Mollusk tolerance in crustacean‐allergic children: Unraveling pediatric shellfish allergy cross‐reactivity patterns

IgE-mediated shellfish allergy (SA) is a variable cause of pediatric food allergy, ranging in prevalence from 1.3% in the United States (US)1 to 2%–5% in Vietnam,2 according to eating habits. A recently published study from Hong Kong reported shellfish as the second most common cause of pediatric anaphylaxis after peanut.3 Taxonomically, shellfish include the phyla Crustacea and Mollusca. The latter further comprise Gastropoda, Cephalopoda, and Bivalvia. Mollusk allergy is less common than crustacean allergy, with a prevalence of 0.5% in children.1 Cross-sensitization to shellfish is commonly reported, partly due to the sequence homology of its major allergen, tropomyosin.4 Tropomyosin is a crucial protein in animal muscle contraction; 91% of its epitopes are conserved among crustaceans, but the IgE epitope-region conservation in mollusks is less than 20%.5 In fact, according to in vitro studies, tropomyosin homology is higher among crustaceans (up to 98%)6 than between crustaceans and mollusks (56%–58%).7 Furthermore, amino acid sequence similarity for mollusk tropomyosin is generally high (85%–91% for gastropods, 91%–100% for cephalopods, 70%–100% for bivalves, and 68%–100% among different mollusks).7 Due to the notion of tropomyosin homology, it is currently standard practice for shellfish-allergic patients to avoid all shellfish in the absence of evidence of tolerance, leading to extensive elimination diets. In fact, 14% of crustacean-allergic patients report having a mollusk allergy,8 while 60% of cephalopod-allergic patients and 40% of bivalve-allergic patients are allergic to crustaceans.9 However, cross-sensitization does not always equate to clinical cross-reactivity and, unfortunately, current recommendations are mainly based on in vitro or survey studies. Studies systematically performing oral food challenges (OFCs) to various shellfish would be very informative but are lacking, partly due to feasibility concerns. Even more so in pediatrics, it is essential to limit the child's diet as little as possible, as these food classes may be a healthy source of protein. The aim of this study was to evaluate mollusk tolerance with OFC in children with a history of crustacean allergy, to contribute to the diagnostic and therapeutic management of children with IgE-mediated shellfish allergy. We performed a retrospective, observational study on children with IgE-mediated crustacean allergy who attended our tertiary pediatric allergy unit in Madrid, Spain, between January 1, 2018 and August 1, 2023. Diagnosis was defined as clinical history of reaction with positive skin tests (Leti Pharma, Barcelona, Spain; major wheal diameter ≥3 mm) and/or specific IgE (sIgE, ≥0.35 kU/L) to crustaceans. As part of the diagnostic algorithm, subjects were investigated for mollusk sensitization with skin tests and sIgE testing and interviewed about mollusk tolerance (bivalves and cephalopods). Those who showed mollusk sensitization and had no history of previous exposure or current tolerance underwent open-label oral food challenges (OFCs) for either one or both classes of mollusks. OFCs were performed according to our Hospital's protocol specified in Appendix S1. Exclusion criteria included home reintroduction of such foods, abnormal medical evaluation, uncontrolled asthma, and ongoing oral antihistamine or corticosteroid therapy. IBM Statistical Package for Social Science software (SPSS, Version 28.0, Chicago, Illinois, USA) was used to perform statistical analyses. Data were reported as medians and interquartile ranges (IQR) for continuous variables, and as percentages for categorical variables. The Mann–Whitney U test and Chi-Square test were used for analyzing differences in continuous and categorical variables, respectively. A p-value <.05 was considered statistically significant. Written informed consent was obtained from the children's parents for all procedures performed, and the study was approved by the Ethical Committee of our hospital. Thirty-six crustacean-allergic children were eligible for mollusk OFC. Their clinical, demographic, and allergic characteristics are shown in Table 1. No significant differences were observed in clinical and immunological variables according to their age (Table S1). Fifteen patients (41.7%) without a history of previous exposure to mollusks were eligible for OFC with both mollusk classes (one bivalve and one cephalopod). Eleven children (30.5%) reported current tolerance to one class of mollusks, while ten children (27.8%) reported a known allergy to one class of mollusks, and in these cases, OFC with the other class of mollusks was attempted (Table S2). Before the OFC, patients were re-interviewed and underwent prick-by-prick (PbP) tests for mollusks. Three patients did not undergo OFC (2 for squid and 1 for mussel) because of a history of recent reaction and increased wheal size at the PbP test. Forty-eight boiled mollusk OFCs were performed (23 squid, 18 mussel, 3 octopus, 2 mussel and clams, 1 cuttlefish, 1 clam). Four OFCs (8.3%) yielded a positive result, three with squid and one with octopus: one patient complained of oral pruritus, one of abdominal pain, and two experienced both symptoms (Table S3). None developed objective symptoms or required medical treatment. When assessing final clinical mollusk tolerance, 23 patients (63.9%) were classified as tolerant, nine patients as partially tolerant (25.0%), and four as non-tolerant (11.1%). Tolerant individuals were defined by tolerance to both classes of mollusks, confirmed by two negative OFCs, or one negative OFC plus clinical history of tolerance of the other class (i.e., current consumption without reactions); partially tolerant individuals were defined as tolerating only one class of mollusks, confirmed by at least one negative OFC; non-tolerant individuals were defined as allergic to both classes of mollusks, confirmed by two positive OFCs to different mollusk classes or a positive OFC to one mollusk class and clinical history of allergy to the other class confirmed by SPT and/or sIgE (Figure 1). Fully tolerant individuals showed significantly lower median sIgE levels for shrimp, squid, clam, and mussel when compared to partially tolerant or non-tolerant individuals (p < .05), approaching statistical significance for Pen a 1 tropomyosin (p = .073), possibly also due to the small sample size (Table 2). However, this difference was not observed when comparing fully tolerant with partially tolerant patients. While substantial information regarding shellfish co-sensitization exist in the literature, published data on clinical cross-reactivity proved by OFC are scarce. Scala et al. recently reported that among 247 patients with self-reported adverse reactions to shrimp, 47.8% had an adverse reaction to mollusks10; positive skin tests and high sIgE levels could identify mollusk reactors, but these were defined based on clinical history and not proven by OFC. Similarly, a study by Vidal et al. showed that shrimp sIgE and Pen a 1 sIgE levels were able to identify mollusk reactors among patients with a history of crustacean anaphylaxis.11 In our cohort, mollusk tolerance in crustacean allergic patients was assessed by performing 48 OFCs, and Pen a 1 was almost statistically higher in the partially tolerant/non-tolerant patients; when available, a complete sIgE study should be performed since it may help to more precisely distinguish reactive from non-reactive patients. The largest pediatric cohort providing results about challenge-proven mollusk tolerance comprised 42 OFC, with a positivity rate of 14%,12 which is slightly above that of our cohort (8.3%), perhaps because our patients had been carefully selected for challenge. Nevertheless, even patients with highly positive skin test and sIgE levels were challenged and they successfully passed the OFC, even though these were more likely non-tolerant or partially tolerant rather than fully tolerant. Vice versa, patients without history of previous mollusk reaction or with history of tolerance to the other mollusk class usually resulted completely tolerant. Therefore, highly positive allergy tests shouldn't prevent us from considering OFC, even more so in the absence of a history of previous reaction. Our study is among the few that assessed mollusk tolerance by means of a standardized OFC protocol in cases of shellfish allergy diagnosis. Moreover, in our pediatric clinic OFCs are proposed to the patients also based on the family's and, when possible, the child's preference, reflecting the real-world component of this study. This study has some inherent limitations, including its monocentric retrospective design, which renders the available data heterogeneous. Furthermore, the small sample size reflects the impossibility of performing OFCs to a full regime during the SARS-CoV-2 pandemic. In conclusion, few patients were non-tolerant, and all patients who reacted during OFC developed mild subjective symptoms, none requiring treatment, confirming the safety of this procedure in selected patients. Mollusk tolerance in pediatric crustacean-allergic patients seems high but must be proven by OFCs to avoid unnecessary dietary restrictions. Studies with prospective design and larger cohorts are needed to confirm this finding. Benedetta Pessina: Conceptualization; data curation; formal analysis; methodology; writing – original draft; project administration. Paula Cabrera-Freitag: Data curation; validation; writing – review and editing. Ana Entrala: Data curation; validation; writing – review and editing. Sonsoles Infante: Data curation; validation; writing – review and editing. Alberto Alvarez-Perea: Supervision; validation; writing – review and editing; writing – original draft; conceptualization; visualization. This research did not receive any specific grant from funding agencies in the public, commercial, or non-profit sectors. The authors have no conflicts of interest to declare. Data S1. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.