Getting rid of dust mites—worthwhile?

Health and medicine explained.
April 25 2008 12:19 PM

Your Health This Week

Getting rid of dust mites—worthwhile?

Dermatophagoides pteronyssinus, or a dust mite. Click image to expand.
Dermatophagoides pteronyssinus, or a dust mite

This week, Dr. Sydney Spiesel discusses whether to get rid of dust mites, how the flu migrates, and what the risks are when taking psychiatric medication while pregnant.

Asthma and dust mites

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Question: Asthma, now the most common chronic disease among children, has been increasing in prevalence worldwide for more than 50 years. All doctors know the common allergens that trigger the condition and the formulas universally recommended by the experts to clear the air. Throw the cat out the window. (OK, folks, lighten up—not literally!) Banish smokers to the back porch. Rip out carpets and damp mop the bare floors. And get rid of pesky dust mites —tiny insectlike creatures found in pillows, comforters, mattresses, and carpets whose corpses and poop collect in house dust. So, does decreasing exposure to dust mites actually decrease the risk of asthma?

New study: A recent review puts together the results of 54 studies, conducted with about 3,000 patients allergic to dust mites, and raises serious questions about the value of the dust mite wars. In each of the studies, patients were randomly assigned to either use or not use one of the standard methods of dust mite riddance. Then they were compared, using a variety of standard tests, to determine whether their asthma improved.

Findings: None of the dust mite attacks made any difference in asthma symptoms. In short, these expert-recommended, often expensive methods didn't benefit patients.

Explanation: Why should that be? As the authors point out, it's implausible to think that completely removing an allergen wouldn't decrease the allergic symptoms it causes. So, most likely, the recommended methods don't remove the dust mites. In fact (as I've pointed out before), some old and forgotten research has shown that the methods we use to fight off dust mites sometimes cause them to be fruitful and multiply.

Conclusion: There's another lesson to be learned from this study. The new watchword of medicine is evidence-based—the idea that the treatments we prescribe should follow from solid scientific research. Yet these findings should give us pause as we confront recommendations, like the recent "U.S. Guidelines for the Diagnosis and Management of Asthma," which gives us an expert but wrong assurance that solid evidence supports their recommendations. Including dust mite abatement.

Watch Dr. Spiesel's new video segment from Slate V:

Solving the mystery of the migrating flu

Question: Recent research on the movement of influenza viruses around the world addresses a mystery: Where do new epidemic strains of influenza come from?

Background: Influenza is an exceptionally successful parasite because it is inherently unstable. The genetic material of this virus is single-stranded RNA, which breaks and mutates relatively easily and for which there are few of the elegant repair enzymes that keep our own double-stranded DNA in good order. In addition, a mechanism called re-assortment mixes and remixes the eight gene segments in each influenza virus particle, constantly producing novel genetic combinations, by drawing genetic material from viruses infecting different species, like bird flu. As soon as our bodies catch on to the exact nature of the invading flu and respond by making protective antibodies, the wily virus mutates into a new form just different enough that our antibodies won't help for the next epidemic.

Prediction: To stay a step ahead of mutation, influenza experts must predict a year in advance the way the virus is likely to change. They alter the annual vaccine formula accordingly, to protect against the next wave of infection. Since influenza causes about 500,000 deaths a year worldwide, accurate prediction matters. Usually—and to my mind almost miraculously—the experts get it right. This year's flu epidemic, however, was an exception. Two out of three of the components of the vaccine didn't match the strains that actually arrived, and so the vaccine was only about 40 percent effective.

New findings: The new research ought to help prevent such an occurrence in the future. By closely studying the evolutionary pattern of the subtle year-to-year changes in about 13,000 influenza A viruses in worldwide circulation in the past five years, the authors show where new strains of virus originate (tropical East and Southeast Asia) and how they spread (first to Australia and Pacific islands, then to Europe and North America, and later to South America). The genetic changes in the virus first in seen in East and Southeast Asia would later show up in places like Australia or America, probably spread by strong travel and trade connections, and last of all in South America, where trade connections to Asia are weaker.

Conclusion: Now that we understand this pattern of distribution, we know exactly where to look for the influenza virus strains that next year's vaccine most likely should guard against. In addition, knowing just where to look for emerging strains will make us better able to capture quickly the occasional, truly dangerous shifts in influenza virus that lead to devastating worldwide epidemics—the kind of outbreak likely if the virulent avian-flu strain makes the leap to human-to-human transmission.

Taking psych meds while pregnant

Question: In the United States every year, at least 500,000 women with a psychiatric illness become pregnant. A substantial fraction of them, perhaps one-third, continue taking medication directed at their condition. Some of these medicines are known, or suspected, to cause harm in pregnancy. How should doctors decide whether to treat a mother with drugs that are potentially harmful to the fetus she is carrying? Which of the psychiatric drugs are safe? And when in pregnancy is it OK to take them? Which can be used by women who are breast-feeding?

Weighing risks: A recent publication (subscription required) of the American College of Obstetricians and Gynecologists weighs the risks of the drugs with the risks of forgoing them. Mothers who go off their meds may not comply as well with prenatal care or may become more likely to drink, smoke, or use other drugs. Once the child is born, a mother with an untreated emotional illness (or whose illness has worsened if not treated during pregnancy) might have trouble sustaining a deep attachment to her child or might give inattentive and unfocused care.

Advice: The paper offers some excellent guidance, for example in charting the increased risk of fetal anomalies from mood-stabilizing medications like Depakote and Tegretol, even though the American Academy of Pediatrics lists the use of these drugs as compatible with pregnancy. Similarly, lithium (used to treat bipolar illness) is associated with a slightly increased risk of congenital heart malformations, as is the antidepressant Paxil. As the paper points out, if the mother's illness is severe enough and other drugs don't work, these somewhat risky medicines may still be the best choice. We don't know the answer to other questions. For instance, might exposure in utero to these medicines cause subtle neurological or behavioral consequences later in life?

Recommendations: The authors find that it's better to use a single medication, even in a slightly higher dose, than a combination. And they note that as a general rule, the most risky time during pregnancy for drug exposure is the third through eighth weeks of gestation. Most valuable, perhaps, is the authors' point that adequate care for the mother often best serves the needs of the fetus.

Sydney Spiesel is a pediatrician in Woodbridge, Conn., and clinical professor of pediatrics at Yale University's School of Medicine.