Genetic evidence for worms preventing diseases such as asthma

Genetic mutations that protect against worm infection may make individuals more susceptible to asthma, a team of scientists studying the inhabitants of a rural fishing village in Conde, Brazil, have found.  Kathleen Barnes, an anthropologist and genetics expert at Johns Hopkins University in Baltimore, Maryland, US, and her colleagues have been studying the relationship between one particular parasitic worm, Schistosoma mansoni, and the incidence of asthma. This worm is common in the region, with up to 85% of locals exposed to it.

Studies have shown that in areas where parasitic worms are endemic, individuals that produce the most worm specific antibodies are the most resistant to worm infection. The most common is an antibody called IgE which triggers inflammation. Barnes’ research has shown that resistant is 30% heritable.

The researchers administered deworming medication to the residents and saw a rise in allergy and asthma rates – from which was hypothesised that the same mutations that help individuals fend off parasitic worms might make them susceptible to asthma and allergies, because if the individual produces a lot of IgE in response to a worm, they might produce a lot of IgE in response to an allergen.

Using blood collected from 850 locals, the team tested for antibodies to the worm to gauge immune response and searched stool samples for evidence of how heavily the individuals were infected. They also focussed on the gene IL-33, the gene linked to inflammation that is overexpressed in people with asthma. They found that five of the genetic markers were associated with worm resistance in the locals of Conde, and these five markers were the exact same markers as were associated with asthma. The researchers have found that the relationship appears to be inverse: a gene variant that conferred risk to asthma confers protection against worm infection.

One explanation is that natural selection favours genetic mutations with a strong resistance to worm infection, which might also increase the risk of other diseases such as asthma – but Barnes says they will have to investigate further to find the gene mutation responsible.

Asthma is becoming more common in developed and developing countries, and many think that the hygiene hypothesis goes some way to explaining why this is – lack of exposure to parasites in childhood doesn’t allow the immune system to strengthen sufficiently, in which case it overreacts when it comes into contact with benign allergens.

Source: American Association for the Advancement of Science


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First Published in Febuary 2011

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