D3.223 - Crossroads of Venom and Bite: A case of Horsefly - Wasp Cross - reactive Anaphylaxis

The horsefly (order Diptera, family Tabanidae) is a hematophagous insect capable of inducing local and, in rare cases, systemic reactions up to anaphylaxis. Three allergens have been identified from the salivary glands of the horsefly: Tab y 1 (apyrase), Tab y 2 (hyaluronidase), and Tab y 5 (antigen 5), showing partial cross-reactivity with Hymenoptera venom allergens. No specific immunotherapy is currently available for systemic reactions caused by Diptera species. This report describes a rare case of dual insect-induced anaphylaxis and explores the potential protective role of venom immunotherapy (VIT).

A 58-year-old male was evaluated for recurrent anaphylaxis at Allergy department. The first episode occurred after a horsefly bite seven years ago, and the second followed a sting by the Vespula species. Both reactions were classified as grade IV according to Mueller. Diagnostic evaluation included skin prick tests and intradermal testing with Vespula spp., Polistes spp., and Apis mellifera venom extracts. Serum specific IgE, total IgE, and baseline tryptase levels were also measured.

Specific allergy work-up demonstrated sensitization to Vespula spp. (0.1 μg/mL) and Apis mellifera (1 μg/mL). Specific IgE levels were positive for horsefly (0.93 kU/L, class 2), Apis mellifera (0.98 kU/L, class 2), Vespula (3.61 kU/L, class 3), and Polistes (3.66 kU/L, class 3). Total IgE was 60.7 kU/L, and serum tryptase was 5.5 μg/L, both within normal limits. Based on these findings, venom immunotherapy targeting Vespula spp. was initiated. During the second year of treatment, the patient experienced aa accidental horsefly bite without systemic symptoms.

Venom immunotherapy to Vespula venom may have contributed to protection against horsefly allergens, possibly due to shared or homologous allergenic epitopes. The findings highlight the clinical relevance of cross-sensitization between Diptera and Hymenoptera species and underscore the need for improved diagnostic tools and targeted therapeutic strategies for Diptera allergies.