Last month, newspaper headlines read that exposure to mouse urine and dander must be added to cockroach feces, dust mites, and air pollution as triggers for asthma attacks. A new Johns Hopkins University study found that common house mice may be a major contributor to asthma among inner-city children. In an eight-city study, researchers found that more than nine out of 10 tested homes had at least one room containing mouse allergens. On December 29, President Clinton justified imposing regulations that would remove 95 percent of the sulfur from diesel fuel and raise its price 6 to 10 cents per gallon on the grounds that "these measures will preserve our environment and protect thousands of children from the agony of asthma and other respiratory diseases."
The developed countries are in the midst of an asthma epidemic. Cases in rich countries have doubled in the last 20 years. Some 15 million Americans suffer from asthma, including 5 million children.
Yet a paradox remains that has been largely ignored in press accounts: Asthma attacks are increasing even as air pollutants and most other allergens have declined in the U.S. and other developed countries.
The EPA reports that particulates like soot and dust declined 22 percent between 1988 and 1995 and that sulfur dioxide concentrations have dropped by 60 percent since 1970. Yet people living in far more polluted cities in Eastern Europe have fewer cases of asthma than do people living in cleaner cities in Western Europe and the United States. "Air pollution may aggravate existing asthma but is not responsible for the asthma epidemic," concluded two Oxford University researchers in Science in 1997.
So if air quality is getting better in the West, why is there an epidemic of asthma? Researchers have uncovered an unlikely culprit as the cause of the asthma epidemic: cleanliness.
Last February, Italian researchers concluded in the British Medical Journal that a "semisterile diet may be at the root of the epidemic of allergic asthma …in developed countries." Dr. Paolo Matricardi and his colleagues compared asthmatic and non-asthmatic cadets in the Italian Air Force. They found that cadets exposed to food-borne infections such as Hepatitis A and Helicobacter pylori were less likely to suffer from respiratory allergies. Earlier studies found that people who had been exposed to intestinal parasites also seemed to be protected from asthma.
Last April, University of Aberdeen researchers in Scotland reported in the scientific journal Thorax that getting measles before age three and having two or more younger siblings protects against asthma. As every parent knows, children regularly bring home novel infections, so having several kids increases the chances that any one of them will infect the others.
Last May, Dr. Andy Liu at the National Jewish Medical and Research Center (NJMRC) found that "children who are allergen-sensitive have less environmental endotoxin in their homes." Endotoxin is part of the cell wall of common bacteria that finds its way into the air and dust when the bacteria die. The higher the exposure to endotoxin, the lower the chance that a child will develop asthma. "A little dust and dirt in the home may help prevent asthma later in life," concluded the NJMRC press release.
The common theme is that if you don't want to risk becoming asthmatic, suffer through some infections in childhood.
Although all of the details are not yet worked out, it seems that infections alter the balance between two types of immune system cells called helper T lymphocytes, TH1 and TH2. TH2 lymphocytes produce the immunoglobulin-E antibody (IgE), which play a role in the production of the histamines that lead to the itching, sneezing, and respiratory distress of asthma. The more TH2 cells, the more IgE -- and the more asthma. Just what the proper balance should be between the two types of helper T lymphocytes is not yet known.
Clearly, the conquest of infectious diseases, especially in children, has dramatically boosted life expectancy over the past 100 years; it is surely one of the great triumphs of modern medicine. However, this triumph has a downside: Our modern, clean, and relatively disease-free environment leads to increased susceptibility to asthma.
That's not the end of the matter, of course. For those people whose immune systems did not get the proper early training, pharmaceutical companies are working on promising therapies that would interrupt the action of IgE and thus prevent asthma attacks. In the meantime, reducing individuals' exposure to allergens such as cockroach feces and mouse dander will help.
"We must improve hygiene to reduce the impact of infectious diseases," concluded the Italian researcher Matricardi. "But at the same time, we must learn how to safely 'train' our immune system, especially during infancy, in order to prevent allergy."
Perhaps in the future, doctors will tell parents to administer tapeworms or measles to tune up their infants' immune systems for the long haul. An ounce of illness may well stave off a pound of problems later on.