Grey alder (Alnus incana) belongs to family Betulaceae which also consists of birch, hazel, oak, and hornbeam tree species. It is a deciduous tree that can grow up to 24 meters. Its flowering season is from February till May and sheds pollen of size 15-40 µm. It usually favors high humidity thus can be seen near riverbanks and marshy lands. This species originated from central Europe and spread to Russia, Siberia, Asia, the US and other parts of the world. Alder is one of the significant contributors for pollinosis and inducer of respiratory allergy such as rhinitis, rhino-conjunctivitis as well as bronchial asthma. The main route of exposure for this pollen is through inhalation. To date, no allergens from grey alder pollen have been characterized. But a major allergen from common alder (Alnus glutinosa) Alt g 1 has been recognized. It shows extensive cross-reactivity with major allergens from tree pollen of the Betulaceae family.

Worldwide distribution

A steady rise has been seen in the number of allergies due to tree pollen in many parts of Europe. Studies have shown that about 10 to 15% of Poland's people and around 15 % in Norway suffer from pollinosis. The major contributors to pollinosis are alder, hazel, and birch trees (5).

In a study conducted from 2002 to 2005 in Croatia, pollens from the Betulaceae family accounted for 34% of the total pollen count in 864 patients with respiratory allergy such as rhinitis, rhino-conjunctivitis as well as bronchial asthma. Alder was responsible for 6% of the above count. The number of patients with polysensitization to the Betulaceae family pollen was 12.88%. The highest rate of allergy (46.22%) was found in the 31 to 50 years age-group, whereas the lowest rate of allergy (23.12%) was found in more than 51 years age groups(6).

In a study conducted in Korea in 2017, alder was the second most found tree pollen allergy in children (n=57) with allergic rhinitis or asthma. The tree pollen count of alder in Busan was 2nd highest with 4078 grains per cubic mm. The sensitization rates for alder was 35.1 percent in these children. Among these, oral allergy syndrome was found in 12.3% of the children (7).

Worldwide distribution

Grey alder has its origin from the central parts of Europe. It has extended to France, Russia, Caucasus, and Siberia eventually. Later it became common in Scandinavia and was found to be present in Britain also. One of its subspecies, hirsuta, has its origin from the central and northeast parts of the Asia region (1). The studies have found that alder pollen has been found to have traveled long distances from the Alps to Fennoscandia and from Belarus, Ukraine, to Rzeszow. It has also been reported in Greece (8). In Canada, it is found mainly in Alberta, British Columbia, Yukon, and the US, particularly in West Virginia, Maryland (9). Its subspecies like rugosa and tenuifolia are formerly from the northern regions of the United States and Canada(1).

Main

The main route of exposure for grey alder pollen is through the airway (2). Alder pollen triggers allergic reaction at the concentrations of 50 grains per cubic millimetres in 24 hours (6).

Alder pollen (a member of the Fagales order) is a significant cause of asthma, allergic rhinitis, and allergic conjunctivitis.

Allergic rhinitis (AR) and Allergic rhinoconjunctivitis (ARC)

Allergic rhinitis is a condition affecting the quality of life of the individual. It is accompanied by other conditions like asthma, otitis media, nasal polyps, sinusitis, and lower respiratory tract infections (10). A study was conducted on 88 patients with respiratory allergy (allergic rhinitis with or without asthma). Sensitization to alder allergen was tested through specific-IgE presence and skin prick testing. The results showed sensitization to alder allergen ranged from 13.6 to 25 percent of the patients (11).

Asthma

In one of the Korean studies conducted in 57 patients, allergic rhinitis was seen in 98.2% of patients. The study also reported other symptoms such as allergic conjunctivitis, atopic dermatitis, bronchial asthma, food allergy, and oral allergy syndrome in 56.1%, 42.1%, 28.1%, 24.6%, and 12.3% patients, respectively. Alder was the second most found tree pollen in patients with allergic rhinitis or asthma. The sensitization rates to alder were 35.1 percent among these patients (7).

Atopic dermatitis

Patients with atopic dermatitis might develop the disease flare-up after exposure to an allergen such as grey alder. The most common sites include areas not covered by clothes, including the head and neck region. In a study done in 125 patients with atopic dermatitis, grey alder sensitization was found in 4.8% (12).

Prevention strategies

Avoidance

Alder allergy can be reduced or avoided by staying indoors even when the pollen count is low (13).

Allergen immunotherapy

A double-blind, placebo-controlled study was conducted in 22 patients with allergic rhinitis to evaluate intranasal immunotherapy's safety and efficacy. They received immunotherapy or placebo for 4 months. Immunotherapy consisted of birch and grey alder allergens. During the pollen season, the patients who took immunotherapy showed fewer sneezing episodes and runny nose compared to placebo and these differences were statistically significant. The requirement for drug treatment was also significantly decreased (14).

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Fabio Iachetti

Last reviewed: November 2020