Box-elder (Acer negundo) is a rapid-growing, wind-pollinated tree, widely used for shading and ornamental purposes. The tree can attain a height of up to 20 m. The flowering season begins during early spring, with production of the seeds in enormous quantities every year. Box-elder tree is widely distributed across the globe, with its prevalence seen in parts of the US, Europe, Canada, Asia, China, Mexico, and Guatemala. Exposure to its pollen can trigger allergic respiratory symptoms, such as asthma, allergic rhinitis, and rhino-conjunctivitis. Studies have identified an association between increased concentration of tree pollens (box-elder and maple tree pollens) and high hospitalization rates due to severe asthma. Furthermore, contact dermatitis as a result of exposure to box-elder pollen oil has also been found in two patients. To date, no allergens have been identified; however, few potential allergenic protein bands have been reported. Cross-reactivity has been found to be high with maples, walnut and limited with other tree pollens, such as American sycamore, cottonwood, birch, English oak, and weeping willow.

Worldwide distribution

Box-elder tree produces allergenic airborne pollens. Aerobiological surveys have observed a positive correlation between elevated air pollen counts and increased risk of hospitalization in patients diagnosed with asthma and other respiratory diseases (7). An atmospheric survey was conducted in three United States (US) regions (Philadelphia, Pennsylvania, and New Jersey) to analyze the contributors of pollen in the air. According to the survey, 75% and 74.2% of tree pollens were detected in Philadelphia and New Jersey (Cherry Hills), respectively, with the leading contributors from the pollen taxa being Acer, Betula, Pinus, Rumex, Quercus, Poacaea, Cupressaceae, and Ambrosia (8).

In a study conducted in the US, the prevalence of sensitization to pollen allergens in young-asthmatic children (6 months-10 years) was assessed. Of all 687 asthmatic children, sensitization to box-elder pollen was found to be 27% (all ages), based on the positive skin prick test (SPT) results. Moreover, in these children, box-elder was found to be the most common sensitizing tree pollen (9).

In a study conducted in New York between the years 1993 and 2000, sera of 371 patients were tested for assessing the hypersensitivity against specific tree pollens, such as box-elder, birch, oak, beech, ash, poplar, elm, and hickory. The results reported a high prevalence of hypersensitivity towards box-elder tree pollen (32.8%) (10).

Risk factors

An interaction between box-elder pollen and atmospheric pollutants, such as SO₂ and NO_2, was observed in a study that established the effects of these pollutants on allergenicity, protein content, and germination rate of box-elder pollen. According to the study, 18 atopic patients in urban areas were sensitized to box-elder, based on an assessment done by SPT. The results pointed out that box-elder pollen antigens were influenced by the levels of atmospheric SO₂ and NO₂. Furthermore, it has been shown that in-vitro exposure to varying levels of these air pollutants might not only elevate pollinosis-related symptoms but also intensify allergenic reactions in box-elder pollen-sensitized individuals (3).

Worldwide distribution

The species from the Aceraceae family are commonly found in the temperate areas of the Northern Hemisphere and can also be found in the eastern part of Asia and China (6). Among all the American maple species, box-elder is found to be the most extensively distributed species, that extends from the eastern coastline of the US to Californian state and from the South of Canada to the mountains of Mexico as well as in Guatemala. In the US, this species is found in small regions of Western and Central America; however, it is not common in the northern part of North America. Furthermore, even though this tree is not native to Europe; however, it has now become naturalized there too (1). 

Main

The route of exposure for box-elder pollen is through inhalation (airway) (11).  

Secondary

Direct skin contact is another route of exposure. This was evidenced in two case studies where, after coming in contact with the pollen oil of box-elder, the patients developed contact dermatitis (12).

Respiratory allergic diseases associated with tree pollens are on the rise, both in terms of severity and prevalence (7). At present, an estimated prevalence of pollen-related respiratory allergy is found to be ~40% (13). Furthermore, it has been reported that the plantation of box-elder trees for ornamental purposes in urban areas poses a high allergic impact on the residents of those areas (7).

Exposure to box-elder pollen can lead to asthma, allergic rhinitis (AR), and rhino-conjunctivitis, and occasionally contact dermatitis (14, 15). 

Asthma and Rhino-conjunctivitis

Airborne pollens are known to trigger allergic reactions, such as asthma and rhino-conjunctivitis (16). Furthermore, sensitization or allergic reactions to the tree pollens may often cause acute exacerbations of asthma, leading to hospitalization (14).

Two studies discussed about the correlation between increased level of tree pollens (including box-elder pollens) and severe asthmatic attack, leading to hospitalization. The first study was a Canadian study that examined the role of different tree pollen allergies and their association with increased hospitalization due to aggravation from severe asthma. The results showed an increased hospitalization (2.1%) for asthma due to box-elder and maple tree pollens (14). Similarly, the second study that was conducted in Portugal reported that, during peak tree pollination season, 31% of hospital admissions were due to asthma (444 out of 1433 every day admissions). Importantly, the correlation was found to be significant between Acer species pollen concentration and increased hospital admissions due to asthma (correlation coefficient=0.92, p<0.01) (7).

In a study conducted on 687 asthmatic-young children (6 months-10 years), the rate of sensitization to tree pollens was assessed. Interestingly, among all the tree pollen allergen extracts analyzed by SPT, box-elder was found to be the most common sensitizing allergen found in young-asthmatic children (12%) (9).  

In another study conducted in Turkey among 89 patients with rhino-conjunctivitis with/without asthma, 35% of patients were found to be sensitized to box-elder pollen, based on the positive SPT results (16).

Allergic rhinitis

In many parts of the world, allergic rhinitis (AR) has proven to be one of the most commonly found chronic diseases, causing a huge impact on patient’s health and quality of life. Furthermore, airborne tree pollens are considered as one of the major causes of developing AR (15). However, limited data on AR caused by box-elder pollen is available.

In a study conducted in Bolivia among 350 patients (aged between 11 to 60 years) suffering from AR, 15% of patients showed sensitization to box-elder pollen allergen, according to the positive SPT results (15).

Contact dermatitis

An association between box-elder pollen and contact dermatitis, although rare, does exist. This correlation was evidenced in two case studies who developed contact dermatitis (erythematous skin lesions on the face, hands, forearms, and the other exposed areas of the body) while coming in contact with the pollens of box-elder tree (during the fall season and in the months of April and May). The results were confirmed with a positive skin-patch test towards box-elder pollen oil in both the patients (12). 

Prevention strategies

Avoidance

Box-elder pollen allergy can be prevented or reduced by using some precautionary measures. One of the main protective measures against these pollen allergens is by minimizing the number of pollens in the respiratory system. Furthermore, during high pollination period, the pollen-allergic patients are suggested to either stay indoors or wear a mask while moving outdoors to avoid exposure to pollen allergens. Moreover, in extreme cases, where the patient’s health condition deteriorates, it is further advised to move to an area where there is less pollination (17).

Allergenic molecules

No allergens have been identified for box-elder tree pollen to date (18). However, some potential allergenic protein bands, with molecular weights of 71, 52, 35, 32, 19, 16, and 14 kDa have been identified in box-elder pollen extract (7).

Cross-reactivity

Box-elder pollen has shown to display high cross-reactivity with closely-related species maple, which belongs to the same family Aceraceae as box-elder (19). According to literature, high inhibition of IgE binding to red maple was exhibited by box-elder (20). Furthermore, in a study conducted in the US, extensive cross-reactivity has been reported between walnut and box-elder (11).

Limited cross-reactivity has been observed between box-elder and other tree pollens. According to a study conducted in Portugal, box-elder pollen demonstrated high-binding affinity with weeping willow (Salix babylonica) and American sycamore (Platanus occidentalis) pollen extracts, however, limited affinity was observed towards cottonwood (Populus hybrida), birch (Betula pendula), and English oak (Quercus robur). This binding affinity may suggest cross-reactivity (although limited) between these tree pollens (7). 

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Fabio Iachetti

Last reviewed: January 2021