Latin name: Populus deltoides
Source material: Pollen
Common names: Cottonwood, Poplar tree
The botanical family willow (Salicaceae) consists of Populus and Salix, the latter being true willows. Populus deltoides is the common designation for species of trees in this genus, but in fact Populus can be divided into 3 distinct groups: cottonwoods, aspens and poplars.
The botanical family Salicaceae consists of Populus and Salix, the latter being willows. Populus is a genus of between 30 and 40 species of flowering plants. Their native distribution covers most of the northern hemisphere, and 9 are native to North America. (1) They do not occur in the southern hemisphere.
Cottonwood is a deciduous tree native to eastern North America. It is the fastest-growing native tree on that continent. In open areas, it typically has a large trunk that divides into upright branches near its base, and ascends to form a broad, open, spreading crown. In plantations, it tends to have a tall, straight, and relatively branch-free bole and a small rounded crown. On good sites, 35-year-old trees may average 40 m in height, while trees 55 to 65 years old may be 53-58 m in height, with a trunk diameter up to 100 cm. The root system is quite expansive and can extend over 30 m from the trunk.
The bark is smooth and yellowish-grey, becoming dark grey and developing deep furrows with age. The leaves are finely toothed, triangular, dark green and shiny on top and paler underneath, with a long, flattened stalk. (1)
Cottonwood is dioecious: each tree is male or female, not both. Cottonwood flowers in early spring, with the exact time of pollen production depending on the region. Populus species are entirely wind-pollinated, as distinct from willows, which are primarily insect-pollinated. Male catkins are 5 to 7 cm long. Propagation is mainly by airborne seed (‘cotton’) from female trees, but detached branches can also take root. (1) The cotton is produced in enormous quantities for about 2 weeks virtually every year, and can be windblown for miles from its origin.
Because of its extensive root system, the tree plays a major role in soil erosion control along rivers, streams, and lakes.
To date no allergens from this plant have yet been characterised.
Extensive cross-reactivity could be expected between species in the genus Populus, as well as with species of the genus Salix, such as willow. (2) Indeed, strong cross-reactivity between cottonwood and willow is shown by skin-test correlations and P-K neutralisation. (1, 3, 4, 5, 6, 7) Moderate cross-reactivity between Salicaceae and Fagales members is shown by P-K neutralisation and passive hemagglutination inhibition. (1, 3, 4)
IgE mediated reactions
Pollen from the cottonwood tree results in asthma, hay fever and allergic conjunctivitis. (8) Estimates from Oklahoma and Colorado in the USA attributed 20% of all hay fever to cottonwood. (1, 9, 10) Lewis et al. showed skin-test reactivity to cottonwood in 60% of atopic patients from 7 different states. (11)
Allergy to Cottonwood pollen has been shown to be among the 10 most important aeroallergies resulting in clinical symptoms in the Eastern Province of Saudi Arabia, where 38% of 1 159 patients were skin-prick test-positive to Cottonwood pollen; (12) and similar findings emerged from among 614 patients with respiratory allergies in Turkey. (13) A later study conducted in 54 patients with seasonal allergic rhinitis living in Ankara/central Anatolia in Turkey reported that 21.8% were skin-prick test-positive for Populus. (14)
In a study in the United Arab Emirates, among 327 adult patients with respiratory, dermatologic and ophthalmologic diseases of suspected allergic origin, cottonwood was found to be the third-most prevalent sensitising allergen (33%) after Mesquite (46%) and grass mix (41%). (15)
A Hungarian study found that 6.8% of hay fever patients were sensitised to cottonwood tree pollen; thus, in this group of patients, cottonwood was not an important cause of hayfever; (5) and the results were similar in an early 1975 study in St. Louis, Missouri, USA. (16) In a study examining aeroallergen sensitisation rates among 209 children of the military in Texas who were undergoing skin testing for rhinitis, the most common allergens were found to be grasses, Alternaria, and cottonwood. (17) Studies of the prevalence of aero-allergen sensitisation in atopic asthmatic adults in Mexico found that 22% were sensitised to Populus spp. (18, 19) A study of allergic patients of all ages reported 21% sensitisation to Populus spp. (20)
Cottonwood pollen has been demonstrated to be a major aeroallergen during the pollen season in Tehran, where the season extends from the first week of February to the middle of October; (21) the pollen is also a major aeroallergen in Fairbanks, Alaska. (22) In other regions, cottonwood pollen may not be a large component of the total aeroallergen measurement. In Madrid, Spain, the largest airborne presence (from 1979 to 1993) was for Quercus spp. (17%), followed by Platanus spp. (15%), Poaceae spp. (15%), Cupressaceae spp. (11%), Olea spp. (9%), Pinus spp. (7%), Populus spp. (4%), and Plantago spp. (4%). The predominant pollens from January to April were tree pollens (Cupressaceae, Alnus, Fraxinus, Ulmus, Populus, Platanus, and Morus). (23) Populus was the eighth-most common allergenic pollen recorded in the atmosphere of Burgos, Spain, (24) and the fourth-most common in Toledo, Spain. (25)
In a study of the impact of different trees on asthma, and of the association between daily hospitalisations for asthma and daily concentrations of different tree pollens in 10 large Canadian cities, statistically significant but small (<2%) effects were observed from Populus (aspen, poplar). (26)
In a study conducted in Mashhad City in Iran (a city with a semi-arid climate in a fairly large country with different geo-climatic conditions), of the prevalence of positive skin tests in 311 allergic rhinitis patients to 27 common regional aeroallergens, the overall rate of sensitisation to any allergen was 81%, of which 43% were sensitised to cottonwood pollen. (27)
Pollen from Populus spp. has been shown to be among the main contributors to aeroallergen load in the air of Vinkovci, in north-eastern Croatia. (28) In the city of Heraklion, Crete, pollen from Populus spp. was the sixth-most prevalent atmospheric pollen. (29) Populus spp. pollen was reported to be a significant atmospheric pollen in Zagreb, Croatia. (30) In Plasencia, Spain, Populus spp. pollen was reported to be the ninth-most prevalent pollen aeroallergen. Out of 210 patients with a diagnosis of pollinosis, 32.3% were sensitised to P. nigra. (31) A study determining the most frequent food and indoor and outdoor respiratory allergens involved in allergic rhinitis in children in the region of Piraeus, Greece, evaluated 50 children (ranging from 6 to 14 years) with symptoms of allergic rhinitis and positive RAST or skin-prick tests; 5.5% were sensitised to Populus alba. (32)
Similarly, pollen from the closely-related white poplar, P. alba, has been reported to be a significant allergen. (33) The pollen of P. alba has been found to be prevalent by aerobiological studies in Turkey. (34, 35) In a study evaluating the clinical importance of poplar allergy in subjects with seasonal allergy living in Ankara, Turkey, which has a high poplar pollen load, allergy to P. alba pollen was reported to result in mild symptoms, while Gramineae (grass) pollen allergy contributed significantly to symptoms. (36) A cross-sectional survey performed in 6 304 patients with asthma and/or rhinitis from 4 regions of China recorded sensitisation to cottonwood, among others. (37)
In a Korean study of 133 patients with allergic symptoms evaluated using 3 different kits for detection of allergen-specific IgEs, depending on the test used, 0 to 3.8% were shown to be sensitised to a poplar mix. (38)
In a study of 371 allergy patients tested serologically for hypersensitivity to prevalent tree pollens in the area surrounding New York, 20.6% were sensitised to poplar. (39) In a study in Westchester County in the state of New York, skin-prick tests to 48 aeroallergens in 100 patients referred for allergic rhinitis showed that 9% were sensitised to cottonwood. (40) A study in Texas 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 skin-prick test to a 53-inhalant-allergen panel. For cottonwood, 40% were skin-prick test-positive, versus 29% for serum-specific IgE. (41)
Immunotherapy may be used in sensitised individuals. (42)
Compiled by Dr Harris Steinman, email@example.com
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