9/23/03 Asthma and parasites: new insightsSummaryIt is well known that many helminth infections induce a strong Th2 cytokine secretion profile resulting in high levels of IgE and eosinophilia. However, several studies have shown that communities in rural Africa where such worm infections are common, extensive and frequently life-long have a very low prevalence of asthma and immediate (IgE-mediated) sensitivity to inhalent allergens This apparent paradox was discussed by Arruda of the University of Sao Paulo in Brazil. He pointed out that several mechanisms have been postulated to underlie this low incidence of respiratory allergy and asthma individuals with certain worm infections,. These hypotheses include 1) strong stimulation of protection of IL-10, a cytokine with potent anti-inflammatory effects or 2) production of antibodies against allergens in the IgG4 subclass a Th2-dependent Ig isotype that is not associated with clinical allergic manifestations. However, certain helminth infections, particularly ascaris, appear to provoke atopic responses. In studies from China and Brazil, current ascaris infection was a strong, independent risk factor for allergic skin test reactivity and wheezing. It may be that helminth infections characterized by passage of larvae of worms such as ascaris and hookworm through the lungs are more likely to induce allergic respiratory reactions. Another possibility is that allergens in certain helminths may induce IgE responses which cross-react with aeroallergens present in agents such as dust mites and cockroaches. One such "shared antigen" would be tropomyosin, a pan-allergen present not only in ascaris but also in cockroaches, dust mites, and even in crustaceans such as shrimp. Reference Respir Med 2003;97:762-9 Editor’s Comments These comments deal with a subject of ongoing interest and investigation. It has been shown that African native children who move to urban areas subsequently have a higher prevalence of asthma and respiratory allergies than their tribal relatives who continue to live in a relatively rural environment. However, these differences may be due in part to environmental exposures in the low income urban settings and not just less exposure to helminth infection. On the other hand, a study from South America showed that the incidence of asthma increased significantly in a population of heavily worm-infected children after their parasitic infections were cleared with intensive therapy. Does this mean that the worm infection somehow prevented asthma with a loss of such protection following anti-helminth therapy? More investigation is obviously needed.
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