Elm is a member of the Ulmaceae family. It is a wind-pollinated deciduous tree found in countries like the US, Canada, Europe, and middle eastern countries. Exposure to elm pollen may induce allergic reactions like allergic rhinitis and bronchial asthma in allergic individuals. A Canadian study reported elm as the prime contributor (ranked first) for an increase in the number of hospitalized patients, based on its pollen concentration potency. No allergens have been identified for elm tree pollen yet. Elm pollen may cross-react with pollen from other species of its own order Urticales such as Morus (mulberry trees) and Urtica (nettle trees). Moreover, elm pollen has exhibited cross-reactivity with melon, peach and pollens originating from birch.

Worldwide distribution

Tree pollen allergens are one of the critical causes of aggravated asthma, leading to further hospitalization. This fact was confirmed by a study conducted in Canada, which associated the surge in hospitalization with an increase in tree pollen concentration from outdoor trees like maple, alder birch, elm, etc. Elm is a significant contributor to the increase in hospitalization due to severe asthma caused by a daily rise in pollen concentration in the cities’ atmosphere (6).

According to a pollen-count study conducted in Spain between 1979-1983, pollen from species of the Ulmaceae family and Ulmus sp. genus accounted for 1.66% of the total airborne pollens (7).

As per a comparative study conducted in the US on 371 allergy patients, elm pollen hypersensitivity was prevalent among 24.6% (8). Graphical information from a study conducted in the US stated approximately 6%-7% of individuals suffering from allergic rhinitis (n=100) showed positive skin prick test (SPT) against elm tree antigen (9).

A study conducted in Buenos Aires (Argentina) on 439 bronchial asthma and/or allergic rhinitis patients analyzed the prevalence of sensitivity to tree pollen. From 401 atopic patients, 309 (77.1%) exhibited positive skin-prick test for at least one of the 12 investigated tree species pollen. Among these, 37.4% showed a positive skin prick test to Ulmus sp., which was highly prevalent, but in correlation, had a lesser percentage of tree population within the study area. Therefore, as per the study, this could be caused by the cross-reactivity with other trees, like Morus (mulberry trees) and Urtica (nettle trees), for which pollination was reported higher than Ulmus (elm) (10).

A Turkish study evaluated pollen sensitization on 54 patients suffering from seasonal allergic rhinitis. Trees were regarded as the most common pollen source, accounting for 95% of pollen's total amount in the atmosphere. Among the subjects, 10 (18.2%) individuals showed positive SPT to elm pollen (Ulmus)(11).

Worldwide distribution

Elm is found in eastern parts of the United States and Canada (3). Elm pollen is abundantly found from January to April in Spain (Europe) (7). Its pollen is also seen in middle eastern countries like Turkey between March and early May (11). 

Main

The main route of exposure for elm tree pollen is through inhalation (airway) (12).

Secondary

Contact urticaria (allergic reaction) was reported in one of the case studies after exposure to the elm tree (13).

Exposure to elm pollen may induce allergic reactions like allergic rhinitis, asthma, and in rare instances, contact urticaria. More than half are allergic to pollens in a population suffering from persistent rhinitis (9). Elm is known to be one of the prime contributors of allergenic pollen in the atmosphere. Some studies reported severe asthma induced by common tree pollens such as elm pollen, which might further increase the patient's hospitalization risk (6). According to studies, allergic rhinitis mediated complications induced by aeroallergens such as tree pollen may lead to a learning disability, cognitive impairment, sleep deprivation, and diminished quality of life (9).

Allergic rhinitis

A cross-sectional epidemiological survey was performed on asthmatic and/or allergic rhinitis patients (n=6304) in 17 cities of China. The result revealed elm tree pollen sensitivity in 4.5% of the total patients with moderate-severe persistent rhinitis (n=339) and 9.1% of 281 patients with moderate-severe intermittent rhinitis (14).

Asthma

In a daily time-series experimental study conducted in Canada, elm tree pollen was responsible for a 2.63% increase in the risk of asthma-mediated hospitalization. The study concluded that the disease burden (such as asthma) is correlated to the tree pollen potency and its variation in the atmospheric concentration (6). Another cross-sectional study in China on patients (n=6304) with asthma and/or allergic rhinitis reported a 4% prevalence of IgE-positivity to elm allergen among 628 patients with severe persistent asthma (14).

Contact Urticaria

In a case study report of a 39-years old male patient without any history of contact dermatitis, contact urticaria occurred after exposure to elm tree (13).

Prevention strategies

Avoidance

One of the main measures to be taken is to cut down the pollen amount in the respiratory system. During pollen season, allergic patients are suggested to stay indoors or wear a face mask outdoors to keep away the pollen allergens. As per the severity of the condition, patients might be advised to move to a non-pollen area (15).

Allergenic molecules

No allergens have been identified for elm tree pollen to date.

Cross-reactivity

Elm pollen is speculated to cross-react with Morus and Urtica pollen, which are species belonging to the same Urticales order as elm (10). Studies have demonstrated cross-reactivity of elm pollen with peach and birch pollen (16).

According to a study, allergy to melon was associated with pollen allergy, as co-sensitization to pollen is usually seen in melon-allergic patients. A significant sensitization to tree pollen like Ulmus (elm) and weed pollen was observed in melon allergic patients  (17). 

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Fabio Iachetti

Last reviewed: December 2020