Walnut Pollen
Summary
Walnut belongs to the Juglandaceae family which consists of 24 different species. Its growth prefers a warm and sheltered site and a long growing season. Soil pH 6 to 7.51 is optimal for growth. The Eastern United States is known as a native place for black walnuts and used for timber. Walnut pollen is moderately allergenic and sensitizes through inhalation. Walnut California, the western species can cause sensitization more than Black walnut. Pollens can cause allergy through inhalation while nuts can cause food. Walnut pollen allergy can induce symptoms such as asthma and rhinitis. Walnut pollens are moderately allergenic and may not lead to severe anaphylaxis. Pollens sensitization was found in young children with rhinitis symptoms. Walnut tree wood has been observed to create dust and can cause occupational allergic contact dermatitis. Prevention measures can be taken by avoiding pollen into the respiratory system during the pollen season. No pollen-related allergens from the Juglans Californica have been determined yet. Walnut pollen has allergen which functions like Bet v 2 (profilin). LTP allergen found in walnut tree pollen is different from the one found in walnut tree nuts. Cross-reactivity is reported between the species from Juglans Carya (Pecan) genus.
Epidemiology
Worldwide distribution
California walnuts are deemed to be western species and are a major cause of pollen allergic sensitization. A significant amount of pollen from trees such as walnut, birch, oak, elm, etc. was observed in a study in Tehran, Iran (1974) which was deemed to induce allergic reactions in sensitized individuals. Lewis and Imber suggested that, in the North temperature region, asthma, allergic rhinitis and allergic conjunctivitis are common symptoms following exposure to tree pollen such as from walnut, oak, elm, etc. An aerobiological survey conducted in Bitlis, Turkey reported atmospheric pollens from Juglans spp. and other tree species. In some patients, the allergic reaction can be due to pollen rather than the plant’s actual fruit or vegetable.
Route Of Exposure
Main
Airway (Inhalation) is the main route of exposure to pollen allergens. Walnut pollens cause inhalant allergies, and the nuts may cause food allergies.
Secondary
Food (Ingestion)
A cross-reactivity reaction usually occurs when patients are exposed to pollen grains after eating fruit or vegetables. The digestive system's allergenicity can enhance due to the release of small soluble allergenic fragments from poorly soluble conglomerate.
Clinical Relevance
A birth cohort study by Lovasi, O’Neil-Dunne et al, was performed for establishing the relationship of urban tree canopy and asthma, wheeze, rhinitis, and allergic sensitization to tree pollen in New York City. This study was positively correlated with asthma and allergic sensitization at the age of 7 years. A higher prevalence of allergic sensitization was found for tree pollen. The mixed tree pollen used for IgE testing included the walnut with other prevalent pollens. Mixed tree pollen sensitization was found to be 19%. Reported asthma diagnosis at age 7 years was found to be 32% while rhinitis at age 5 years was found in 2%.
Allergic rhinitis
The nuts from walnut trees consist of lipid transfer protein (LTP) allergen but to date similar, LTP allergen has not been found in pollen from walnut trees. Cross-reactivity of LTP appears only in food through ingestion, but not through pollens inhalation. Walnut pollens can cause inhalant allergy. Dermatitis, asthma, and rhinitis were found in Central American walnut (Juglans olanchana) due to dust but not through pollen. This pollen inhalation resulted in immediate bronchospasm.
Wong, Wilson et al. conducted a study in children to detect pollens that trigger early sensitization in young children suffering from symptoms such as allergic rhinitis. The study included 123 pediatric cases (age ≤ 8years). It was observed that more than 50% were sensitized to at least one aeroallergen like walnut. Among 2 to 3-year-old children, 75% (16 children) were sensitized to walnut. In the contrast, walnut didn't sensitize children from the age group of 4 to 7 years.
Asthma
A case report of asthma and rhinitis induced due to Central American walnut dust is reported in a 48-year-old man with disease severity. The intradermal test showed a negative result, but dust inhalation induced immediate bronchospasm response. De Roos et al conducted a retrospective study for tree and grass pollen sensitization which includes walnut species as one of the pollen allergen. The most prevalent exposure-response trend was observed for walnut, ornamental, ash, oak and sweetgum trees with asthma exacerbation.
Atopic dermatitis
Walnut tree wood has been observed to create dust and may cause occupational allergic contact dermatitis.
Prevention And Therapy
Prevention strategies
Prevention measures can be taken by avoiding pollen into the respiratory system during the pollen season. Allergic patients should stay indoors or wear masks outdoors to avoid exposure. Patients should move to live in a pollen-restricted area.
Cross-Reactivity
Cross-reactivity is reported between the species from genus Juglans and on a moderate level was observed with the genus Carya (Pecan). The walnut tree nut consists of a lipid transfer protein allergen (LTP).
But similar LTP allergen is present in walnut tree pollen has not been found yet. Cross-reactivity due to LTP allergens appears to be relevant only in foods ingested, not in inhaled pollens.
References
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