Egg allergy: involvement of the early intestinal microbiota
According to the World Health Organization, food allergies are the fourth most common chronic disease worldwide. In France, they currently affect 3.5% of the general population, and almost 10% of children. In the same way as asthma, atopic dermatitis or rhinitis –other allergies which have multiplied in recent decades– they originate in a combination of environmental, societal and medical factors. Since the spectrum of allergic reactions ranges from simple redness (or erythema) to death, we need to take them very seriously. As a result of its link with the immune system, the study of the intestinal microbiota could provide insights and lead to preventive measures other than the exclusion of the allergen responsible or desensitization, when the latter approach is possible.
Egg allergy is the second most common food allergy in children, just behind cow’s milk allergy. It affects almost 10% of children with an allergy under three years old. Although variations in the early intestinal microbiota have already been associated with sensitization or the onset of allergies to certain foods in the scientific literature, what is the situation for egg allergies?
The very rapid rise in the incidence of food allergies, incompatible with the time-line of genetic evolution, forces researchers to broaden their thinking to better understand the processes at work. While allergies, as other diseases, have been re-examined in the light of the growing understanding of the microbiota in recent years, some studies have taken a new approach by focusing specifically on the link between the early intestinal microbiota and egg allergy11. The study of children aged between three and sixteen months at enrolment showed that 46% of them were allergic to eggs exclusively and 71% were sensitized to eggs. Their intestinal microbiota was characterized by stool sample analysis, supplemented by blood and skin tests to monitor the progression of their sensitization/ allergy. They were followed up regularly until the age of eight: most allergic children were no longer ill a few years later.
Intestinal microbiota of allergic children: surprising results… actually not so surprising
This long-term follow-up shows that the early intestinal microbiota of children with egg allergy is more diversified than that of the control group. At first sight, this is an astonishing discovery in view of what is usually said in the literature, in which an opposite theory is sometimes advanced with respect to other pathologies such as obesity. But comparable results have previously been documented for asthma and the respiratory microbiota12. Enough to alert the researchers, who know the common origins of the two allergies and who did not settle for microbial diversity alone to explain the role of the microbiota in the disease. The intestinal microbiota of children with egg allergy is moreover composed of families of bacteria distinct from that of healthy children, some (Lachnospiraceae and Streptococcaceae) being more abundant in the first group. Others, like lactic acid bacteria whose protective effects against allergy have already been observed in animals, were more abundant in the second group. Detailed genetic analyses also allowed researchers to note that certain bacteria present in the intestinal microbiota of the children with egg allergy modified the metabolism of purines–molecules present in the organism which are strongly involved in some biological reactions and already associated with peanut allergy in children13.
The role of resident bacteria still needs to be clarified
At the end of the eight-year study, the egg allergy had disappeared in 60% of the children initially affected. However the researchers did not note a significant difference in the early intestinal microbiota according to whether the allergy has ceased or persisted. This result remains to be confirmed by larger studies, which should also explore the role of resident bacteria. In any event, the discoveries already made open the way to a preventive or therapeutic strategy for this widespread allergy.
11 Fazlollahi M., Chun Y., Griechin A. et al. Early-life gut microbiome and egg allergy. Allergy. 2018;1–10
12 Huang YJ, Nelson CE, Brodie EL, et al. Airway microbiota and bronchial hyperresponsiveness in patients with suboptimally controlled asthma. J Allergy Clin Immunol. 2011;127:372-381. & Marri PR, Stern DA, Wright AL, Billheimer D, Martinez FD. Asthma associated differences in microbial composition of induced sputum. J Allergy Clin Immunol. 2013;131:346-352
13 Kong J, Chalcraft K, Mandur TS, et al. Comprehensive metabolomics identifies the alarmin uric acid as a critical signal for the induction of peanut allergy. Allergy. 2015;70:495-505