Owen Dyer London
The results of the world抯 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抯 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抯 College, London
and one of the authors of this month抯 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抯 needed is research specifically tailored to
answer this question," he said.
http://www.bmj.com/cgi/content/full/323/7313/592/c