Cottonwood is a deciduous, fast-growing tree that can attain a height of around 175 to 190 feet. This tree is claimed to possess an average life-span of 55 to 65 years. The tree originated from North America and is prevalent in countries like the US, Canada, Sweden, United Kingdom, Netherlands, India, China, Nepal, Australia, Pakistan, and New Zealand. This tree can release pollens in abundance, which might be responsible for the manifestation of allergic reactions. Exposure to cottonwood pollen can elicit allergic rhinitis and asthma among allergic individuals. To date, no cottonwood allergens have been identified in the IUIS database. Cross-reactivity has been reported between tree species from Salicaceae and Fagales order. Species from the Populus (cottonwood) and Salix (willow) genus are also reported to exhibit strong cross-reactivity. 

Worldwide distribution

Aeroallergens (like cottonwood pollens) are regarded as one of the potential contributors of allergic diseases (allergic rhinitis, atopic dermatitis, and asthma) in sensitized individuals upon exposure (acute or chronic) (9).

Cottonwood pollen has been reported to be one of the crucial pollens prevalent in the atmosphere during the tree pollen season in Tehran (Iran) (10).

A study was conducted in the United Arabic Emirates (UAE) on 327 patients (with ophthalmologic, dermatologic, and respiratory diseases) to find the correlation between allergen-specific IgE and skin test reactivity in asthmatic patients. 244 patients responded positively towards the skin prick test (SPT) for the investigated aeroallergens. 33.1% of 244 patients were reported to exhibit positive SPT towards cottonwood pollen (11).

A United States (US) based study was conducted on 345 military children with rhinitis to analyze the aeroallergen mediated sensitization rates. The study confirmed cottonwood tree allergen to be responsible for sensitization in 32.8% of this study population through SPT (12). 

Worldwide distribution

The species from the genus Populus are usually prevalent around the Northern Hemisphere (floodplain forests in several ecoregions) (13). Populus Deltoides (Cottonwood) originated from the US and Canada. It is considered an exotic species in countries like Sweden, the United Kingdom, Netherlands, India, China, Nepal, Australia, Pakistan, and New Zealand (4).

Main

The route of exposure for cottonwood pollen allergen is through inhalation (14). 

Cottonwood tree pollen might induce symptoms like allergic rhinitis and asthma in allergic patients (15).

Allergic rhinitis (AR) 

A US-based study conducted on 562 military children (≤18 years) with rhinitis underwent SPT with an 8-test screening panel (various allergen mixes; n=209) or a 51-aeroallergen standard panel (n=345). 8 patients were excluded from the study based on positive diluent or negative histamine control. 80.3% (277/345) of the patients showed positive SPT towards at least one aeroallergen. 32.8% of the 345 patients exhibited positive SPT towards cottonwood tree allergen (12).

A study conducted serum specific-IgE testing against 16 common aeroallergens among 2268 patients with asthma and/or rhinitis in China. The rhinitis patients were subdivided based on the severity of rhinitis, which included mild intermittent (MII, n = 175), moderate-severe intermittent (MSI, n = 281), mild persistent (MIP, n = 596), and moderate-severe persistent (MSP, n = 339). The rate of prevalence of serum specific-IgE positivity towards cottonwood tree pollen in rhinitis patients was reported as 5% in MII, 6.8% in MSI, 2.7% in MIP, 4.5% in MSP, and with a p-value of 0.050 (15). d

Asthma

In a China-based study, serum specific-IgE testing was conducted on 2268 patients with asthma and/or rhinitis. The asthmatic patients were subdivided based on the severity of asthma, which included mild intermittent (MII, n = 405), mild persistent (MIP, n = 313), moderate persistent (MOP, n = 335), and severe persistent (SEP, n = 628). The rate of prevalence of serum specific-IgE positivity towards cottonwood tree pollen in asthmatic patients was reported as 2.8% in MII, 2.1% in MIP, 3.6% in MOP, and 3.4% in SEP (15). 

Prevention strategies

Avoidance

One of the main measures to be taken is to cut down the pollen amounts in the respiratory system. During the pollen season, allergic patients are suggested to stay indoors or wear a mask when outdoors to keep away from pollen allergens. In worsening conditions, patients may be advised to move to a non-pollen area (16).  

Allergenic molecules

To date, no allergens have been identified and listed by the IUIS database (8).

A research study predicted 28 proteins as candidate allergens from mature poplar pollen through proteomic analysis. As per the study, four small heat shock protein (Hsp) and four Hsp70 allergenic molecules were identified from P. deltoides pollen. Similar Hsp allergenic proteins have also been found in soybean, penicillium, cystic echinococcosis, corn dust, and wheat dust. Protein spots similar to Olive (Ole e 1) allergen, thioredoxin proteins, and profilin (similar to allergens in wheat, maize, birch, etc.) have also been identified from P. deltoides pollen (17).  

Cross-reactivity

Pollen from cottonwood (P. deltoides) exhibits high cross-reactivity with willow (Salix spp.) (14). Additionally, moderate cross-reactivity has also been observed between the species of order Salicales and Fagales (18).

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Fabio Iachetti

Last reviewed: January 2021