Commonly grown weeds are rich sources of pollen and are causing allergy to many people globally. Russian thistle (Salsola kali), belonging to the family of Amaranthaceae which is well known for its allergenic potential. Russian thistle or Saltwort (prickly) pollen came into the limelight as serious allergens due to the extensive desertification brought by the drastic climate change in the last few years. Russian thistle is prevalent in dry, arid and semi-arid regions, although it has naturalized to regions of North America, Africa, Australia, Europe, and Asia. The peak pollen counts are generally seen between August and September. The characteristic feature of Russian thistle is the formation of “tumbleweed” upon maturing. Russian thistle pollen accounts for as a major aeroallergen in some of the regions. About 70% and nearly 50% of patients with pollen allergic symptoms tested positive in Spain and UAE, respectively for Russian thistle. Being an aeroallergen, inhalation of the Russian thistle pollens leads to allergic rhinitis (AR), asthma, rhino-conjunctivitis.

Sal k 1, a pectinmethylesterase, is the most well-characterized and the most potent allergen marker of Russian thistle pollen and is present in more than 50% of individuals prone to it. Other minor allergens include Sal k 2, a protein kinase homolog, Sal k 4,  a profilin, and Sal k 5, which shows high cross-reactivity to Chenopodium album. The first case of allergy to Russian thistle was reported in Arizona, which to date reports a significant number of S. kali pollen allergies. Some regions in Spain like Murcia, Aragón, Andalucía, Levante, and Zaragoza reported for major cases of Russian thistle pollen allergy.

It shows a high level of cross-reactivity with other members of the family Amaranthaceae and with other species like Olea, Kiwi fruit, Birch pollen, etc. Allergen immunotherapy with Russian thistle pollen extracts has proved to be an effective treatment option for allergic reactions caused by this pollen.

Nature

Plants belonging to the family Amaranthaceae are ubiquitous in nature. The pollen of S. kali, (commonly known as Russian thistle or Saltwort), is one of the best-known allergens of this family. It belongs to the genus Salsola. (1)

Habitat

Russian thistle/saltwort is particularly found in salty and semi-arid soils (1). The plants are known to have originated in Europe and Asia but have established themselves globally in arid and semi-arid lands, wastelands, cultivated fields, along beaches, sandy shores, in deserts, or invaded grasslands. (1)

Taxonomy

Living environment

Russian thistle/saltwort can thrive in hostile climatic conditions and can resist droughts(4). Owing to their resistance to drought, they are also found in Arabic and Mediterranean countries(1, 4). It is also specifically used as ornaments and in greening programs in dry regions and deserts (1).

Worldwide distribution

The genus Salsola from the family Amaranthaceae is the best characterized genus from an allergenic point of view. Salsola encompasses various allergenic species of which Russian thistle/saltwort is the most commonly known and characterized allergen (4).

Within the family, 10-30% cases of allergic sensitization are caused by Russian thistle/saltwort. In various regions of Spain, it has become one of the major causes of seasonal allergies(5). In Iran, it has become the main cause of pollinosis (4).Russian thistle/saltwort is a prominent allergen and in a study of 263 patients representing respiratory disorders in the UAE, 45.2% tested positive for the presence of it (6). Sal k 1 is the major potential allergenic marker of Russian thistle/saltwort pollen. In Spain, which accounts for a high number of Russian thistle/saltwort allergies, Sal k 1 is found in 70% of patients showing sensitivity to Russian thistle/saltwort pollen and about 18% in general pollen sensitized patients (5).

Risk factors

Amaranthaceae pollens are very well known for causing respiratory allergies(1). Shafiee and colleagues demonstrated the development of hypersensitivity to Russian thistle pollen in 9 patients through positive skin prick tests after the discovery of the pollen’s ability to cause allergy(7).

Pediatric issues

On studying aeroallergen sensitivity in 3066 pediatric patients with asthma and rhinitis, Russian thistle/saltwort was found to be the second-highest cause of AR and asthma in pediatric population (8)

Worldwide distribution

Russian thistle/ saltworts can be easily found in the United States where rainfall is not abundant and also across coasts in Europe, central Australia, North Africa, North America, and Arab countries(1, 4). Russian thistle/ saltworts are native to Norway, the Baltic States, and in regions of Finland, Poland, Germany, in UK, Spain, France, and Portugal. The widespread distribution of Russian thistle has caused it to naturalize in the United States in California, Oregon, Florida and Washington, Canada, Mexico, Chile, Argentina, Australia, New Zealand and Southern Africa, Indonesia, Japan, and China (2).

Main

Allergic reaction to Russian thistle pollen occurs upon direct inhalation of the pollens which subsequently can cause respiratory disorders (7).

Secondary

Direct exposure to the plant parts can cause mechanical irritation and the pollens can manifest an allergic reaction by piercing through the skin. This causes dermatitis in exposed and sensitized individuals (9).

Allergic rhinitis

The most commonly reported implications of Russian thistle allergy include AR, rhino-conjunctivitis and asthma in prone individuals (7).

AR is the most common respiratory symptom presented in Russian thistle allergies. In a multicentre study conducted in regions of Spain, with 1536 patients having a history of seasonal or perennial rhinitis or asthma, 556 patient (60% women and 40% men) showed sensitization to the members of Amaranthaceae family with monosensitization being present in 0-5% patients among different regions (10). Amaranthaceae pollens showed a prevalence of as high as 31% in patients complaining of AR (10). In another study conducted by Barber et al. understanding the patient’s sensitization profile in areas with mixed pollens, Salsola was the most common cause of seasonal allergies in most of the semiarid regions (11).

Similarly, in Iran Russian thistle/saltwort allergen was found in 72.5% of patients with AR, making it the widespread weed pollen allergen in the region of Mashhad (12).

Asthma

Pollen counts for Russian thistle are highest in August and September. In a study conducted to understand the correlation between pollen counts and allergy symptoms in patients in the region of Zaragoza, which records a high Russian thistle pollen count, it was found that there was a considerable time lag between the times of high pollen counts and manifestations of the first symptom. The time lag was even higher for patients who have asthma(13). In Arizona, the USA, where the allergy to Russian thistle/ saltworts was discovered, the prevalence of allergic symptoms like AR and asthma are very high and Russian thistle/ saltworts contribute to it much (14).

Other diseases

Russian thistle is also known to cause dermatitis in few sensitized individuals, but the occurrence of dermatitis is only limited to those who come in direct contact with the plant. The mechanical rubbing can pierce the skin and cause dermatitis (9).

Allergen immunotherapy

Allergen immunotherapy (AIT) is considered to be one of the important treatment options that may help in providing long-lasting relief to IgE-mediated allergies (15). Studies have been conducted using modified allergen extracts for immunotherapy. A depigmented and glutaral dehy-polymerized extract of Russian thistle showed positive results in immunotherapy with a significant reduction in rhino-conjunctivitis, asthma and skin sensitization reactions in patients (7).

Prevention strategies

Avoidance:

• Borger et al. studied the various physical, chemical, and biological control methods of managing the growth of Russian thistle (2).
• Avoiding direct exposure to weed pollen is the most effective strategy to reduce the incidence of allergies. The allergy incidence rates are highest at the end of August and early September (13).
• Exposure to aeroallergens can be controlled to a certain level indoors using basic measures like closing windows in the peak months, using HEPA filters in air conditioners, staying indoors when pollen rates are recorded high and maintaining basic health hygiene (16).

Allergenic molecules

The following allergens have been well- characterized:

• Sal k 1: 37 kDa, pectin methylesterase, with more than 20 isoforms.
• Sal k 1 is the major allergen of all, present in 80% of Russian thistle/ saltworts sensitization cases and is the first allergen to be identified.
• Sal k 2: 36 kDa, protein kinase homologue.
• Sal k 3: 45kDa, fragmented protein.
• Sal k 4: a profilin.
• Sal k 5: Ole e-1 like protein (4).

Biomarkers of severity

Sal k 1 was characterized as a biomarker for Russian thistle/saltwort pollen since a counterpart is not found closely related Chenopodium album (4).

Cross-reactivity

A very significant cross-reactivity is observed between Russian thistle pollen and other members of Amaranthaceae (5). Marked cross-reactivity is also observed between Salsola, Chenopodium and Mercurialis annua and Salsola, Lolium and Olea (7, 17). Sal k 5 shows high cross-reactivity with Chenopodium album (4).

Allergen from Birch pollen, Kiwi fruit and Ole e11 from olive trees are all pectin methylesterase. Ole e 11, shows a 54% identity in amino acid sequence to Russian thistle (18, 19).

Within the genus, S.incanescens show a significant cross-reactivity to Russian thistle/ saltworts so much that in S.incanescens niche areas, Russian thistle/ saltworts markers are extensively used to detect S.incanescens allergies in patients (20).

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Christian Fischer

Last reviewed: October  2020