Latin name: Ulmus americana
Source material: Pollen
Common names: Elm, white elm, American elm
See also: Cedar elm (U. crassifolia) t45
The elm family, Ulmaceae, contains 6 genera, the most important being Ulmus, Planera, and Zelkova. (Hackberries [Celtis spp.] used to be included in the elm family, but are more closely related to Urticaceae and have been placed in a separate family, Celtidaceae.) There are about 45 species of elm worldwide, found in the temperate northern hemisphere, with about 4 introduced species to the USA, and 6 native to North America east of the Rocky Mountains. There are 25 species of Ulmus. (1, 2, 3)
White elm can be found mostly in forests in central and eastern North America, where it represents a major source of pollen. White elm is native to North America and has a wide range, from all of the eastern states through the Central Plains. (4)
The corresponding European species, wych elm (U. glabra) and smooth elm (U. carpinifolia), grow wild or cultivated throughout the north temperate regions. In the 1930s, Dutch elm disease devastated white elm trees in North America as well as in Europe. The disease, a fungal blight, was caused by the ascomycete Ophiostoma ulmi and transmitted through the native elm bark beetle, Hylurgopinus rufipes. (5) All elms are susceptible to some degree to the fungus, especially American elm (U. Americana) and September elm (U. serotina). Chinese elm (U. parvifolia), Siberian elm (U. pumila) and red or slippery elm (U. rubra) are highly or moderately resistant. (1, 3)
White elm is a deciduous tree growing up to 40 m tall, and is well known for its vase-shaped crown. The bark is furrowed and light or dark grey. The dark-green leaves are alternate, simple, and have doubly-toothed margins.
The flowers appear in late winter or early spring as drooping, hairy, greenish-red clusters of 3 to 4 flowers, while other species of the genus flower in autumn. Oval and flat fruit are produced. These are up to 1.5 cm long and have papery wings.
White elm grows in woods and hedges and by roads and streams, occurring on moist uplands and bottomlands. In bottomlands, the trunk is more likely to be buttressed than in uplands. American elm was widely planted as an ornamental in urban settings because it was stress-tolerant, fast-growing, and beautiful.
No allergens have yet been characterised.
Extensive cross-reactivity between the different individual species of the genus could be expected, as well as to a certain degree between members of the family Ulmaceae. (6) However, to date this has not been documented.
In a Spanish study, individuals with Melon allergy were found to be especially prone to sensitisation and asthma to several tree and weed pollens, predominantly Ulmus and Ambrosia. (7)
IgE mediated reactions
White elm pollen can induce asthma, allergic rhinitis and allergic conjunctivitis. (8, 9)
The importance of Ulmaceae pollen in asthma, allergic rhinitis and other allergy conditions has been demonstrated by numerous studies from around the world. In Washington, DC, the Ulmaceae have been shown to have 2 distinct pollinating times, with the later-flowering species of elm pollinating from August until October, coinciding with ragweed season. April is usually the month with the highest weekly average concentrations. (10)
Elm tree pollen has also been demonstrated to be an important aeroallergen in Salamanca, Madrid, and Badajoz, Spain. (11, 12, 13)
Elm tree pollen has also been demonstrated to be an important aeroallergen in 9 districts of northern China. (30) In one district, elm pollen was the dominant aeroallergen detected. Other Chinese studies concur. (14)
Elm tree pollen has also been shown to be an important aeroallergen in Tehran, Iran, where the pollen season extends from the first week of February until the middle of October. (15)
In Siena, in central Italy, elm tree pollen was found to be an important aeroallergen in March, but not during the rest of the tree pollen season; (16) whereas in Modena, in northern Italy, the elm family contributed little pollen to the air (about 1% of the total pollen recorded). (9) Similarly, elm tree pollen was not shown to be an important aeroallergen in Missouri, USA. (17) In Tampa, Florida, in the USA, where the major tree pollen season occurs from December until May, with a minor season occurring from October to November, elm tree pollen is important during the minor season. (18)
In a study in Westchester County in the state of New York of skin-prick tests to 48 aeroallergens in100 patients referred for allergic rhinitis, 65% had a positive SPT to at least 1 aeroallergen. Alternaria was positive in 8%, cat hair in 18%, birch tree in 20% and grass mix in 24%. The highest positive SPT frequency was to house dust mite (30%). Others were for maple (13%), birch (20%), oak (18%), white ash (10%), sycamore (5%), hickory (10%), beech (16%), sweet gum (16%), and cottonwood (9%). Sensitisation to white elm was documented in 6%. (19) A study in Cincinnati identified atopic parents and determined the prevalence of sensitisation to 15 aeroallergens in their infant offspring. For American elm, for tested parents with symptoms (n = 1 134), 16.5% were positive; and for infants with a skin-prick test-positive parent to any of the 15, and symptoms (n = 680), 1% tested positive. (20)
In a New York study of 371 allergy patients tested serologically for hypersensitivity to prevalent tree pollens in the surrounding area, the highest rate of hypersensitivity was to oak (34.3%), birch (32.9%), and maple (32.8%) tree pollens, with a rate of 24.6% for elm being recorded. (21) In a Canadian study that evaluated the impact of different trees on asthma, along with the association between daily hospitalisations for asthma and daily concentrations of different tree pollens in 10 large Canadian cities, it was reported that for an interquartile increase in daily tree pollen concentration, percentage increases in daily hospitalisation for asthma were 2.63% for Ulmus. (22)
A number of studies conducted in Texas demonstrated sensitisation to white elm tree. The first study, which examined the prevalence of positive skin-test responses in a symptomatic military population with a wide age range of patients and large number of allergens, recruited 1 137 patients aged 4-79 years old who underwent a standard skin-prick testing panel of 53 aeroallergens. Results indicated that 81.6% of patients had at least one positive skin test; 9.2% of patients were monosensitised. The average number of positive skin tests peaked in the 10- to 19-year age group at 13.1. The most common allergens were grasses, mountain cedar, and dust mites. Twenty-six per cent were sensitised to American elm. (23)
Two of the authors also evaluated aeroallergen sensitisation rates in military children in Texas who had undergone skin-prick testing for rhinitis. Of 209 patients, 35.4% had at least 1 positive result. The most common allergens were mould mix (16.3%), cat (13.2%), dust mite mix (11.4%), tree mix (9.4%), and grass mix (9.4%). The prevalence of sensitisation to pollen from trees was: cottonwood (33%), mountain cedar (31%), oak (31%), white Ash (30%), mesquite (29%), pecan tree (28%), mulberry tree mix (27%) and elm 24%. Sensitisation to olive tree, birch, box elder and maple mix, hackberry, sycamore, cypress, palm tree, walnut tree and pine tree varied between 23% and 6%. (24)
In an allied study, researchers compared serum-specific IgE testing to skin-prick testing for 53 inhalant allergens using subjects > or = 18 years old with chronic rhinitis, and who had at least 1 positive ST to a 53-inhalant allergen panel. The mean number of positive skin-prick tests and serum-specific IgE results were similar. For American elm tree, 36% were skin-prick test-positive, versus 31% for serum-specific IgE testing. (25)
A study conducted in West Virginia, with the purpose of determining the prevalence of sensitisation to common indoor fungal species in an atopic population in West Virginia, recruited 102 patients who were evaluated for IgE reactivity to a panel of skin-prick test reagents used for routine allergy testing. Of the 102 eligible patients, 68% had at least one positive skin test. The most prevalent positive SPTs were to dust mites, cats, vernal grass, and short ragweed. Fourteen per cent were found to be sensitised to elm tree pollen. (26)
In a study assessing the prevalence of skin sensitivity to airborne pollens of 12 species of trees in patients with asthma and/or rhinitis living in Buenos Aires, consecutive patients with bronchial asthma and/or clinically-defined allergic rhinitis were recruited. Of 439 patients aged 10 to 60 years, 401 (91.3%) were skin-prick test-positive to at least one of the allergens. Of the 401 atopic patients, 309 (77.1%) were skin-prick test-positive for at least 1 of the 12 tree species studied, and only 10 (2.5%) to all 12 species of trees. Ulmus sp was the most prevalent skin-prick-positive tree (37.4%), followed by Platanus sp (30.2%) and Morus sp (25.2%). Ulmus, although not constituting a significant percentage within species of trees in the study area, was found to have the highest percentage of skin sensitisation recorded. The authors postulated that this could have been as a result of potential cross-reactivity with other trees, e.g. with Morus and Urtica, for although the three species pollinate in the spring, pollination for the latter two species is markedly higher than for Ulmus. (27)
Aerobiology studies often do not draw distinctions between the various species of Ulmus, but this may not be problematic; considering the close relationship between these species, cross-sensitisation or cross-reactivity is possible. Pollen concentration in the atmosphere of Lublin (in eastern Poland) was reported to be the highest for Betula, Pinaceae and Alnus; and the lowest for Ulmus, Fagus and Corylus. Significantly, total pollen grains for Ulmus varied almost twofold between successive years. (28) Pollen from the Ulmus species has also been reported to be an important allergen in Zagreb, Poland. (29) In aerobiology studies of 9 districts of northern China, the most common aeroallergens in spring were often Ulmus, Populus and Salix, (30) findings supported by other studies. (31)
The aim of a Turkish study was to document sensitisation to pollens and to evaluate their clinical importance in patients with seasonal allergic rhinitis living in Ankara/central Anatolia. The study consisted of 54 subjects. Trees were the most common pollen source, accounting for 95% of the total amount, followed by grasses (3%) and weeds (2%). Sensitisation to Gramineae, to at least one weed, and to tree pollens were 100%, 85.2% and 94.4%, respectively. The most common positive skin tests among tree pollens were to Oleaceae (59.2%), Aesculus (57.4%); and Tilia (42.5%), despite low pollen counts. Chenopodiaceae (88%) and Plantago (63%) were the most sensitised weed pollens, with high pollen counts. 21.8% were SPT-positive for Populus. Ten patients (18%) were sensitised to Ulmus. (32)
IgE antibodies to elm tree pollen were detected in pollen-allergic individuals with allergic rhinitis in Jena, Germany. (8)
Delayed and prolonged contact urticaria as a result of contact with elm tree has been reported. (33)
Compiled by Dr Harris Steinman, firstname.lastname@example.org
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