http://bmj.com/cgi/content/full/323/7313/592/c
Owen Dyer London
The results of the world’s largest asthma study seem to discount the notion
that environmental allergens could account for the huge variation in asthma
rates across Europe.
The research, published in the September issue of the European
Respiratory Journal (2001;18:598-611), identifies no single cause for the
much higher rates of the disease in some countries but links paracetamol use
with increased susceptibility to asthma.
The European community respiratory health survey was a cross sectional study
of atopy, asthma, and other chronic respiratory disorders that was carried out
in 22 countries around the world (though mostly in Europe) during the early
1990s and that involved some 140 000 participants.
Its most startling finding was the uneven geographical distribution of these
respiratory symptoms. There was a fourfold difference in the prevalence of
nasal allergy between the most and least affected countries, a sixfold
difference in the prevalence of current asthma, and an eightfold difference in
the prevalence of wheeze.
Britain, Ireland, Australia, New Zealand, and the United States had the most
asthma, while low rates were found in Iceland, Norway, Spain, Germany, Italy,
Algeria, India, and the former Warsaw Pact countries.
A strong genetic association was found in both asthma and atopic
sensitisation. Overall, children whose mothers had asthma were at 3.2 times the
average risk of developing the disease, and the odds ratio was 2.9 if the
father had asthma. If both parents had the disease, the risk was seven times
normal. Study of segregation models indicated that at least one major gene for
asthma exists but that no single gene model can account for the disease. Having
a large number of siblings seems to reduce the risk of asthma and of allergy.
Allergic sensitisation accounted for only 13% of the overall variation in
bronchial responsiveness. House dust mites and cat allergens were the most frequent
allergic causes of airway constriction. Occupational causes were blamed in
5-10% of cases. Use of gas stoves was associated with a mildly increased
incidence of asthma in British and French women, but not in men or children in
these countries, and not in other countries, where different gases may be used.
The connection between asthma and paracetamol was first proposed in a
previous analysis of the respiratory health survey’s data, which found that the
prevalence of wheeze in 13-14 year olds increased by 0.5% per gram of drug
taken per person per year. The authors of that study proposed a mechanism
whereby paracetamol strips reduced glutathione from the airway, shifting the
balance of oxidant and antioxidant and causing inflammation. Other researchers
have postulated that an unexplained protective effect of aspirin is reduced in
countries where there is a preference for paracetamol.
The paracetamol researchers noted that asthma and allergy symptoms were most
prevalent in countries with per capita sales averaging over 20mg a year. Since
these were mostly anglophone countries, apart from Scandinavia and France, they
attempted to control for the "anglophone effect," and found that much
of the association between paracetamol and asthma vanished, although there was
still a clear association with eczema in adolescents.
Peter Burney, a public health scientist at King’s College, London and one of
the authors of this month’s paper, said it was difficult to separate the
anglophone effect from paracetamol use because such a large proportion of the
paracetamol sold worldwide was consumed by anglophones. "What’s needed is
research specifically tailored to answer this question," he said.
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