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Day Care May Boost Immunity
To Asthma
by Susan Okie, Washington Post, 08/24/00
Infants who go to day-care
centers or who have older siblings are less likely than those who don't to
develop asthma later in childhood, perhaps because they are exposed to more
germs, researchers say.
The new findings provide strong
support for the provocative but increasingly accepted theory that exposure
to microbes early in life may help the immune system mature properly,
lowering the risk of asthma and allergies.
Small families, good sanitation
and widespread antibiotic use — all of which reduce childhood exposure
to bacteria and viruses — may be part of the reason for the dramatic
increase in asthma and allergies seen in the United States and other
industrialized countries over the past three decades.
"I think it's a fascinating
area. It's probably not just specific to asthma," said Robert A. Wood, an
associate professor of pediatrics at Johns Hopkins University School of
Medicine. In the future, he added, if additional research clarifies the link
between early exposures and fine-tuning of the immune system, "it's
extremely promising that you'd be able to expose a newborn to the right mix
of safe bacteria and potentially turn the allergy system way down or
completely off."
In the new study, babies who
entered day care before the age of six months had only 40 percent the risk
of asthma seen in those who were not exposed to day care or older siblings.
An estimated 17.3 million
Americans suffer from asthma, a chronic and often progressive disorder in
which small air passages in the lungs become temporarily blocked, causing
difficulty breathing. The figure has more than doubled since 1980, when
there were 6.7 million asthmatics. Asthma, which often develops during
childhood, is one of the most common chronic illnesses in the United States,
causing almost 500,000 hospitalizations and more than 5,000 deaths annually
and costing an estimated $14.5 billion per year.
Asthma experts, who have been
puzzled by the sharp rise in the disease's frequency, said the large,
long-running study by Arizona researchers helps explain previous, seemingly
discrepant findings about the effects of day care on the disorder. The new
study found that infants who were exposed to other children had more
wheezing episodes during their preschool years, chiefly because they
suffered more colds and other infections. But they were less likely to
develop asthma later on, perhaps because early experience with bacteria and
viruses favorably influenced their immune systems. Wheezing is common in
children before the age of 3, and most infants and toddlers who wheeze do
not go on to become asthmatic.
Researchers at the University of
Arizona's Respiratory Sciences Center studied 1,035 children from birth to
age 13, collecting detailed information on the number of older siblings, day
care exposure, frequency of wheezing, asthma episodes diagnosed by a doctor
and other factors. The children received skin tests and blood tests for
allergies at the ages of 6 and 11.
In addition to the reduced risk
of asthma for babies who were in day care before the age of six months, the
study found that those who entered day care in the second six months of life
also had a somewhat lower risk, said pediatrician Thomas M. Ball, one of the
authors. Starting day care after a child was 1 year old did not reduce the
asthma risk, the researchers report in today's issue of the New England
Journal of Medicine.
Having older siblings also
appeared to be protective – and the more, the better. For instance, among
children with two older siblings, the asthma risk was 70 percent of that
seen in children with none; among those with three or more, it was 60
percent.
Researchers do not know what it
is about having siblings or being in day care that reduces a baby's later
risk of asthma, although they suspect that exposure to bacteria —
possibly including common types that don't cause disease — may play a
role. Other studies have found that living on a farm with animals or in a
house with a dog reduces children's chances of developing asthma, said
Sandra C. Christiansen of Scripps Research Institute in La Jolla, Calif.,
who wrote a commentary accompanying the study.
Exposure in the early months of
life appears to be key, Christiansen added. Cells of the immune system
called helper T cells come in two types, and type 2 has been implicated in
asthma and allergies. All babies are born with an immune system that makes
mainly type 2. Soon after birth, the system normally switches over to
producing predominantly type 1. People whose immune systems don't make this
change, and who continue to produce mostly type 2 cells, appear to be more
likely to develop asthma and allergies. Researchers theorize that being
exposed to common microbes may prime an infant's immune system to make the
switch.
The sharp increase in asthma has
been fairly uniform throughout the U.S. population, although blacks and
members of some Hispanic groups have slightly higher rates of the disorder
than whites, said Stephen C. Redd of the federal Centers for Disease Control
and Prevention. He said members of ethnic minorities have much higher rates
of death and hospitalization from asthma than whites, probably reflecting
poorer health care.
Factors such as air pollution
and allergens (such as cockroaches and dust mites) don't explain the
increase, although both can trigger attacks in people who have the disorder,
said Anne L. Wright, a research professor of pediatrics at the University of
Arizona and a co-author of the new study.
The study's implications seem to
contradict the conventional wisdom that babies should be kept away from sick
children who might infect them. Ball said that if he sees a healthy infant
whose parents have a history of asthma or serious allergies, "I would inform
them that they might be better served by exposing that child to other
children early in life. It's up to them how they want to do that."
Hopkins' Wood disagreed. "In
terms of going out and seeking a germ factory — a day-care center
where everyone has a green nose — I'm not sure that's the right thing
to do," he said. "But . . . reassurance that exposure to normal childhood
illnesses is not a bad thing, and could even have some beneficial effects in
the long run, is a good concept."
Although the number of children
in day care has increased significantly in the United States in recent
years, researchers speculate that it hasn't been enough to offset other
factors. Only day care attendance in the first six months of life appears to
be highly protective, and only about 7 percent of U.S. infants under a year
old are in day care.
Christiansen, in her commentary,
suggests that the study's results may help assuage the consciences of
working parents. "For those of us who share the furtive guilt of having left
marginally ill toddlers at day care," she wrote, "these findings . . . offer
a sense of relief."
-reproduced from The Washington Post,
08/24/00
The study is Siblings, day-care
attendance, and the risk of asthma and wheezing during childhood,
in The New England Journal of Medicine, August 24, 2000. The abstract from
the Journal is
available online.
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