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|Route of Exposure||Inhalation|
|Latin Name||Corylus avellana|
|Synonyms||Cobnut, European filbert, European hazel, Filbert, Giant filbert; Hazelnut|
Hazel (Corylus avellana) is a wind-pollinated tree that attains a height between 4 to 8 meters and can sustain up to 80-90 years. The leaves of this tree are rounded with a length of around 6-12 cm. It is prevalent in forests, natural habitats, open areas, and mountainous regions. Exposure to hazel tree pollen may induce allergic conditions (allergic rhinitis, allergic conjunctivitis, asthma, and atopic dermatitis) among sensitized individuals. Cor a 1, Cor a 6, and Cor a 10 have been reported as potential allergens from hazel tree pollen. Hazel pollen exhibits a high degree of cross-reactivity with species from the Betulaceae family (alder, birch, and hornbeam), the Oleaceae family (ash, olive, lilac, and privet), and saffron.
Hazel (Corylus avellana) is a deciduous, wind-pollinated, monoecious shrub (small tree) that grows to a height of 4 to 8 meters and a lifespan up to 80 - 90 years. The bark of this tree is greyish with large whitish patches. The leaves are rounded, hairy (both surface), double toothed edge, and 6 to 12 cm in length. This tree’s flowering usually begins during early spring, even before the leaves appear (1). In Poland, the flowering season falls between January end and late March (2). Hazel trees play an essential role in biocenosis by providing a shaded environment and decomposing litter, thus promoting soil formation. The tree also provides nuts as a natural food source for rodents and birds (3).
Hazel is found in natural habitats, mountainous regions, open areas, and forests (3). The January and February temperatures (like in Poland) are favorable for the development of flowers. Weather factors such as humidity, temperature, sunlight exposure, precipitation, and speed of wind are primarily known to impact hazel pollen concentrations. It is capable of developing in areas with ample sunlight, which are periodically shaded. It can grow on various soil compositions like dry, moist and fertile, stony clay, humid-mineral, or sandy clay (3). Usually rich, fertile soil with neutral or low acidity and area with high temperature and moderate climatic conditions are suitable for its growth. In countries such as Turkey, more than 700mm of rainfall with an average temperature of 13-16º C is ideal for this tree’s cultivation (1).
Hazel (Corylus avellana) belongs to the Betulaceae Family and order Fagales. Birch (Betula), alder (alnus), and hornbeam (Carpinus) are other important members of the same family (2). Pollen from species of the genus Corylus (Corylus avellana L., C. colurna L., and C. maxima Mill) are reported to possess familiar morphology (3).
|Taxonomic tree of Hazel (4)|
Hazel tree produces smooth, spherical (polar view: triangular; equatorial: elliptical) pollen grains of size 26 to 31 µm (polar axis) and 25 to 31 µm (equatorial axis). Their shell is usually thin, rough with large protrusions on the surface. These pollen grains comprise three pores of diameter ranging between 2 to 3 µm (5). Cor a 1, Cor a 6 and Cor a 10 allergenic proteins from this pollen have been identified (6), which may induce allergic symptoms such as allergic rhinitis, conjunctivitis (2), asthma (7), and skin reactions (8) in sensitive individuals.
A constant surge in the number of allergies due to tree pollen in many countries is evident. Studies have revealed that about 10 to 15% population of Poland and around 15 % of people in Norway suffer from pollinosis. The major contributors to pollinosis in the northern and eastern regions of Europe are alder, hazel, and birch trees (9).
A study was performed in Canada to analyze the association between tree pollen and patients' hospitalization due to aggravated asthma. The study revealed a trivial (< 2%) but statistically significant (p<0.05) correlation between hazel pollen (along with other species like alder, birch, ash, and poplar) and hospitalization of patients with severe asthma (10).
A four-year study (2002 - 2005) in Croatia revealed 34% of the average total pollen count was from the Betulaceae family species pollen, and Corylus sp. (like hazel) pollen accounted for 7% of the total pollen count. Among 864 sensitized patients, hazel pollen was reported as one of the critical triggers of respiratory allergy-related symptoms (11).
In a study conducted in 10 Canadian cities, Corylus (Hazelnut) along with Alnus (alder) and Betula (birch) tree pollens were responsible for a 1.45% increase in the risk of hospitalization due to asthma (10).
A Poland-based study on 625 atopic individuals (with atopic dermatitis, allergic rhinitis, and allergic conjunctivitis) reported 71 patients (11.36%) as skin prick test (SPT) positive towards Corylus sp. (hazel) (2).
A study demonstrated significant relation between the severity of atopic dermatitis (AD) with the sensitization level towards aeroallergens (like hazel pollen), specifically among pediatric patients with AD. The severity of this disease further depends on the extent of the breakdown in skin barrier function (2).
Hazel trees can be found in Europe, North Africa, Asia Minor, and North America (1). Daily hazel pollen concentrations can be affected significantly due to changing weather conditions. In Poland, hazel pollen is considered one of the primary contributors to airborne allergy during the initiation of the growing season. Elevated pollen concentration in the air many a time surpasses the threshold levels, which further manifest allergic symptoms. Hazel and alder pollen is the first pollen grains to emerge in Poland's atmosphere (3).
The routes of hazel tree pollen exposure are through inhalation and skin contact (2, 3).
Among hazel pollen allergic individuals, atopic dermatitis symptoms are majorly induced through direct skin contact rather than inhalation (2).
C. avellana (Hazel) pollen is capable of inducing allergic reactions at a concentration level of 20 to 30 grains per m3 per day (5). Several pollen species (birch, poplar, hazel, oak, and Artemisia) sensitive patients suffering from allergic rhinitis show symptoms such as congestion, runny nose, itching and sneezing (12). These chronic allergies cause physical weakness and reduce mental concentration. To overcome these issues, patients need to undertake several measures such as changing their lifestyle or occupation, following a specific diet, and avoiding allergens, continuing life-long symptomatic therapy, including immunotherapy (13).
In a study conducted on 52 patients exhibiting allergic rhinitis (hay fever), 50% (26/52) were allergic to several tree pollens, like hazel, birch, oak, poplar, Artemisia (12).
A Central Europe based study was conducted on 625 atopic individuals suspected of allergy towards airborne pollen. The results reported 14.4% (90/625) of patients were sensitized to tree pollen allergen, and 11.36% (71/625) revealed positive SPT for Corylus sp. (hazel) pollen. Among these hazel sensitized patients, 53% (38/71) had a history of atopic dermatitis, 37% (26/71) with disseminated eczematous lesions, and 10% (7/71) had rhinitis and conjunctivitis (2).
A study was conducted on 15-year records of allergenic pollens associated with 1311 allergic patients with asthma in Thessaloniki, Greece. An average yearly concentration of 3% was reported for Corylus spp. (hazel) pollen. 9.6% (126/1311) sensitive individuals exhibited positive skin sensitivity towards hazel pollen (7).
According to a study, 53% of patients exhibiting positive SPT towards hazel pollen allergens also demonstrated AD correlated symptoms (2).
A study was conducted between 2018-2019 in the Czech Republic on 81 patients (aged ≥ 14 years) with atopic dermatitis. Positive sensitization to hazel pollen allergen (rCor a 1.0101) was observed in 45.7% (37/81) of these patients (8).
A 62-years-old Turkish female working on the farm was exposed to hazelnut leaves and complained about intermittent fever, respiratory distress, and cough for the last 20 years. Patterns of chronic exogenous allergic alveolitis with predominant CD8 cell was revealed through Broncho-alveolar lavage (BAL) testing. Serum analysis confirmed elevated titers of IgG antibodies against mold fungi derived from the culture of the hazel-nut husk (14).
For two years, an Italy-based controlled study was conducted on 36 patients exhibiting sensitivity to either grass and hazel or birch and grass allergen. The patients reported conditions like nasal, respiratory, and ocular symptoms. Pre-seasonal subcutaneous immunotherapy (SIT) with grass combined with non-injective sublingual immunotherapy (SLIT) with hazel/birch pollen extract in sensitized patients exhibited enhanced tolerance during the pollen exposure season. A decline in the priming effect induced by hazel/birch on symptoms during grass pollen exposure was evident from the two-year observational follow-up (16).
Avoiding exposure to aeroallergen, such as hazel tree pollen, can prevent the development of allergic symptoms. Measures such as monitoring the hazel tree pollen concentrations in the air, spreading information to the public, planning activities according to the hazel pollen count in the air, and avoiding areas with high pollen counts can help the individuals (2).
One of the main measures to be taken is to cut down the amount of pollen entering the respiratory system. During pollen season, allergic patients are suggested to stay indoors or wear a mask outdoors to keep away from pollen allergens. In worsening conditions, patients may be advised to move to a non-pollen area (15).
Hazel tree pollen contains allergenic proteins, which include Cor a 1 (17kDa), Cor a 6 (35kDa), and Cor a 10 (70 kDa). (6).
In a European study, 65 patients were recruited with positive food challenges to hazelnut. The study revealed 96.9% and 50.7% of patients showed positive IgE - response towards 18kDa allergen (similar to Cor a 1) and 35 kDa allergen, respectively. The 18 kDa allergen has been identified as a homolog to Bet v 1, a major birch pollen allergen (17).
The following aeroallergens found in hazel pollen are listed in the IUIS database (6).
|Allergen||Biochemical Name||Molecular Weight (kDa)||Allergenicity|
|Cor a 1||Pathogenesis-related protein, PR-10, Bet v 1 family member||17||
|Cor a 6||Isoflavone reductase homolog||
|Cor a 10||Luminal binding protein||
kDa: kilodaltons, IgE: Immunoglobulin E
Cor a 1 is present as food and inhalant pollen allergen from hazelnut. Cor a 1 is a major hazel tree pollen allergen in the Betulaceae family and order Fagales. It shows evident cross-reactivity and homology to Bet v 1, a marker allergen for Fagales order. Studies have shown that Bet v 1 can inhibit IgE-reactivity of sera from patients sensitive to pollen from other species under order Fagales. (19).
There exists a high degree of cross-reactivity and homology between the major pollen allergens found in hazel and the allergens from species of the Betulaceae family, such as alder, birch (3) and hornbeam (2).
Cross-reactivity has also been documented between species of order Fagales (birch, alder, hazel, and beech) with species of the Oleaceae family (ash, olive, lilac, and privet) (19).
A study revealed that Cro s 2 (a 34-kDa recombinant saffron profilin) possesses 79% of amino acid sequence homology with profilin from Corylus avellana (hazel tree pollen) (20).
Author: Turacoz Healthcare Solutions
Reviewer: Dr. Fabio Iachetti
Last reviewed: January 2021