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Parasite May Fend Off Asthma

Ethiopians with hookworm show less sensitivity to dust mites in British study

THURSDAY, Nov. 1, 2001 (HealthDayNews) -- It's axiomatic that parasites don't pay their bills, but a new study shows that blood-sucking hookworms may at least leave a tip, in the form of reduced sensitivity to asthma-causing dust mites.

British researchers say people infested with the tiny worms are about half as likely as those free of the parasites to suffer asthma and related breathing attacks. The study, which appears in the Nov. 3 issue of The Lancet, is the latest to show that certain organisms appear to ward off the lung disorder, most likely through their self-protective ability to suppress the immune system.

Led by Dr. John Britton, from the University of Nottingham, the researchers compared asthma rates in almost 12,900 Ethiopians; 9,800 of them lived in towns and the remainder lived in the countryside.

People who reported wheezing attacks in the previous year were twice as likely to have hookworms in their feces as those who said they breathed freely. Protection from asthma seemed a bit stronger in people with more severe hookworm infection, the researchers say.

Exposure to dust mites, a known asthma risk, significantly increased wheezing, but the effect was much greater among those living in urban areas. And among those living in the country who had significant hookworm infestation, there was no link between wheezing and dust mites -- suggesting that the protective effect of the worms muzzles the aggravation of the mites.

"There's something happening in the lung that we don't understand yet, possibly driven by the parasites that traverse the lung," says study co-author David Pritchard, an immuno-parasitologist at the University of Nottingham.

Even after controlling for hookworm exposure and other breathing risk factors, asthma was about three times more common in the city dwellers than in the rural residents, the researchers say. The reason for the disparity isn't clear, but might have something to do with the burning of fossil fuels, particularly kerosene, in urban areas, the scientists say.

Pritchard says scientists working to eradicate hookworm in the developing world should consider the latest findings. Vaccine researchers are now trying to come up with a way to immunize people against the parasites, but doing so too effectively might lead to more asthma, he says.

Hookworms are a leading cause of disease in both women and children in the developing world, causing anemia, mental and physical retardation, premature birth and a host of other problems. The World Health Organization estimates that 44 million pregnant women carry the worms. Drugs can destroy the parasites, but re-infection is common in areas of high infestation.

Yet "it may be possible that worm burden of a tolerable level may have some beneficial effect," says Pritchard, who admits that view is an anathema in the vaccine field.

Pritchard and his colleagues are now trying to find the exact means by which hookworm infection reduces allergy to dust mites. If they can isolate a chemical the worms use to suppress the local immune response, that might serve as a useful therapy against asthma, he adds.

Dr. Peter Hotez, principal investigator on the Hookworm Vaccine Initiative, adamantly opposes the notion of anything but a full-frontal attack on the pest.

"It's the second most important parasite in humans next to malaria," says Hotez, chairman of the department of microbiology and tropical medicine at George Washington University in Washington, D.C..

What's more, there's evidence that hookworms and related parasites may in fact provoke asthma. Dutch scientists in the mid-1990s showed that a similar worm, called toxocara , exacerbated breathing problems among people in the Netherlands.

And toxocara is common here, too, says Hotez, who recently conducted a study that found 51 percent of Hispanic children in Bridgeport, Conn., were infected with the worms. "We think it's an emerging cause of asthma in the United States," he says.

What To Do

An estimated 15 million Americans suffer from asthma, including 5 million children. Although easily controlled with medication, the disorder can be deadly.

For more on asthma, check out the American Lung Association.

To learn more about ways to help prevent asthma flare-ups, visit the American Academy of Allergy, Asthma & Immunology.

To find out more about the hookworms, check out the Hookworm Vaccine Initiative.

SOURCES: Interviews with David Pritchard, Ph.D., professor of parasite immunology, University of Nottingham, England; Peter Hotez, M.D., Ph.D., professor and chair, department of microbiology and tropical medicine, George Washington University Medical Center, Washington, D.C.; Nov. 3, 2001, The Lancet
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