Allergies, Asthma and the Hygiene Hypothesis

If you're a conscientious parent who tries to keep everything and everyone
fastidiously clean, look out. Recent research indicates that you may want
to let your little grubmeisters enjoy their dirt. Cleanliness may not be
next to godliness after all.

In this century, there has been a huge increase in the numbers of children
with asthma and allergies worldwide, particularly in developed countries.
In the United States, asthma is the most common chronic disease of
childhood, affecting nearly 5 million children and adolescents. Reports
show a 75 percent increase in asthma from 1980 to 1994, with an astounding
160 percent rise in children under the age of four, according to the U.S.
Centers for Disease Control and Prevention.

In 1989, an epidemiologist writing in the British Medical Journal suggested
that the rise in allergies may be due to declining family sizes and higher
standards of cleanliness that provide young children with less exposure to
dust and germs. This, in turn, gives their still-developing immune systems
less practice in fighting off intruders. The result, as the theory goes, is
that the under-challenged immune system wants to be utilized, so it's
primed to see harmless substances like dust and pollen as dangerous
invaders, leading to allergies and asthma.

Recent studies show that there may be some truth to this theory, which has
come to be known in medical circles as the "hygiene hypothesis." Now, a
comprehensive study in the New England Journal of Medicine (August 2000)
has produced some fascinating results. In this study, scientists at the
University of Arizona followed a group of 1,035 children from infancy until
they were as old as 13. They found that in babies under six months, those
who were in daycare or who had older siblings had higher rates of illness,
including asthma and wheezing. But by the time these children were 6 to 13
years old, they were 40 percent less likely to suffer from asthma. That
means that children who are susceptible to asthma may be less likely to
develop a chronic form of the condition if their immune systems get a
workout at a very young age.

At what age is a child most likely to benefit from exposure to other kids?

"It does seem like there is a window of opportunity to impact the
developing immune system," says Thomas M. Ball, M.D., M.P.H., assistant
professor of Clinical Pediatrics at the University of Arizona and one of
the authors of the study. "It appears that this window occurs during the
first year of life."

"For a healthy infant, our study suggests that getting the exposure, and
the resultant infections, early in life might prevent the development of
allergies and asthma later," notes Ball.

Can bacteria be beneficial to your child?

Ball speculates that what actually protects children is contact with
endotoxins, which are toxic substances found in bacteria that separate from
the cell body when the bacteria dies. "In other words, maybe infants don't
need to be infected; maybe they just need to come into contact with
microbes and bacteria--specifically, endotoxins. After all, being in
daycare puts a child in contact with a lot of bacteria."

Along this line of thinking, Graham A.W. Rook, a researcher at the
University College London Medical School and an advocate of the hygiene
hypothesis, believes that instead of contact with disease-causing agents,
children need contact with innocuous microbes found in soil and untreated
water, particularly organisms called mycobacteria. The main problem may be
that kids are just too squeaky clean, says Rook.

Can animals actually help prevent allergies?

Interestingly, a child's exposure to animals appears to play a significant
role in whether or not she develops allergies and asthma. A May 2000 study
in the American Journal of Respiratory and Critical Care Medicine reported
that among almost 1,200 rural teenagers in Canada, those who grew up on
farms were 40 percent less likely to have asthma than their urban and
suburban counterparts.

"Recent studies have shown a protective effect from living on a farm,
particularly if there is contact with poultry, livestock, or domestic
animals," says Ball. "What do day care, siblings, and animals have in
common?" It sounds inelegant, but the answer is probably contact with trace
amounts of feces, which are loaded with endotoxins, says Ball.

Can your house be too clean?

In the past, children grew up playing outdoors and slept with the windows
open. Now houses are better insulated, antibacterial cleansers abound, and
kids spend a lot more time indoors. Their exposure to the outdoors and all
the microorganisms living there is much more limited, says Dr. Pierre
Ernst, of McGill University Health Center in Montreal, who was a researcher
in the Canadian study.

For now, what appears clear is the relationship between exposure to
bacteria at an early age and decreased rates at an older age of allergies
and asthma. Researchers are examining this link, and in the process they
hope to unlock the secrets of rising allergy and asthma rates. Until then,
the hygiene hypothesis suggests that hypervigilance in the clean department
may do more harm than good. Isn't that the best news you've heard in a while?