t25 European ash

Allergens within Tree Pollens

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  • Latin name: Fraxinus excelsior
  • Family: Oleaceae
  • Common names: European ash, Common ash, European common ash
There are 4 important genera in the Oleaceae family: Olive (Olea), Ash (Fraxinus), Lilac (Syringa), and Privet (Ligustrum).
 
Related species:
F. americana - White ash
F. pennsylvanica - Green or Red ash
F. tomentosa - Red or Pumpkin ash
F. quadrangulata - Blue ash

Pollen

A tree species producing pollen, which often induces hayfever, asthma and conjunctivitis in sensitised individuals.

Allergen Exposure

Geographical distribution
Fraxinus is a genus of about 65 species, which are distributed in northern temperate regions. The hardy White ash, for example, is native and common in eastern North America and also thrives in Europe. The European ash (Fraxinus excelsior) is native to most of Europe, with the exception of northern Scandinavia and the southern Mediterranean.
 
European ash is a broad, spreading, deciduous tree, capable of reaching 35 m in height. It is distinguished from other species of Ash in that it has black and not brown buds. The bark is light-grey (smooth in younger trees, rough and scaly in older specimens). The dark green leaves are 20-35 cm long, pinnate compound, with 9-13 leaflets. The leaflets have sharply-toothed margins. The leaves of the European ash are often among the last to open in spring and the first to fall in autumn in the event of an early frost. They usually drop off while still green, but on some cultivars may turn yellow first.
 
European Ash is dioecious (male and female flowers are distinct but – most of the time, in this case - grow on separate trees). Unusually, a tree that is male one year can produce female flowers the next, and a female tree can become male. The unornamental, purple or greenish-white, petal-free springtime flowers (the female ones somewhat longer than the male ones) open before the leaves.  Ash sheds copious pollen. The tree is entirely wind-pollinated. Ash pollen load may vary extensively between years (1). In Europe, European ash flowers in April and May.
 
The fruits are 4-5 cm long, oblong, and winged (known as "Ash keys" because they hang in bunches). They turn brown and remain on the trees until the following spring, when they are blown off and carried away by the wind.
 
European ash tree pollen may often be overlooked as a cause of pollinosis, as its flowering season coincides with that of Birch. Because of its close family relationship with Olive tree, its pollen is a significant cause of respiratory allergy through cross-reactivity. Observations from Switzerland suggest that European ash tree pollen may be, at least locally, as important as Birch in the elicitation of spring pollinosis (2).
 
Environment
European ash grows best on deep, well-drained, moist soils with other hardwoods. It is often utilised as a landscape tree and is found in gardens and parks.
 
Unexpected exposure
Because of its high resistance to splitting and its flexibility, European ash wood is the traditional material for tool handles, tennis rackets and snooker cues.
 
The bark contains the bitter glucoside fraxin, the bitter substance fraxetin, and tannin, quercetin, mannite, and malic acid.

Extract of Fraxinus excelsior combined with other ingredients, e.g., extracts of Trembling poplar tree (Populus tremula) and Goldenrod (Solidago virgaurea), has been used for the management of mild to moderate rheumatic complaints (3).
 
Allergens
European ash tree pollen contains a number of components related to allergens found in other species e.g. Fra e 1, a major allergen related to the major Olive allergen Ole e 1; the panallergen profilin Fra e 2; a 2-EF-hand calcium-binding protein, Fra e 3; a pectinesterase-like molecule; and an allergen sharing epitopes with Group 4 grass pollen allergens (4).
 
Allergens characterised:
  • Fra e 1 (2, 4-8)
  • rFra e 1 (5-6)
  • Fra e 2, a profilin (2, 4)
  • Fra e 3, a calcium-binding protein (2, 4)
  • Fra e 9, a 1,3-beta-glucanase (9)
Fra e 1 is a major allergen for Ash pollen-sensitised individuals in northern and central Europe. It belongs to the Olive tree Ole e 1-like family and has a high degree of cross-reactivity with taxonomically related members (5). Approximately 70-80% of Ash pollen-sensitised individuals will have Fra e 1-specific IgE (2, 6).
 
Approximately 75% of European ash-sensitised patients were shown to have serum-specific IgE to rFra e 1, and 29 of 30 Ash-sensitised patients were shown to have skin-specific IgE to rFra e 1 (6).
 
About 50% of Ash-allergic individuals have been shown to be sensitised to Fra e 2, a profilin, and in the same study, the calcium-binding protein Fra e 3 was shown to be a major allergen (4).
 
In a study evaluating the frequency of IgE-binding to Fra e 1 and pollen panallergens, Fra e 1 sensitisation was found in 100% of monosensitised patients (n=6), 93% of oligosensitised patients (n=16), but only 44% of polysensitised patients (n=25). Specific IgE against Fra e 2, Fra e 3, and carbohydrate epitopes in the 3 groups was found in 0/0/17%, 0/19/31%, and 32/72/60%, respectively. The study found that only 20% of positive skin-specific IgE to European ash resulted from cross-sensitisation to pollen panallergens (2).

Potential Cross-reactivity

The genus Fraxinus belongs to the family Oleaceae. It is closely related to Jasmine (Jasminum), Lilac (Syringa), Privet (Ligustrum), Forsythia (Forsythia) and Olive (Olea). 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 (10). Indeed, cross-reactivity has been documented among Olive (Oleaeuropaea), White ash (Fraxinus americana), Privet (Ligustrumvulgare), and Russian Olive tree (Elaeagnus angustifolia) pollen allergens (11). A high degree of cross-reactivity has also been demonstrated between Olive tree (Olea europaea), European ash (Fraxinus exselsior), Privet (Ligustrum vulgare) and Phillyrea angustifolia, although there was not 100% identity among these species (12). Similar results have been reported by other studies (2, 13-15). Six common protein bands were found to be responsible for the cross-reactivity, with apparent molecular weights of 49.6, 40, 36.7, 19.7, 16.7, and 14 kDa (15). A study using recombinant Fra e 1 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). Further clarity on the relationship among the Oleaceae pollen allergens was achieved in a study evaluating the common epitope determinants in Olive and other Oleaceae pollens: European ash, Privet, Lilac, and Forsythia; 18- and 20-kDa proteins were present in each pollen except Forsythia. Serum-specific IgE for Forsythia was mainly directed at 50- to 55-kDa protein bands (16).
 
A study comparing the profiles of Olive and Ash pollen allergens investigated the degree of cross-reactivity using Spanish and Austrian allergic patients selectively exposed to Olive or Ash pollen. Both groups exhibited an almost identical IgE-binding profile to both pollen allergens, with major reactivity directed against Ole e 1 and its homologous Ash counterpart, Fra e 1. Extensive cross-reactivity was demonstrated between Olive and Ash pollen allergens. However, whereas cross-reactions between profilins and calcium-binding allergens also occurs between unrelated plant species, cross-reactivity to Ole e 1 was confined to plants belonging to the Oleaceae. The study concluded that Ole e 1 is a marker allergen for the diagnosis of Olive and Ash pollen allergy (7). As Fra e 1 has strong cross-reactivity with Ole e 1 (5), some authors have suggested that the cross-reactivity is so pronounced that immunotherapy with Ole e 1 would protect European ash-allergic individuals (15).
 
Since European ash pollen contains a profilin and a calcium-binding protein and both are panallergens, cross-reactivity with other plants containing these protein families may result. In a study evaluating sera of 40 Ash pollen-allergic individuals, 30% had IgE antibodies to several high-molecular-weight Ash pollen allergens cross-reactive with Timothy grass and Olive pollen (4).
 
Ole e 9, an Olive tree pollen allergen and a 1,3-beta-glucanase, has been shown to have counterparts in pollen from European ash and Birch tree pollen, Tomato, Potato, Bell pepper, Banana and Latex (9). Specific cross-reactivity between Ash and Birch has been reported in another study (17).

Clinical Experience

IgE mediated reactions
European ash is a common cause of asthma, allergic rhinitis and allergic conjunctivitis (2, 11, 18) in particular in Central and Northern Europe, as has been recognised in recent years (2, 4). The tree is regarded as contributing to important health problems (12, 19). In patients living near Strasbourg, Ash pollen induces nearly 4% of the total sensitisation of the allergic population (6). The relevance of Ash pollen as a cause of sensitisation may have been undervalued because of the overlapping of the tree’s pollination period with the pollination of other better-documented trees, such as Birch (20).
 
In an eastern Austrian study of 5,416 consecutive patients sensitised to any pollen, approximately 18% were shown to have skin-specific IgE for Ash (2). Ash tree pollen has also been demonstrated to be an important aeroallergen in Zurich, Switzerland, contributing greatly to the overall pollen count (21).
 
The daily pollen concentration measured in the atmosphere over a 6-year period in Badajoz, in southwestern Spain, demonstrated that pollen from Fraxinus species was one of the most important aeroallergens (22). In Madrid, Spain, pollen from Fraxinus species was shown to be one of the dominant pollens from January to April (23). 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 (24).
 
In Plasencia, Caceres, Spain, aerobiological studies reported that although the most common pollens detected were Quercus, Poacea, Olea, Platanus, Pinus, Cupresaceae, Plantago, Alnus, Populus, and Castanea, in 210 patients with a diagnosis of pollinosis, 68% were sensitised to European ash tree pollen. In context, the percentages of sensitisation were the following: Dactylis glomerata 80%, Olea europea 72%, Fraxinus excelsior 68%, Plantago lanceolata 63%, Chenopodium album 61%, Robinia pseudoacacia 49%, Artemisia vulgaris 44%, Platanus acerifolia 37%, Parietaria judaica 36%, Populus nigra 32%, Betula alba 28%, Quercus ilex 21%, Alnus glutinosa 21%, Cupressus arizonica 8% and Castanea sativa 7% (25).
 
Pollen from this species was also shown to be important in Mexico City (26).
 
In a population in St. Louis, Missouri, USA, in a study using skin-specific IgE determination for pollen from 12 wind-pollinated tree species, it was reported that the closely related White ash was found to have sensitised individuals, although it was shown to be less reactive than other tree species (27).
 
Although Ash tree pollen may not be a major component of aeroallergens in a given community, individuals who are Olive tree pollen-allergic may also be found to be sensitive to Ash tree pollen as a result of cross-reactivity. In a study of 503 patients with allergic rhinitis in the southern part of Switzerland (Canton Ticino), patients were evaluated by skin-specific IgE for sensitisation to common allergens. The authors suggested that, out of the 54% who were positive to Olive tree, a great many would be allergic to Ash tree pollen (28). Similarly, in a study of Olive tree pollen allergy, almost all patients exhibited concomitant skin sensitivity to both Ash and Olive pollen (15).
 
Rhinitis and occupational asthma from exposure to Ash wood dust has been reported (29). A study described an 18-year-old man who worked in a furniture factory and reported rhinitis and asthma after exposure to Ash wood dust. A bronchial provocation test induced a dual asthmatic response. Intradermal testing with Ash wood extract elicited a positive immediate response. IgE antibodies to Ash wood were found in the patient's serum (30).

Compiled by Dr Harris Steinman, harris@zingsolutions.com.

References:

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    3. Klein-Galczinsky C. Pharmacological and clinical effectiveness of a fixed phytogenic combination trembling poplar (Populus tremula), true goldenrod (Solidago virgaurea) and ash (Fraxinus excelsior) in mild to moderate rheumatic complaints. [German] Wien Med Wochenschr 1999;149(8-10):248-53.
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2006



Further reading