Worldwide distribution
Pollen from the Olive tree is the most significant allergy-causing pollen in the region of the Mediterranean basin (6-10). In Southern Europe, pollen from Olive trees forms the most significant cause of Type-I respiratory allergy after grass pollen allergens (3). Olive pollen accounts for the majority of cases of pollinosis in Mediterranean countries. In many Spain and Italy provinces, Olive tree pollen is the primary cause of pollinosis (4). In the South-West Mediterranean and some North America regions, Olive pollen allergens form the most significant cause of respiratory allergy (9, 11).
In Spain, the second most prevalent cause of allergy after grasses is Olive (4, 12). The annual pollen counts of Olive in Southern Spain are twice as high as those in Central Spain and eight times higher than in Northern Spain. In Northern Spain, the concentration and potency of Olive pollen were lower than those in the Mediterranean countries (6). In olive-growing areas, high sensitization to olive has been observed. A total of 58% of people in Toledo, 78% in Ciudad Real, and almost 97% population in Jaen are sensitized to pollen from Olive trees in Spain (10). Another study in Spain found that out of 195 patients in the age range of 14-76 years, with pollen allergy and suffering from rhinitis and/or asthma, 53% were monosensitized to Olive tree pollens (13). The sensitization for Olive pollens was the highest (16.5%) amongst all tree allergens in 532 children with allergic symptoms and age less than 15 years from 21 primary care centers in Italy and Spain (14). Olive was the least sensitizing agent amongst Parietaria, dust mites, grass pollen, and Olive pollens for 462 elderly allergic patients in Campania (Southern Italy) (15).
Out of 952 allergic patients (of age 41 years) from Monchengladbach (North Rhine-Westphalia) and Munich (Bavaria) in Germany, 26% of patients were allergic to Olive pollens. The percentage of patients with Olive allergy was two times higher in Bavaria than in NRW (p= 0.000) (8).
In Northern Greece, Olive pollen was the least sensitizing allergen affecting 231 (14.71%) out of 675 school-going children in the age range of 6-17 years (16).
Risk factors
There is a significant variation in the pollination period of Olea, depending on the geographical location. The pollination period for Olive pollens is between Mid-April and June-end in Spain and Italy (4). During the pollination period, there is an increase in the number of allergic individuals and individuals with severe symptoms due to the rise in pollen production (4).
The cross-reactivity between seasonal allergens may cause allergic reactions throughout the year in individuals sensitized to pollen from the Oleaceae family. As a result, aggravation of the allergic response induced by Olive during winters or spring could be due to allergens of the Ligustrum genus during summers. Thus chronic symptoms in patients are sensitized to pollen allergens from the Oleaceae family (17).
Plants from the Oleaceae family are widely used as ornamental plants and trees in cities. Since the flowering season of each of these genera differs, the urban dwellers who are sensitized to Olive pollen may present allergenic reactions during the winter (resulting from pollens of ash), the spring (due to allergens from olive pollen), and early summers (as a result of privet pollens) (3).
