Oak Pollen
Summary
Oak (Quercus alba) tree is a wind-pollinated, deciduous, long-lived (up to hundreds of years), and slow-growing tree. The flowering season of this plant falls during spring. It is reported to produce large quantities of pollen. The Oak species are majorly found in deciduous forests of the Northern Hemisphere. The predominant species of the white oak group found in North America is Q. alba which is now also found in Europe, and certain parts of Asia and Africa. Exposure to oak pollen may induce symptoms such as allergic rhinitis, allergic rhino-conjunctivitis, and asthma in sensitive patients. The major allergen listed officially in the database of the World Health Organization/International Union of Immunological Studies Allergen Nomenclature Sub-Committee is Que a 1 (a Bet v 1 homolog, Pathogenesis-related protein). Studies have also identified calcium-binding protein and profilin allergenic molecules from this pollen. Various studies have shown significant cross-reactivity between oak pollen and pollen from other related tree species such as birch, alder, hornbeam, hazel, European chestnut, sawtooth oak, Mongolian oak as well as with pollen from unrelated tree species like ginkgo and grass species like timothy grass.
Epidemiology
Worldwide distribution
The oak and beech species from the Fagaceae family are known to be of allergenic significance, among others. Oak is capable of causing tree pollinosis, especially in areas with numerous oak tree population. The oak pollen allergy is of particular significance in Europe and North America. The prevalence of tree pollen allergy, due to species from the Fagales order (oak, hornbeam, hazel, alder, and birch), is determined to be 20% amongst the European allergic population.
In a study on sera from 102 individuals allergic to Fagales tree pollens, oak tree pollen sensitization was found to be 72% across different areas of Europe based on positive SPT. It was found to be 69% (n=38) in Austria, 76% (n=27) in France, 60% (n=19) in Sweden and 84% (n=18) in Switzerland. Also, the prevalence of sensitization to oak tree pollen was reported to be 14% among pollen-allergic patients in Spain while in Sweden, it was 70-80% among pollen-allergic patients especially in the spring season.
Allergy due to pollens from the oak species is also found to be of significance in Switzerland (Zurich), the Iberian peninsula, Turkey, South Africa (Cape Town), Florida (Tampa), Mexico city, Japan, and Korea.
A United States (US) based study identified oak as one of the heaviest pollinators along with maple, hemlock, birch, juniper, and pine species. In the same study, the most common skin prick test (SPT) reactions were found positive to oak along with other tree pollens like willow, ash, maple, beech, and birch.
A study was conducted on 371 allergic patients in New York City to analyze the sensitization rate to tree pollen. The results showed the highest prevalence (34.3%) of hypersensitivity towards oak tree pollens.
In Korea, in a study, the sensitization to white oak along with birch pollens among respiratory allergic patients was found to have increased from 6.7% in 1999 to 9.6% in 2008 owing to widespread oak trees in Korean forests. Further, another study’s evaluation on 12 allergic Korean patients found white oak sensitization in 83% based on measurement of serum specific immunoglobulin E (IgE).
Route Of Exposure
Main
A significant route of exposure for oak pollen is through inhalation.
Clinical Relevance
Oak allergy may induce respiratory symptoms such as asthma, allergic rhinitis, and conjunctivitis in sensitive individuals.
Allergic Rhinitis (AR) and Allergic Rhino-conjunctivitis (ARC)
A US-based study conducted on 562 military children (≤18 years) with rhinitis underwent SPT with an 8-test screening panel (various allergen mixes) (n=209) or a 51-aeroallergen standard panel (n=345). 8 patients were excluded from the study based on positive diluent or negative histamine control. 80.3% (277/345) of the patients showed positive SPT towards at least 1 aeroallergen. Out of these, 31% were reported to show positive SPT towards oak tree (mix of red, white, and Virginia live) extract allergen.
A study on 12 allergic Korean patients found white oak tree pollen sensitization in 10 of them in addition to sensitization to other allergens. Allergic conjunctivitis and allergic rhinitis were observed in 30% and 90% respectively of allergic patients having sensitization to white oak pollens.
Another study was conducted in Korea on 976 children from an urban and suburban region with or without ARC. Out of these, overall 19.3% (53/274) of children with allergic conjunctivitis reported positive SPT towards oak allergen.
Asthma
A study was conducted in 10 Canadian cities to examine the role of different tree pollen allergies and its association with increased risk of hospitalization due to aggravation from severe asthma. The results showed an increased risk of hospitalization (2.3%) for asthma due to Quercus (oak) tree pollen along with Castanea (chestnut) and Fagus (beech) tree pollen.
In another study conducted on woodworkers from a furniture factory, the results showed the occurrence of bronchial hyperresponsiveness as well as sore throat in these workers due to occupational exposure to oak (Quercus spp.) and beech (Fagus spp.) wood dust.
A study on 12 allergic Korean patients found white oak tree pollen sensitization in 10 of them in addition to sensitization to other allergens. Asthma was observed in 40% of allergic patients having sensitization to white oak pollens.
Other topics
Oak processionary caterpillar and moth are prevalent in many European countries. Their larvae contain toxic hairs (setae) which may induce serious reactions like lepidopterism (lesions, toxic dermatitis, itchy papules), urticaria, conjunctivitis, respiratory (upper tract) symptoms, pharyngitis, anaphylaxis, and malaise after its contact. In the study, 42 people were observed to develop such symptoms as severe pruritus, Quincke edema, dyspnea, bronchoconstriction, etc. due to contact with an infested oak tree. However, very few of them were reported to have contact with caterpillars.
Prevention And Therapy
Allergen immunotherapy
A randomized, double-blind, placebo-controlled trial was conducted in 219 birch allergic patients (aged 18-65 years) with moderate to severe ARC with or without asthma. Oak sensitization was found in 75% of the patients based on positive SPT. The sublingual allergen immunotherapy induced IgG4 antibodies in patients who reacted to birch as well as oak showing a high correlation (r = 0.78) between them. The immunotherapy tablet was able to significantly (p=0.03) reduce ARC symptoms induced by birch or oak tree pollen in birch allergic patients. The significant reduction of ARC symptoms due to exposure to oak confirms the clinical relevance of cross-reactivity between birch and oak.
Prevention strategies
Avoidance
One of the preventive measures to be taken is to cut down the amounts of pollen entering the respiratory system. During pollen season, allergic patients are suggested to stay indoors or wear masks outdoors to keep away from pollen allergens. In worse situations, patients may be suggested to move to a non-pollen area.
Cross-Reactivity
The studies have demonstrated significant cross-reactivity between oak pollen and birch pollen. Members of the birch family (Betulaceae) has been shown to have strong cross-reactivity with oak. It has demonstrated that Bet v 1 and Bet v 2 of birch pollens possess most of the IgE epitopes of Fagales pollens including oak pollen allergen Que a. Sera from European patients (Austria, Switzerland, France, and Sweden) displayed similar IgE reactivity to birch, beech, and oak pollens. Cross-reactivity is also shown between oak pollen allergen and timothy grass pollen allergen Phl p 4 in IgE-inhibition experiments.
Amino acid sequence homology has been shown among Fagales allergens like Que a 1 (oak), Bet v 1 (birch), Aln g 1 (alder) and Car b 1 (hornbeam). Further, a study reported amino acid sequence identity of Que a 1 with other PR-10 allergens like Bet v 1 (68%), Aln g 1 (60%), Car b 1 (58%), Cor a 1 of hazel (74%), Fag s 1 of beech (74%) Mal d 1 of apple (69%), Pru av 1 of cherry (66%) and highest similarity of 88% with Cas s 1 of sweet chestnut.
Studies have also shown sequence homology and cross-reactivity between oak pollen and alder, hornbeam, hazel, and European chestnut pollen.
Significant cross-reactivity was shown between oak pollen and sawtooth oak, Mongolian oak as well as common silver birch pollen in a Korean study. Furthermore, 86% IgE-binding to ginkgo pollen proteins were inhibited oak pollens in an immunoblot analysis.
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