Latin name: Salix caprea
Source material: Pollen
Common names: Willow, Goat willow, Great sallow, Pussy willow
The family: Salicaceae contains the genera Populus (Aspens, Cottonwoods, and Poplars) and Salix (Willow)
The family Salicaceae contains the genera Populus (Aspens, Cottonwoods, and Poplars) and Salix (Willow). The genus Salix, which contains around 400 species of deciduous trees and shrubs, comprises Willows, sallows and osiers (1). They are found primarily on moist soils in cold and temperate regions of the Northern Hemisphere. Most species are known as Willow, but some narrow-leaved shrub species are called osier, and some broader-leaved species are called sallow. Some willows (particularly arctic and alpine species) are low-growing or creeping shrubs (2).
Willows are cool-climate trees and are common in most of Europe, North America, western temperate Asia, and northeast Africa. In eastern Asia they are replaced by related species. The tree is uncommon in the tropics.
Willow is a deciduous shrub or small tree, usually attaining between 3 m and 15 m in height. It may grow in the form of a large and upright shrub or a multi-stemmed small tree. The bark is yellowish-brown, becoming dark brown as the tree grows older. The green leaves are oblong and irregularly toothed.
Willow is among the first trees flowering in spring. The flowers are dioecious (individual flowers are either male or female, but only a single sex is to be found on any one plant) and appear in catkins. The male Willow produces white, 2.5 – 5 cm-long flowers on a bottlebrush-like catkin. The catkins are produced early in the spring, often before the leaves, or as the new leaves open. Pollination is by insects.
The fruit is a small, cylindrical, beaked capsule containing numerous tiny (0.1 mm) seeds. The seeds are furnished with long, silky, white hairs, which allow the seeds to be widely dispersed by the wind (2).
Willow occurs in wet environments, such as riverbanks and lake shores, and in drier sites where bare soil becomes available due to ground disturbance. Willow bark is used as an herb. The wood is light and firm and yields salicine (salicylates), which is used in headache tablets and muscle-pain ointments.
No allergens from this plant have yet been characterised.
The family Salicaceae contains the genera Populus (Aspens, Cottonwoods, and Poplars) and Salix (Willow). Extensive cross-reactivity between the species in the genus Salix and in the genus Populus can be expected (3-4). This has been demonstrated between Cottonwood and Willow (5). Through P-K neutralisation and passive hemagglutination inhibition, moderate cross-reactivity between members of Salicaceae and of Fagales has been shown (6).
Ole e 9, a major Olive tree pollen allergen, shows 39%, 33%, and 32% sequence identity with 1,3-beta-glucanases from Wheat, Willow, and Arabidopsis thaliana, respectively (7).Whether the 1,3-beta-glucanases from Willow were allergenic was not evaluated for.
Willow tree pollen can induce asthma, allergic rhinitis and allergic conjunctivitis (2,8-11). As a close relationship exists between this Willow and other species, these species, where they commonly occur, may also induce allergic symptoms (12-13).
Willow tree pollen is an important aeroallergen in many parts of the world. This has been demonstrated in Turkey and other parts of the eastern Mediterranean region (6), at the Rabka health resort in Poland (14), and in Switzerland (5). Pollen from Salix spp. was reported to play a role in allergic rhinitis in Eskisehir, Turkey (10). In another Turkish study of 614 respiratory-allergic patients, Willow, Poplar, Olive and Cypress pollens were among important inhalant allergens causing skin test positivity (6). Other aerobilogical studies from various regions in Turkey have documented the presence of Salix spp. pollens in the air (15-18).
Pollen extracts of Box elder, Willow and Hickory elicited the largest number of allergic reactions in a Missouri, USA, population skin-tested with pollen from 12 wind-pollinated tree species (7). Salix pollen was also recorded in Anchorage, Alaska (19).
Measurement of daily pollen concentration over a 6-year period in Badajoz, in southwestern Spain, demonstrated high levels of Willow pollen, along with the pollen of another family member, Populus (20). A study of the common airborne pollen allergens in the city of Salamanca, Spain, also confirmed the presence of Salix pollen in the air (21).
Willow pollen (and the pollen of the family member Cottonwood) has been demonstrated to be an important aeroallergen in Tehran, with the pollen season extending from the first week of February through the middle of October (22).
In 9 districts of northern China, the most common aeroallergens included the pollen of Willow and its family member Populus (23); the findings were similar in Seoul, Korea, where the pollen from these trees was recorded from March to May (24).
Phytodermatitis due to contact with Willow has been documented (25).
Anaphylaxis has been described in a 32-year-old atopic patient after the ingestion of a pollen compound prepared in an herbalist’s shop. A few minutes after ingestion, generalised pruritus, diffuse erythema, facial oedema, cough, hoarseness and dysphonia occurred. The patient was shown to be sensitised to pollens from Artemisia vulgaris, Taraxacum officinalis and Salix alba, which were found in the preparation (26).
As Willow bark contains acetylsalicylic acid, adverse reactions to the ingestion of herbal products made from Willow bark may be attributed to allergy to Willow when in fact the allergy is to acetylsalicylic acid (27).
Mabberley DJ. The Plant Book. Cambridge University Press: Cambridge. 1997
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