By Ben Hirschler

LONDON, Aug 25 (Reuters) - Childhood allergies are on the rise around
the world, including in many developing countries where asthma, eczema
and hay fever are emerging as important public health problems,
scientists said on Friday.

Asthma, in particular, is responsible for millions of children missing
school, ending up in hospital or even dying.

The underlying cause of the condition is inflammation of the airways
but just what triggers the problem and why some people develop asthma
and others do not is still poorly understood.

What is clear, however, is that more and more young children are
suffering allergic disorders, with the prevalence of all allergies
increasing notably in the past decade.

Researchers who carried out the International Study of Asthma and
Allergies in Childhood in 1991 repeated the survey in 2002 and 2003 and
found widespread increases in prevalence in 56 countries, especially
among young children.

The study -- details of which were published in the latest edition of
the Lancet medical journal -- surveyed parents of 193,000 children aged
6-7 years and 305,000 children aged 13-14 years in countries as diverse
as South Africa, Brazil, Iran, Canada and Sweden.

The increases were greatest for eczema in the younger age group and for
hay fever in both age groups. In the older age group, however, where
prevalence of asthma had been high, there were some signs of decreases.

Nonetheless, in Britain -- one of the developed countries with the
worst allergy epidemics -- asthma prevalence went up to 20.9 from 18.4
percent, hay fever increased to 10.1 from 9.8 percent and eczema to 16
from 13 percent between 1991 and 2003.
Professor Innes Asher of the University of Auckland, New Zealand, lead
author of the study, said the rise in prevalence in many countries was
"concerning", especially as allergies often occurred in large
population centres.

"Although changes in mean annual prevalence to the order of 0.5 percent
might sound small, such changes could have substantial pubic health
implications, especially since the increases took place most commonly
in heavily populated countries," he said.

Experts say a host of factors including air pollution, diet, lifestyle
and exposure to bacteria in early life are likely to be linked to the
rise in asthma and other allergies, but the reasons are likely to vary
from place to place.

Given the diversity of the condition, the Lancet said in a editorial
that asthma was in fact unlikely to be a single disease and the term
should be abolished altogether.

A separate study published in the journal reinforced the benefits of
combination medicines in tackling asthma.

Dr Klaus Rabe of Leiden University in the Netherlands said a 12-month
study involving more than 3,000 people showed that giving asthma
patients a top-up dose of AstraZeneca's (AZN.L: Quote, Profile,
Research) two-in-one drug Symbicort to relieve their symptoms was more
effective than giving them a single drug top-up.

All patients in the study were already on a maintenance dose of
Symbicort, which combines a corticosteroid with a long-acting beta
agonist.