134 Spellerberg et al.: Silver Birch Pollen and Human Health RESULTS Pollinosis and Asthma Tree pollens are among the most important allergen sources. Silver birch and some related tree species have been de- scribed as the most potent and frequent allergen sources (Mothes et al. 2004; Mothes and Valenta 2004). This has been recorded in the literature for many years. Birch pollen is well known to be a significant aeroallergen (an allergen dis- persed by wind). This pollen is known to be a notable cause of hayfever and pollen-related asthma (see for example Emberlin et al. 2003). In New Zealand, birch pollen is the most common tree pollen causing seasonal rhinitis (see the Glossary for defini- tions of medical terms) together with food allergy (the oral allergy syndrome). The incidence of cases is increasing. Ten years ago at the Auckland Allergy Clinic, one or two patients a year with food allergy were being seen and now it is ap- proximately 100 per year. In Northern Europe, between 39% and 78% of atopic pa- tients are sensitized against birch pollen (Eriksson et al. 1998), and in northwest Spain, between 13% and 60% of individuals who are immunosensitive to pollen grains re- spond positively to its allergens (Cotos-Yanez et al. 2004). In Vienna, more than 25% of the population have allergies caused by pollen from trees, including birch (Mothes and Valenta 2004). In some countries, the birch pollen allergy is increasing. In Belgium, for example, among patients with respiratory allergy, the frequency of birch pollen sensitization increased significantly from 13% in 1975 to 1979 to 34% in 1992 to 1995 (Stevens et al. 2003). Although sensitization to pollen from other tree species does occur, an allergy against such trees is seldom found in the absence of birch pollen allergy. This has been found to be the case in Sweden where silver birch is common (Eriksson et al. 1984; Eriksson and Wihl 1987). Exposure to birch pollen has implications for babies and young children. Exposure to high levels of birch pollen in infancy increases the risk of sensitization to the same allergen as well as the risk of asthma (Kihlstrom et al. 2002). Early pollen contacts, particularly during the first 6 months of life, increase the risk of pollen allergies for a period of 20 years (Bjorksten et al. 1980). It has also been shown that exposure of the mother during pregnancy to high levels of birch pollen results in a tendency toward increased risk of sensitization to the same allergen and symptoms of atopic disease in children (Kihlstrom et al. 2003). So significant are the effects of birch pollen on health that there has been research on shifts in the timing of the birch pollen season (Emberlin et al. 2002). Although birch pollen allergens have been implicated as asthma triggers, it appeared for some time that the pollen grains were too large to reach the lower airways where asth- matic reactions occur. However, research has shown that ©2006 International Society of Arboriculture when the highly allergenic birch trees are flowering and ex- posed to moisture followed by drying winds, they can pro- duce particulate aerosols containing pollen allergens (Taylor et al. 2004). Food Allergies (adverse reactions to food) and Crossreactivity Between Food and Pollen Crossreactions between food and aeroallergens occur as a result of common allergenic structures, epitopes, on the pro- tein molecules. One of the most well-known crossreactions is that between birch pollen and a variety of vegetable foods. The foods that most often give symptoms in patients with birch pollen allergies are nuts (hazelnut, brazil nut, and wal- nut), kiwifruit, and also fruits belonging to the botanical fam- ily Rosaceae (apple, peach, almond, pear, nectarine, plum, cherry, apricot), as well as carrots, celery, and potato peel (Eriksson 1978; Eriksson et al. 1982; Ortolani et al. 1988; Anhoej et al. 2001; Eriksson et al. 2003; Ghunaim et al. 2005; Osterballe et al. 2005). Approximately 70% of individuals with birch pollen aller- gies have symptoms of crossreactions to food (Eriksson et al. 1982). There is a positive relationship between the degree of birch pollen sensitization and the occurrence of food hyper- sensitivity; the more pronounced the birch pollen allergy, the greater the probability for the food allergy. Among those with the highest degree of sensitization to birch pollen, 90% report a food allergy (Eriksson et al. 1982). In New Zealand, previous studies have shown that patients who are sensitized to birch pollen will develop symptoms (Crump, pers. obs.). Recently, for example, at the Auckland Allergy Clinic, 64 patients attending the clinic for immuno- therapy for hayfever were interviewed followed by skin prick tests to fresh kiwi fruit, apple, and banana. Nine of those patients had known food allergy and of those, eight were allergic to silver birch. From the skin prick tests on the 64 patients, 37 patients were sensitized (21 to kiwi fruit, 10 to apples, and 6 to bananas). In Northern Europe, allergy against crossreacting food is more common in patients with a “pan pollen allergy” (i.e., sensitization against grasses, mug wort [Artemisia vulgaris], and birch) than those with only a monopollen allergy or a combination of grass plus birch, grass plus mug wort, or birch plus mug wort pollen allergy (Ghunaim et al. 2005; Osterballe et al. 2005). The kiwi fruit is one of the fruits that crossreact with birch pollen (Aleman et al. 2004). In the last few decades, the prevalence of the kiwi allergy is increasing in some European countries and was ranked among the top 10 foods among individuals with food allergies in Sweden, Denmark, and Estonia (Eriksson et al. 2004). Among patients with birch pollen allergies with crossreactive food allergy, 51% reported symptoms on eating kiwi fruit (Eriksson et al. 2003). The sensitivity can be so pronounced that symptoms can appear
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