Latin name: Salsola kali
Source material: Pollen
Family: Amaranthaceae (Chenopodiaceae)
Common names: Russian Thistle, Prickly Saltwort, Prickly Glasswort, Tumbleweed
Closely related species: Prickly Russian Thistle - Salsola kali ruthenica
Saltwort or Russian thistle originated in Europe and Asia, but has become naturalised and common throughout most arid and semiarid regions of the world, including the central and western regions of Canada and the United States. It is commonly found on the coasts of Europe, North Africa, Asia, North America and Australia.
Saltwort is a rounded, erect annual, intricately branched, growing from 0.3 to 1m in height and from 0.3 to 1.7m in diameter. The stems are ridged and usually red- or purple-striped. The young plants are fleshy and tender, with narrow, dark-green, and fleshy 15mm to 5cm-long pointed leaves. These leaves drop off. The mature leaves are short, broader, stiff, awl-shaped, and tipped with stiff spines.
The plants flower in late summer and autumn. The inconspicuous whitish, green or pinkish flowers are clustered at the base of the leaves along the upper branches. The flowers are hermaphrodite (have both male and female organs) and are pollinated by wind. The winged, 2mm grey to brownish-yellow seeds are cone-shaped, coiled utricles with the coiled embryo visible, and are held in the leaf axils until after plant death. After the seeds mature in late autumn, the plant stem separates from the root, becoming a ‘tumbleweed’. The plant is then blown by wind. Seeds fall to the ground as the plant tumbles, and further dispersal is accomplished when wind scatters the winged seeds. These plants are prolific seeders.
Saltwort or Russian thistle is most common along beaches and sandy shores, in cultivated fields, waste places, and disturbed grassland and deserts. The largest populations occur in semiarid regions.
Pollen grains of the Chenopodiaceae-Amaranthaceae families are very similar, although the plants are very different in appearance. In Europe the highest pollen counts are detected at the end of August and beginning of September. However, significant pollen counts have been demonstrated during May to October, and patients sensitive to Chenopodiaceae may be symptomatic during this period. (1)
Members of the Amaranthaceae and Chenopodiaceae families, e.g. Russian thistle (Salsola kali/pestifer) and Lamb’s quarter (Chenopodium album), survive in aggressive climatic conditions such as dry summers and mild winters. The genus Salsola is one of the most well- known genera of the Chenopodiaceae family, and its species – especially S. kali (formerly S. pestifer) and S. incanescens – are common throughout arid and semiarid regions. (2) These species are also cultivated in desert countries such as Saudi Arabia, Kuwait, and the United Arab Emirates, as a part of ground-greening programs or to avoid erosion of drained zones. They are also spreading throughout areas of the United States and the temperate regions of southern Europe. (3)
In Spain, S. kali is very common in Toledo, Aragon, Andalucia, Murcia and Levante. (4, 5, 6, 7, 8, 9) In these areas, the pollen load may represent up to 5% of the total pollen, being responsible for many allergic sensitisations.
Chenopod has been reported to cause allergy in desert countries, where it is well adapted. (10, 11, 12, 13) The genus Salsola is one of the well- known genera of the Chenopodiaceae family and its species, especially S. kali (formerly S. pestifer) and S. incanescens, are common throughout arid and semiarid regions of Iran.
A significant feature of Chenopod sensitivity is its concomitant appearance with other pollinoses, which probably explains the lack of attention that this allergy has received. (3, 14, 15)
The allergen profile of Salsola has been determined, but not fully characterised. Three allergens were initially isolated, with masses of 39 kDa, 42 kDa, and 14 kDa. (15) The two larger of these are glycoproteins, and were found to be immunologically identical. (16) Other predominant bands were subsequently identified at 15, 17, 18, 29, 58, 69, 74 and 97 kDa. Protein bands with molecular weight of between 36 and 90 kDa were more frequently recognised, especially those with molecular weight of 43, 57, 69 and 81 kDa. (17)
An earlier study, utilising human IgE- and IgG-specific antibodies in in vitro studies, recognized twenty distinct protein bands of Russian thistle pollen extract. Molecular weights ranged from 12.2 kDa to 85 kDa. Allergic subjects had varying individual patterns of protein band recognition. (18)
The following allergens have been characterised:
Sal k 1, a 43 kDa protein, a pectin methylesterase. (17, 19, 20, 21, 22, 23)
Sal k 2, a protein kinase homologue. (19, 24)
Sal k 3, a methionine synthase. (19, 25)
Sal k 4, a 14 kDa protein, a profilin. (3, 19, 26, 27)
Sal k 5, an Ole e 1-related protein. (19, 28)
Of the 39 individuals with a positive allergen-specific IgE determination to S. kali, 26 (66.6%) had detectable IgE antibodies to Sal k 1. (20)
Sal k 1 was recognised by IgE from all 11 patients allergic to Saltwort. Two other allergens of 23 and 14 kDa were isolated and found to be present in 27% (n= 3) and 9% (n= 1), respectively, of the 11 subjects. (21)
Extensive cross-reactivity among the different individual species of the genus may be expected, as well as among members of the families Chenopodiaceae (15, 29, 30) and Amaranthaceae (30, 31) to a certain degree. In particular, high cross-reactivity is well-known to occur between Chenopodium album (Goosefoot) and S. kali. The correlation of sensitisation to C. album in patients sensitised to S. kali has been reported to vary between 70-96% and 70-75% by skin-prick test or RAST respectively. (1, 6, 32)
S. incanescens pollen has been shown to have a close allergenic relationship with S. kali; and it has been shown that in S. incanescens-rich areas, S. kali pollen extracts could be used as a diagnostic reagent for patients allergic to S. incanescens pollen. (2)
However, Sal k 1, a pectin methylesterase, does not have a counterpart in the closely-related family member C. album. (21) A minor allergen from Birch pollen also belongs to the pectin methylesterase protein family. (33) In addition, an allergenic pectin methylesterase was identified in Kiwi fruit. (34) Ole e 11, a pectin methylesterase from Olive tree pollen, has a low amino acid sequence identity with foods such as carrot (23%), orange (25%) and tomato (24%), and higher identity with pectin methylesterase allergens in pollen from Arabidopsis thaliana (57%) and S. kali (54%). (35)
A significant degree of cross-reactivity has been demonstrated between saffron and Lolium, Salsola, and Olea. (36)
A significant but low antigenic cross-reactivity between Annual Mercury (M. annua) and Salsola kali, Olive tree (Olea europaea), Fraxinus elatior, Ricinus communis, Parietaria judaica and Artemisia vulgaris has been demonstrated by several in vitro techniques. (37)
Saltwort pollen commonly induces asthma, allergic rhinitis and allergic conjunctivitis in sensitised individuals. (31, 36,38, 39, 40, 41, 42, 43)
In Sicily, 13.7% of 343 allergic patients were shown to be sensitised to Salsola pollen. Only one patient was monosensitised. (31) In 263 United Arab Emirate nationals with a respiratory disease suspected of being of allergic origin, 45.2% were positive to pollen from the Chenopodiaceae subfamily, specifically Salsola kali. (12)
In a Spanish multicentre study, sensitisation to Chenopodiaceae was found in 555 of 1536 patients; only a few patients were monosensitised (between 0% and 5%, depending on region). The prevalence of asthma was 41% in the total group, compared to just 20% in the 555 patients sensitized to Chenopodiaceae. Although pollen from this family accounted for just 1.6% of all pollen collected, the mean prevalence of positive skin tests in hay fever patients was high (31%), occupying third place (with regard to pollen sensitisation) after grass and olive pollens. (44)
An earlier Spanish study conducted in Madrid evaluated the sensitisation profile of individual allergens in 891 pollen-sensitised patients, in a region with high exposure to Olive pollen. From a panel of 13 purified allergens, which included the most relevant allergens in the area, as well as minor Olive allergens and pan-allergens, Olive tree pollen allergy (Ole e 1 sensitisation) was found to be the leading pollen allergy in the area. Profilin sensitisation was the second most prevalent cause of pollen allergy (associated mainly with grass allergy). Saltwort pollen allergy was the third most common cause of pollen allergy in the area (28.8%) as inferred from Sal k 1, the main allergen of Salsola kali pollen, which was found to be the third most common allergen. Sensitization occurred mainly in central and eastern regions. In some areas, Salsola was the most frequent cause of pollinosis. ( 45) In a more recent study Spanish study of 179 pollen-sensitised patients who underwent SPT with pollen extract and allergen-specific IgE determination against different allergens, 51(28.5%) were skin prick test positive to S. kali and 22 (12.3%) sensitised to Sal k 1. (46)
In another Spanish study, conducted in Zaragoza, sensitisation to Chenopodiaceae was the second most prevalent cause of sensitisation, affecting 42% of the patients with clinical sensitivity to pollen. (1, 47) Symptoms in monosensitised patients were shown to lag high pollen counts by almost two weeks. (1, 48)
In the Alicante Province of Spain, a review of 3066 paediatric patients with asthma or rhinitis attending three hospitals in this coastal region in the south-east of Spain found that the main aeroallergens in this region, in descending order of importance, were: Olive tree, mites, Saltwort, Alternaria, Cat, Dog and grasses. (49)
In an European study, IgE reactivity to the major mugwort allergen Art v 1 in relation to allergens from other weed pollen and cross-reactive components in mugwort-sensitised subjects was studied. The major finding was that the prevalence of IgE reactivity to Art v 1 was significantly higher in samples from North Europe than in those from North America. IgE to Art v 3 in mugwort-allergic patients was more common in North Europe than in South Europe and North America, while IgE to Sal k 1 was similar between the areas. (50)
In the third National Health and Nutrition Examination Surveys (NHANES III) conducted in the United States from 1988 through 1994, 10 allergen and 2 control skin-prick tests were administered to all subjects aged 6 to 19 years, and a random half-sample of subjects aged 20 to 59 years. Of these, 54.3% of the population had positive test responses to one or more allergens. The prevalence of sensitisation to dust mite was 27.5% (the highest recorded); lower percentages were 26.9% for Perennial rye, 26.2% for Short ragweed, 26.1% for German cockroach, 18.1% for Bermuda grass, 17.0% for Cat, 15.2% for Saltwort, 13.2% for white oak, 12.9% for Alternaria alternata, and 8.6% for Peanut. (51)
In Tucson, Arizona, USA – a city that has a high prevalence of allergic rhinitis and asthma – Russian Thistle contributes largely to the overall aeroallergen load. (52) Salsola pollen has also been found to be an aeroallergen in St. Louis, Missouri, and in other parts of the Midwest of the USA. (53, 54)
Salsola has also been shown to be a prominent allergen in Iran. (39, 55) In a study conducted in Mashhad City, the second largest city in Iran, with a semiarid climate, skin-prick tests performed for 27 common regional aeroallergens in 311 patients with allergic rhinitis found that the overall rate of sensitisation to any allergen was 81%. S. kali was the most prevalent allergen, with 72.5% of subjects sensitised. Of the atopic patients, 97% were sensitised to S. kali and/or White Ash tree (Fraxinus Americana). (56) S. incanescens is reported to be more abundant than S. kali in Mashhad; however, S. incanescens pollen has recently been shown to have a close allergenic relationship with S. kali, and in S. incanescens-rich areas, S. kali pollen extracts could be used as a diagnostic reagent for patients allergic to S. incanescens pollen. (2)
A study, conducted in patients with asthma and rhinitis residing in Riyadh in Saudia Arabia, evaluated skin prick test results of 139 Saudi nationals with a mean age of 27 +/-12 years. Seventy-five percent of patients reacted to one or more allergen extracts. Of the outdoor allergens, sensitisation to Prosopis juliflora was demonstrated in 72.1%, to Bermuda grass in 53.8%, to Chenopodium album in 47.1%, to Rye grass in 36.5% and to Saltwort in 36.5%. (57)
Similarly, in a study in the nearby arid country of Kuwait, most allergic patients were reported to become sensitised to pollens; the strongest and most frequent reaction being to the closely-related Saltwort plant, Salsola imbricate. This plant is found growing extensively in most areas of the country, flowering mainly in autumn, when the most common pollen is of the Chenopod-Amaranth type and when most patients with seasonal allergic rhinitis become symptomatic. Patients with asthma or allergic rhinitis (451 patients aged 5 to 60 years) were tested (by means of skin-prick tests) for a battery of allergens; and of these, 403 (89.4%) had a positive test result to at least one allergen, with most being sensitised to Salsola pollen (76.7%), followed by Chenopodium album (57.6%), and 38.2% to Bermuda grass. D. pteronyssinus was positive in only 37.5%, and American and German cockroaches in 33.2% and 22.3% respectively. (58)
Many pollen-allergic patients develop allergy to plant foods, attributed to cross-reactivity between food and pollen allergens. A study analysed the differences among pollen-allergic patients with and without plant food allergy in 806 recruited from 8 different hospitals. Each clinical research group included 100 patients (50 plant food-allergic patients and 50 pollen-allergic patients). A panel of 28 purified allergens from pollens and/or plant foods was used to quantify specific IgE. Six hundred and sixty eight patients (83%) of the 806 evaluated had pollen allergy: 396 patients with pollen allergy alone and 272 patients with associated food and pollen allergies. A comparison of both groups showed a statistically significant increase in the food and pollen allergy subgroup in frequency of: (a) asthma (47 vs. 59%; (b) positive skin test results to several pollens: Plantago, Platanus, Artemisia, Betula, Parietaria and Salsola; (c) sensitization to purified allergens: Pru p 3, profilin, Pla a 1 - Pla a 2, Sal k 1, PR-10 proteins and Len c 1. (43)
Russian thistle may cause dermatitis in persons who come into direct contact with it. The dermatitis appears to be due only to mechanical irritation from plant floral bracts, which pierce the skin and stimulate an urticarial reaction. (59)
Compiled by Dr Harris Steinman, email@example.com
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