Latin name: Fraxinus americana
Source material: Pollen
There are 4 important genera in the Oleaceae family: Olive (Olea), Ash (Fraxinus), Lilac (Syringa), and Privet (Ligustrum).
Fraxinus is a genus of about 65 species, which are distributed in the northern temperate regions. The hardy White ash is native and common in eastern North America. It also thrives in Europe.
White ash is a tall, deciduous, long-lived tree, growing to a height of between 18 m and 40 m. The trunk is long, straight and free of branches for most of its length, and the crown is narrow and pyramidal when the tree is grown in a mixed stand. Specimens grown in the open have a short trunk with a rounded crown. The bark is ash-grey to brown, with a diamond pattern. The leaves are compound and 20 to 38 cm in length. Winged fruits hang in bunches throughout the winter and are shaped like keys.
White ash is dioecious (male and female flowers are distinct and grow on separate trees). Clusters of yellow-orange flowers appear in April and May, the male flowers blooming first, long before the leaf buds appear. Pollen season may start as early as January. Ash sheds copious amounts of pollen. Ash is entirely wind-pollinated. The pollen is carried by wind as far as 110 m from the point of dispersion.
White ash grows best on deep, well-drained, moist soils along with other hardwoods. Ash is an important timber tree, having light and elastic wood. The US exports Ash timber all over the world. White ash was used as snakebite preventive.
No allergens have been characterised to date.
However, from the closely related European ash tree (F. excelsior), the following allergens have been isolated to date:
- Fra e 1
- Fra e 2 (a profilin)
- Fra e 3 (a calcium-binding protein) and
- Fra e 9 (a 1,3-beta-glucanase) (1).
An extensive cross-reactivity among the different individual species of the genus could be expected, as well as to a certain degree among members of the family Oleaceae (2).
Cross-reactivity among the pollen allergens of Olive (Olea europaea), White ash (Fraxinus americana), Privet (Ligustrum vulgare), and Russian olive tree (Elaeagnus angustifolia) has been documented (3). A high degree of cross-reactivity has also been demonstrated among Olive tree (Olea europaea), European ash (F. exselsior), Privet (Ligustrum vulgare) and Phillyrea angustifolia, although there was no 100% identity among these species (4). These results are consistent with those of other authors (5).
A study using recombinant Fra e 1 from the closely related European ash (F. exselsior) reported an 82%, 88%, and 91% identity with, respectively, Syr v 1 (Lilac tree), Ole e 1 (Olive tree), and Lig v 1 (Privet tree) (6), suggesting possible cross-reactivity between pollen from this tree and these pollens.
Cross-reactivity is also known to occur among members of the Fagales family (Birch, Alder, Hazel, and Beech) and among members of the Oleaceae family (Ash, Olive, Lilac, and Privet); a study reports on specific cross-reactivity between Ash and Birch (7).
Pollens from White ash can induce asthma, allergic rhinitis and allergic conjunctivitis (1,3,8-9).
Pollen from the Ash tree is an important aeroallergen. In a European study of 5,416 consecutive patients sensitised to various pollens, 17.6% had a positive skin-prick test to Ash (1.)
Ash tree pollen has also been demonstrated to be an important aeroallergen in Zurich, Switzerland, contributing greatly to the overall pollen count (10). Although Ash tree pollen may not always be a major component of aeroallergens, because of cross-reactivity individuals who are Olive tree pollen-allergic may be found to be sensitive to Ash tree pollen. Thus, among 503 patients with allergic rhinitis in the southern part of Switzerland (Canton Ticino) who were tested by skin prick for sensitisation to common allergens, of the 54% who were positive to Olive tree, a great many would be allergic to Ash tree pollen (11).
The daily pollen concentration measured in the atmosphere over a 6-year period in Badajoz, in southwestern Spain, demonstrated pollen from Fraxinus
species to be among the most important aeroallergens (12). In Madrid, Spain, pollen from Fraxinus
species was shown to be among the dominant pollens from January to April (13). In Cordoba, Spain, skin-prick tests were carried out on 1,500 pollen-allergic patients with an extract of Fraxinus
pollen, and 59% were positive. The great majority of the patients were polysensitive; only 8% were found to be monosensitive (14).
Pollen from this species was also shown to be important in Mexico City (15), Sivrihisar, Turkey (16), Vinkovci in northeastern Croatia (17), and Mar del Plata, a city in Argentina (18). The presence of pollen from this tree was also demonstrated in an aerobiological study of 10 large Canadian cities (19).
In a study of 371 allergy patients tested serologically for hypersensitivity to prevalent tree pollens in the area surrounding New York, 26% were found to be sensitised to Ash (20). In a population in St. Louis, Missouri, USA, who were skin-tested with pollen from 12 wind-pollinated tree species, White ash was found to have sensitised some individuals, although it was shown to be less reactive than other tree species (21).
Rhinitis and occupational asthma from exposure to Ash wood dust have been reported (8,22).
Compiled by Dr Harris Steinman, email@example.com
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