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PEDIATRICS Vol. 108 No. 2 August 2001, p. e33
Received Jun 27, 2000; accepted Apr 10, 2001.
From the David Hide Asthma and Allergy Research Centre, St.
Mary's Hospital, Newport, Isle of Wight, United Kingdom.
Background. Atopy is defined as the
genetic propensity to develop immunoglobulin E antibodies in response
to exposure to allergens and assessed by skin prick test responses to
common allergens. Although it is generally agreed that atopy is an
important risk factor for allergic diseases such as asthma, rhinitis,
and eczema, the extent to which atopy accounts for these diseases is
controversial.
Objective. We aim to describe the prevalence of
sensitization to common allergens and investigate the degree of
association of atopy (as defined by positive skin prick test to 1 or
more common allergens) to asthma, rhinitis, and eczema in a birth
cohort at the age of 4 years.
Methods. A birth cohort of 1456 children was recruited
over a 14-month period (1989-1990). These children have been seen
previously at 1 and 2 years of age. At 4 years, 1218 children were
reviewed and an interview was administered or postal questionnaire was completed for the presence of allergic diseases (asthma, rhinitis, and
eczema). Additionally, in 981 children, skin prick tests with a battery
of 12 common allergens were performed. Allergens were house dust mite
(Dermatophagoides pteronyssimus), grass pollen mix, cat,
dog, Alternaria alternata, Cladosporium
herbarum, cow's milk, hen's egg, soya, cod, wheat, and
peanut. A mean wheal diameter of at least 3 mm greater than the
negative control was taken as positive. This analysis is confined to
the 981 (67% of the original population) who also had skin prick tests
to the standard battery. Results. Children who were skin prick-tested at 4 years
were similar in most characteristics to the rest of the population,
except that they had a higher prevalence of allergic disease. Allergic disorders (asthma, rhinitis, and eczema) were present in 276 (28.1%) of 981. One hundred ninety-two (19.6%) children were atopic (positive reaction to 1 or more allergens). Sensitization to inhalant allergens was relatively common (19.2%) as compared with food allergens (3.5%).
House dust mite (11.9%), grass pollen (7.8%), and cat (5.8%) were
the most common positive reactions. A test to the 4 most common
allergens (house dust mite, grass pollen, cat, and A
alternata) could detect 94% of the atopic children.
Sensitization to the 4 most common allergens was strongly associated
with the presence of allergic disorders. There was a graded effect with the potent allergens, such as house dust mite, having the greatest impact. For example, 50% of children sensitized to house dust mite had
asthma as opposed to 44% sensitized to cat, 42% sensitized to grass
pollen, and 32% sensitized to A alternata. Overall,
68.4% of children sensitized to house dust mite had asthma, eczema, and/or rhinitis. The respective figures for grass pollen, cat, and
A alternata were 64.9%, 66.7%, and 57.4%. The
proportion of children sensitized to cat was not higher in households
with cat ownership (households with cats: 5.1% [19/374]; households
without cats: 6.2% [36/580]; not significant [NS]). Similarly, no
difference was seen in sensitization to dog in households with and
without dogs (households with dogs: 1.8% [5/282]; households without
dogs: 2.8% [19/673]; NS). Boys were atopic more often than girls at this age (male: 112 of 497 [22.5%] vs female: 80 of 484 [16.5%]; OR: 1.47, 95% CI: 1.07-2.02). Male preponderance was observed with
most allergens, but this was statistically significant only for house
dust mite (male: 75/497 [15.1%] vs female: 42/484 [8.7%]; OR:
1.87; CI: 1.25-2.79) and grass pollen (male: 51/497 [10.3%] vs
female: 26/484 [5.4%]; OR: 2.01; CI: 1.23-3.29). An independent effect of allergen sensitization on asthma was observed
only with house dust mite with an OR of 8.07 (CI: 4.60-14.14). The
highest independent risk for rhinitis was sensitization to grass pollen
(OR: 5.02; CI: 2.21-11.41), and for eczema, sensitization to peanut
(OR: 4.65; CI: 1.02-21.34). The majority of children (98/192) were sensitized to >1 allergen. A
graded effect was observed with the risk of allergic disease in the
child increasing with the number of positive skin prick test reactions.
This effect was consistent throughout the spectrum of allergic diseases
(asthma, eczema, and rhinitis). Nearly 80% of the children with
positive skin test reactions to 4 or more allergens had asthma, eczema,
and/or rhinitis compared with 20%, if they were nonatopic.The prevalence of atopy in asthmatic children was 44%. With an OR of
4.56, the population-attributable risk was calculated to be 35%.
Fifty-five percent of children with rhinitis were atopic, and the OR of
rhinitis was 5.85. Therefore, 46% of the cases of rhinitis could be
attributable to atopy. The population-attributable risk of atopy for
eczema was 32% (the prevalence of atopy in children with eczema: 43%;
and the OR for the development of eczema: 3.86).
Conclusion. Atopy is closely associated with asthma,
rhinitis, and eczema at 4 years of age, with a direct and linear
relationship. However, the proportion of cases of allergic disease
attributable to atopy is <50%. We propose a model for the development
of allergic disorders, where 30% to 40% of cases of allergic disease
(asthma, eczema, and rhinitis) in early childhood are attributable to
atopy and 60% to 70% of cases could be accounted for by organ-based
and other factors.
2 tests were used to test the univariate association
between each allergic disease and positive skin test. Multiple logistic regression analysis was performed to obtain the adjusted odds ratios
(ORs) and 95% confidence intervals (CIs) for the independent effect of
sensitization to each allergen on allergic disease, adjusting for the
effect of sensitization to other allergens. To ascertain how much of
allergic disease is attributable to atopy, we estimated the
population-attributable risk. This was calculated with the formula:
P(R 
1) where R is the OR for the allergic disease under
consideration and P is the proportion of atopy in children with that
disease.
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