the study?
Wheezing respiratory illnesses were very common during the first three years of life, with 454 episodes documented in the whole study group. For 97% of these episodes, nasal samples were obtained. In 90% of the samples, viruses were detected, with rhinovirus (the cause of common cold) being by far the most common, identified in 48% of cases.
Respiratory syncytial virus (the common cause of bronchiolitis – an inflammatory airways infection which occurs in babies under one years old) was the second most common virus, occurring in 21% of samples.
In the 48 illnesses which involved multiple viral infection, rhinovirus was present in 60%. Children diagnosed with asthma had a significantly increasing number of rhinovirus infections with each year of life (one through to three) compared with children without asthma at six who had had far fewer infections and a significant decrease in number over the years.
Twenty-eight per cent of children had asthma (based on the defined criteria) by age six. Among these, 48% had intermittent asthma, 34% had mild persistent asthma and 18% had moderate persistent asthma.
Analyses were carried out on the risk of diagnosis of asthma and any link with rhinovirus infection or respiratory syncytial virus only, as these were the viruses most commonly identified. When compared with children who were not infected with either of these viruses, children who had wheezing illness in the first three years of life were 9.8 times more likely to have asthma diagnosed by age six if they had rhinovirus infection. They were 2.6 times more likely if it was respiratory syncytial virus infection; and 10 times more likely if it was infection with either rhinovirus or respiratory syncytial virus.
In the first year of life, wheezing illness with rhinovirus infection and allergen sensitivity both independently increased the risk of asthma at age six years (2.8 times and 3.6 times respectively). But for the third year of life, the risk of asthma was far greater if there was a wheezing illness with rhinovirus infection (25.6 times) compared with the risk from allergen sensitivity (3.4 times). Almost 90% of children who had a wheezing illness associated with rhinovirus in their third year of life had asthma diagnosed by age six.
Other non-viral factors significantly associated with asthma at age six were having older siblings in the house, and having food sensitivity during the first year of life.
What interpretations did the researchers draw from these results?
The authors conclude that of the community–acquired viral infections which cause wheezing in infancy and childhood, rhinovirus was the most significant predictor of the subsequent development of asthma at six years.
What does the NHS Knowledge Service make of this study?
Asthma has a wide variety of risk factors, both genetic and environmental, and these environmental risk factors include exposure to bacterial and viral infections. Therefore it is not surprising to find that those who already have some inherited disposition towards asthma and then develop a wheeze during a viral illness may be more likely to go on to develop asthma. Although this study demonstrates associations of the common cold with asthma, it does not mean that the cold is the cause of asthma. There are some points to consider:
- The group of children included in the study were already at higher risk. They were selected on the basis of having a parent or parents with either asthma or respiratory allergies. Therefore risk among this group (who may be more predisposed to developing asthma) cannot be considered to be representative of other groups.
- The size of the group was relatively small, and larger observational studies would be needed to provide confirmation of the results.
- The study has only considered asymptomatic infection and symptomatic wheezing illnesses in the community that did not require hospitalisation. Had more serious respiratory infections been considered, different viruses might have been detected and found to correlate with risk.
- Asthma at age six (diagnosed by fulfilling certain criteria over the past year of life) does not necessarily mean that the condition will persist into later childhood or adulthood.
The common cold is an unavoidable infection and most of us will suffer from repeated episodes during our lifetimes. It should also be noted that wheeze during an infective illness is extremely common in childhood, and it does not necessarily mean that a child has asthma or will develop asthma in the future. Asthma is always a difficult condition to diagnose in children. Although wheeze is the most well-known symptom, it can present in many different ways, and parents should be aware of other possibilities. For instance, sometimes a persistent nocturnal cough may be the only symptom.