White ash is a wind-pollinated tree, belonging to the genus Fraxinus of the family Oleaceae. The trees can grow at varied altitudes i.e., from plains to mountains, and can survive both, in low light and water levels. Fraxinus Americana (White ash) is widely distributed across Europe and North America and expanded to China, Pakistan, India, Afghanistan, Morocco, and Algeria. According to some data, where a large patient population (n=1,006) was taken, the prevalence of allergy to white ash was reported to be ~40%. Further, inhalation has been found to be the primary route of exposure to white ash pollen allergen. Ash pollen allergy can be prevented by avoiding stepping out and playing outdoor activities during peak hours and peak months of pollination It is one of the most prevalent allergens leading to asthma, allergic rhinitis, rhino-conjunctivitis, and atopic dermatitis. Closely related to olive, lilac, and privet, due to similar glycoproteins and high sequence identity, with limited cross-reactivity observed with tree, grass and weed pollens.

Worldwide distribution

White ash is a native plant of Europe that is widely distributed in the entire continent (7).

It can be found throughout the world, that includes France, North America, North East Asia, China, Northern regions of Pakistan, India, Afghanistan, Morocco, and Algeria (8).

In a study conducted between the years 1997 and 1999 in Central Europe, a total of 12812 patients with suspected inhalant allergy were included. Of these patients, a total of 5416 (42.3%) were found to be pollen sensitized. Among the pollen sensitized patients, 17.6% (n=920) of patients were sensitive to ash pollens. Furthermore, in these 920 ash pollen sensitized patients,  9.5% (n=87) were reported to be mono-sensitized, 47.6% (n=438) were oligo-sensitized, and 42.9% (n=395) were polysensitized individuals (9).

In a cross-sectional study conducted in Saudi Arabia, that included 209 patients with allergic clinical presentation, 1.9% of patients displayed positive serum immunoglobulin (sIgI) for white ash (10).

In another cross-sectional study conducted between October 2010 and February 2014, a total of 1,006 allergic patients, aged between 1 to 86 years were included in the study. Of these patients, ~40% were found to be allergic to white ash. Furthermore, ash was considered to be one of the most prevalent allergen in spring (15.5%), summer (46.1%), autumn (22.7%), winter (15.7%) (11).

A survey was conducted in 476 adult allergy patients in Germany, that reported the prevalence of ash allergy (found by prick test reactivity) to be higher in North-Rhine Westphalia (28%) when compared to the Bavaria region (19%) (12).

Risk factors

Ash pollination period varies according to geographical regions. Pollination season begins early in January in California and South-central states, in April in the rest of the United States, and in May in Canada (2).

In recent years, sensitization with ash pollens is perceived as a recurrent cause of spring pollinosis. It is more frequent in polysensitized patients. The impact of allergy due to ash pollens is high due to its overlapping pollination period (birch or grasses pollen) and also because it is not included in routine screening assay procedures for inhalant allergies (13, 14). The rise in pollen production also increases the number of cases of allergic individuals (15). 

Living environment

White ash trees grow at diverse altitudes i.e., from plains to rocky mountains. It can be found either in forests, riverside seeps, slopes, lakes or meadows. Ash tree forests are known as frasinete (Fraxinetum) and are found to be rare (3).

Worldwide distribution

White ash trees are native to North America but are present throughout the world. It can be found from Nova Scotia in the west to Minnesota in the east, and Texas in the south to Florida in the north. It is also cultivated in Hawaii (16). It is comprised of approximately 65 species and grows in temperate areas of the Northern hemisphere (2).

In Europe, it is found from Ireland to the Russian Federation and from Scandinavia to Spain, Italy, and the Balkans. In Romania, it is considered the principal member of the Oleaceae family (3).

Main

The main route of exposure is through airway inhalation. Pollens from white ash trees are wind-pollinated. These pollens are carriers of allergens, however, exposure to the pure allergen is rare, instead, it is exposed through the particles of allergen pollen (17).  Intact pollens cannot enter the lower respiratory tract; and thus, affect the nasal or nasopharyngeal mucous membrane. However, particles of pollens can reach the lower respiratory tract, mainly through respiration by mouth (17).

Inhalation and penetration of pollen allergen particles to the lower respiratory tract is a direct cause of symptoms of pollinosis (17, 18).

White ash dispersion takes place, both by insects as well as wind (ambophily) (19). 

Allergic rhinitis

The most common allergic disease that has an adverse effect on patients’ quality of life is allergic rhinitis (AR). The prevalence and pattern of sensitization vary geographically (20).

A cross-sectional study was conducted (October 2010 and February 2014) in Iran that evaluated the prevalence of ash pollen allergy in patients with AR (n=699; aged 1-86 years). The study reported that 61.1% of patients with AR developed ash pollen allergy (11).

Similarly, in a study conducted in Europe that investigated the prevalence of sensitization to ash pollen allergens in 5416 allergic patients, 17.6% of patients were sensitized to ash pollens (9). In another study performed in Iran, 97% of atopic patients were reported to be sensitized to white ash pollens (20).

Asthma

In a cross-sectional study involving 163 patients (aged between 1 to 86 years) with asthma, the prevalence of allergy to ash pollen was found to be 48% in these patients (11).

In a retrospective, observational study, a total of 113 patients, received nasal provocation test with ash pollen. Of these patients, 58% were white ash allergic to clinical symptoms, whereas 42% were without clinical symptoms. Furthermore, 49% and 45% of these patients reported asthma, respectively (21).

Atopic Dermatitis

In a cross-sectional study that enrolled 40 patients with atopic dermatitis, the prevalence of ash pollen allergy was reported to be 44.4% in these patients (11). 

Other diseases

Ash pollen allergy is commonly not prioritized by allergists. In Europe, birch-pollen allergy usually causes symptoms of rhino-conjunctivitis, and since the pollination period of ash and birch overlap (April/May), it is usually assigned to birch pollen (21).

A cross-sectional study was conducted between October 2010 and February 2014 to study the prevalence of ash pollen allergy in patients with urticaria (n=115; age 1-86 years). Based on the results it was reported that 37.5% of patients with urticaria developed ash pollen allergy (11).

Other topics

In a retrospective study conducted between December 2009 and March 2017, a total of 113 patients underwent nasal provocation and skin prick testing to ash pollen. According to the results, the nasal provocation test found that 58% of 113 patients being sensitized to ash were allergic with clinical symptoms, whereas 42% of patients were only sensitized without showing any symptoms (21).

Allergen immunotherapy

As majority of the patients with ash pollen allergy exhibit polysensitization, identifying the right allergen that causes sensitization is extremely essential for choosing proper allergen immunotherapy (AIT) since molecular diagnosis changes the indication for AIT (22).

Prevention strategies

Avoidance

Ash pollen allergy mainly takes place due to wind pollination. One should avoid going out and play outdoor activities during peak hours and peak months of pollination (23). 

Cross-reactivity

Close phylogenetic relationships among Oleaceae members such as olive, lilac, and privet lead to cross-reactivity. Their main allergens, Ole e 1 (88%), Syr v 1 (82%), and Lig v 1 (91%), belonging to Ole e 1–like protein family, are responsible for high amino acidic sequential similarity with ash pollen allergen (Fra e 1) (7, 24). It is often suggested ash pollen allergic symptoms might be due to sensitization to cross-reactive allergens from other related or non-related sources. Various in vitro studies have shown cross-reactivity between ash pollen and birch pollens (7).

A study was conducted among 40 ash pollen-allergic patients to determine the profile of allergen components in ash pollen. According to the results, allergic symptoms to the pollens of ash are a consequence of sensitization to cross-reactive allergens from tree, grass and weed species (1).

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Fabio Iachetti

Last reviewed: December 2020