Fennel, fresh
Summary
Fennel is a herb widely used in cooking, which can be eaten fresh or as a seed. Clinical reports of fennel allergy are rare and limited to case reports, with the majority being in oral allergy syndrome (OAS) patients who are typically pollen-sensitized, especially to birch and mugwort. Cross reactivity has also been observed to peach and celery allergens.
Epidemiology
Worldwide distribution
Allergy to spices is infrequent, accounting for about 6.4% of all food allergies in adults. Accordingly, mono-allergy to fennel is unusual, even among those sensitized to other Apiaceae.
A large clinical study was carried out to determine the prevalence of Apiaceae based on carrot, celery, and fennel hypersensitivity in patients with birch pollen allergy. A total of 196 birch pollen-hypersensitive patients with OAS caused by different vegetable foods were examined, of which only one patient had Apiaceae allergy alone.
Another study addressed allergic sensitization to coriander, caraway, celery or fennel by skin prick testing in 589 patients, where 32% of children and 23% of adults were positive. However, only 3 patients (0.5%) were thought to have a clinical allergy to fennel.
Clinical Relevance
Allergic rhinitis
Clinical reports of fennel allergy are rare, with the majority being birch and mugwort pollen-sensitized patients. This phenomenon is part of the general pollen-food allergies). There are adult case reports of such highlighting the cross-reactive nature of fennel. For example, a 20-year-old man with a history of mild grass pollen allergy experienced anaphylaxis (generalized urticaria, dysphonia, angioedema of the lips, palmar-plantar itch) after ingesting raw fennel, which resolved following the administration of systemic steroids and oral cetirizine. Another 41-year-old man with seasonal rhinitis experienced episodes of OAS and dyspnea after ingestion of raw and cooked fennel. Both were positive to fennel after skin prick testing. Similarly, A 34-year-old woman with a pollen allergy reported OAS with the ingestion of both fennel and cucumber.
Cases of allergy to fennel are rarely reported in children. One case study has been described in an 11-year old boy with seasonal allergic rhinitis following contact with toothpaste containing fennel. He presented with sneezing, nasal congestion, sinus pain, vomiting and abdominal pain, difficulty breathing, and palpitations. His serum specific IgE test result to fennel was positive as well as a skin prick test to fennel-flavored toothpaste.
Other topics
One case of allergic contact dermatitis to raw fennel (along with parsnip and carrot) has been reported in a 51-year old man. Contact with fennel caused intermittent rashes which resolved naturally after 3 days.
Cross-Reactivity
Cross reactivity has been demonstrated to birch (Bet v 1) and peach (Pru p 3). Homologous protein allergens may also exist with mugwort pollens, accounting for the mugwort-celery-spice syndrome. A 60 kDa allergen, highly homologous to a celery allergen (Api g 5), has been recognized in fennel by patients’ IgE. Inhibition experiments showed a high degree of cross-reactivity between this fennel allergen and a homologous mugwort pollen allergen. The cross-reactivity seems to be clinically relevant as there is improvement of a patient’s OAS to fennel, cucumber, and melon after pollen-specific injection immunotherapy with a product containing grass, mugwort (50%) and ragweed (50%) allergens. Fennel allergens also show homology and cross-reactivity with other Apiaceae family members, such as celery (Api g 5).
References
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- Asero R. Fennel, cucumber, and melon allergy successfully treated with pollen-specific injection immunotherapy. Annals of Allergy, Asthma & Immunology. 2000;84(4):460-2.
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- Borghesan F, Mistrello G, Amato S, Giuffrida MG, Villalta D, Asero R. Mugwort-fennel-allergy-syndrome associated with sensitization to an allergen homologous to Api g 5. Eur Ann Allergy Clin Immunol. 2013;45(4):130-7.