Parasites may protect against allergies
NEW YORK | Tue Dec 7, 2010 3:18pm EST
NEW YORK (Reuters Health) - Children infected with hookworm or other intestinal parasites may be less likely than uninfected children to have allergies, a new research review finds.
The study, published in the journal Allergy, gives some support to the idea that our increasingly germ-free surroundings may be contributing to a worldwide increase in allergies and asthma in recent decades -- a theory known as "the hygiene hypothesis."
It's thought that exposure to viruses and other pathogens early in life may help nudge the immune system toward a normal infection-fighting mode, and away from a tendency to overreact to benign substances, which is the basis of allergies.
Studies so far, however, have come to conflicting conclusions about the hygiene hypothesis. Some, for example, have linked early attendance at daycare, where kids swap germs freely, to a lower risk of developing allergies and asthma -- which would seem to support the hygiene theory. But others have found no such protective effect.
For this latest analysis, Dr. Johanna Feary and colleagues at the University of Nottingham in the UK analyzed 21 previous studies on the association between parasitic infection and children's allergy risk.
The research, which included almost 29,000 people, most of them children, was conducted in South America, Africa, Cuba, Vietnam and Turkey -- regions where intestinal parasite infections are common. The majority of subjects were infected by the geohelminth family of parasites -- which includes hookworm, roundworm and whipworm.
When Feary's team pooled the results of all the studies, they found that participants with any current parasitic infection were 31 percent less likely than others to display reactions when exposed to common allergens like dust mite or cockroach proteins in a skin test.
Such skin responses indicate that the immune system is reacting to the proteins, though that does not necessarily mean the person has a full-blown allergy that might bring on sneezing, congestion or other symptoms with greater exposure to the substance.
The findings also do not prove that intestinal parasites, themselves, are protective against allergies. Further research on the relationship between the two is still needed, according to Feary and her colleagues.
Since allergies are common in developed countries, and on the rise in developing ones, the researchers note, it is increasingly important to understand how environment might be affecting allergy risk worldwide.
Curbing parasitic infections in the developing world remains an important goal, Feary and her colleagues point out. Nonetheless, they add, these findings raise the question of whether eradicating such infections could have the unintended effect of boosting allergy rates in countries where health services are already overstretched.
SOURCE: link.reuters.com/cyb98q Allergy, online November 18, 2010.
Atopy and current intestinal parasite infection: a systematic review and meta-analysis
1. J. Feary, 2. J. Britton, 3. J. Leonardi-Bee
Article first published online: 18 NOV 2010
© 2010 John Wiley & Sons A/S
Keywords: * allergen skin sensitization; * atopy; * helminth; * IgE; * parasite
To cite this article: Feary J, Britton J, Leonardi-Bee J. Atopy and current intestinal parasite infection: a systematic review and meta-analysis.
Allergy 2010; DOI: 10.1111/j.1398-9995.2010.02512.x00: 00–00.
Background: The rate of increase in prevalence of allergic disease in some countries implies environmental exposures may be important etiological factors. Our aim was to undertake a systematic review and meta-analysis of epidemiological studies to quantify the association between current intestinal parasite infection and the presence of atopy and to determine whether this relation is species specific.
Methods: We searched MEDLINE, EMBASE, LILIACS and CAB Abstracts (to March 2009); reviews; and reference lists from publications. No language restrictions were applied. We included studies that measured current parasite infection using direct fecal microscopy and defined atopy as allergen skin sensitization or presence of specific IgE. We estimated pooled odds ratios (OR) and 95% confidence intervals (95% CI) using data extracted from published papers using random-effects model.
Results: Twenty-one studies met our inclusion criteria. Current parasite infection was associated with a reduced risk of allergen skin sensitization OR 0.69 (95% CI 0.60–0.79; P < 0.01). When we restricted our analyses to current geohelminth infection, the size of effect remained similar OR 0.68 (95% CI 0.60–0.76; P < 0.01). In species-specific analysis, a consistent protective effect was found for infection with Ascaris lumbricoides, Tricuris trichuria, hookworm and Schistosomiasis. There were insufficient data to pool results for atopy defined by the presence of specific IgE.
Conclusion: Intestinal parasite infection appears to protect against allergic sensitization. Work should continue to identify the mechanisms of this effect and means of harnessing these to reduce the global burden of allergic disease.
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