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g10 Johnson grass

Whole Allergen
Code g10
LOINC 6152-3
Family Poaceae (Gramineae)
Genus Sorghum
Species Sorghum halepense
Route of Exposure Inhalation
Source Material Pollen
Latin Name Sorghum halepense
Other Names Aleppo grass, Arabian millet, Egyptian millet, Evergreen millet, False guinea, Morocco millet, Syrian grass
Categories Grass Pollens

Summary

Johnson grass (Sorghum halepense) is a perennial plant native to the Syria and Mediterranean region. This grass is best adapted to warm, humid climates, particularly areas with high moisture levels, such as irrigation canals, field edges, cultivated lands, pastures, and orchards. The pollination period starts from 6 - 9 weeks after the emergence of the grass and remains throughout the growing cycle. This grass was introduced to the United States as a forage crop around the 1830s. This invasive species is present worldwide, in countries like India, Australia, Turkey, the US, etc. Exposure to Johnson grass pollen may induce allergic reactions such as allergic rhinitis and asthma among sensitized individuals. Allergens Sor h 1, Sor h 2, Sor h 7, Sor h 13, and Sor h 23 from the Johnson grass pollen have been identified. Johnson grass pollen exhibits cross-reactivity with maize, Bahia, timothy, sweet vernal, Bermuda grass pollen which are species belonging to the same family Poaceae and other grasses like Cottonwool, Kikuyu, and English bunch. 

Allergen

Nature

Johnson grass (Sorghum halepense) is a perennial plant that possesses erect, unbranched flowering stems attaining a height between 0.5 - 2.5 m and 0.5 – 2 cm in diameter. This plant consists of fleshy rhizomes (roots) partially coated with brown scale-like covers. The leaves are hairless, 2 - 5 mm in length, with the base surface, frequently covered in a waxy secretion. Each leaf consists of a prominent mid-vein with rough projections on the lower edges of the surface. The grass consists of flat, hairless leaf blades (20 - 60 cm: length; 1 - 3.3 cm: width) that are green and rolled in a bud. In Ontario (Canada), this grass usually starts developing in May, and the flowering season initiates 6 - 9 weeks post-development of the grass and remains throughout the growing season (1). In Nebraska, the flowering starts from July until October (2). The grass has a high seed-producing capacity, which was reported to be approximately 80,000 seeds per growing cycle (3). It is categorized as an invasive, widespread weed in around 53 countries (3). This is due to its aggressive nature, which damages principal crops, hosts numerous pathogens, insects, etc., and even triggers fire hazard (4).

Habitat

This grass flourishes in a wide variety of habitats, including pastureland and wasteland sites (5). Johnson grass is prevalent in temperate and tropical climates worldwide. However, humid summers, warm temperatures, and rainfall in the subtropics are the best thriving conditions for this grass. It is incapable of adapting to low-temperature conditions as rhizomes are intolerant to freezing temperatures (1). It can thrive in a vast range of environmental conditions and is resistant to numerous primary herbicides and microorganisms (3).

Taxonomy

Johnson grass belongs to the family Poaceae and Genus Sorghum. Some of the clinically relevant species from the Poaceae family, observed in subtropical and temperate climatic areas (like Queensland, Australia) are Bahia grass (Paspalum notatum), Bermuda grass (Cynodon dactylon), Maize (Zea mays), Timothy grass (Phelum pratense) and Ryegrass (Lolium perenne) (6). 

Taxonomic tree of Johnson grass  (7)  
Domain Eukaryota
Kingdom Plantae
Phylum Spermatophyta
Subphylum Angiospermae
Class Monocotyledonae
Order Cyperales
Family Poaceae
Genus Sorghum
Species Sorghum halepense

 

Tissue

Johnson grass pollen is a circular, medium-sized (26-50 µm), dry (8) trinucleate (9) grain and is heteropolar in nature (8). To ensure the success of wind pollination, pollens are released in enormous quantities, resulting in an extensive negative impact on health while manifesting conditions like atopic dermatitis, asthma, and rhinitis (10). Some of the critical proteins identified from this pollen are Sor h 1, Sor h 13, which have been recognized as major allergens responsible for triggering allergic reactions in sensitized individuals (11, 12).

Epidemiology

Worldwide distribution

Grass pollens (such as Johnson grass) in subtropical regions are some of the most common causes of respiratory allergies like allergic rhinitis and asthma. In a study conducted in Queensland, large numbers of subtropical grass pollens (liken Bahia, Johnson, and Bermuda) contributed heavily to the pollen count, resulting in allergic respiratory diseases (6). 

A study conducted in the United States to analyze the impact of aeroallergens on 817 individuals reported, 22.8% showed positive skin test reactions towards the Johnson grass pollen allergen (13). 

In Thailand, a study conducted on 100 children aged 0-16 years suffering from asthma revealed 14% were sensitized to Johnson grass (14). Another study in Thailand conducted on 100 patients (aged 10-59 years) diagnosed with allergic rhinitis observed 21% demonstrated positive results on positive skin prick test (SPT) for Johnson grass (15). 

In Turkey, Johnson grass has been identified as one of the important grass pollens, with positive SPT in 41% of the individuals out of 614 allergic patients with respiratory allergy (16). 

Environmental Characteristics

Worldwide distribution

Johnson grass was brought from Turkey to South Carolina as a foraging crop around the 1830s. Colonel William Johnson further sowed this grass to his rich river bottom farm located in Alabama. Eventually, this grass was named after him and is commonly known as Johnson grass (17). 

Although originating from Syria and Europe's Mediterranean region, Johnson grass is prevalent in areas like India, the Mediterranean, the Middle East, Australia, South America (Central and Northern regions), all over the United States, and southern Canada. It is also found in Samoa (introduced from Guam), Pacific islands, Indonesia, Hungary, Uganda, Romania, Turkey, Thailand, Philippines, New Zealand, Japan, and China (4). 

Route of Exposure Section

Main

The main route of exposure for Johnson grass pollen is through inhalation (airway) (13).

Clinical Relevance

Allergic rhinitis (AR) 

A Queensland based study on 64 patients allergic to grass pollen (including Johnson, Bahia, and Bermuda) reported AR in 89.1% and asthma in 45.3% of individuals. Eleven sera of patients with AR showed positive SPT response to Johnson grass pollen and IgE-reactivity to a 30 kDa protein (Sor h 1). The study observed IgE- reactivity towards Sor h 1, Sor h 13, Sor h 2, and Sor h 23 in sera of allergic patients (11). 

In a Taiwanese study, 419 patients suffering from AR symptoms were enrolled; among these, 74.7% (313/419) were observed to have positive SPT towards 30 investigated allergens (perennial allergens and pollen). Sensitivity to total pollen allergens and Johnson grass pollen allergen was reported in 28.2% and 10% of patients, respectively. Patients allergic to perennial and pollen allergens showed symptoms such as nasal obstruction, sneezing, rhinorrhea, and itchy eyes and nose (18). 

Asthma

Skin testing with 14 aeroallergens was conducted in a Thailand based study on 100 asthmatic patients (≤ 16 years). The study reported, 14% of the patients were sensitive towards Johnson grass pollen (14).

A study in Nebraska revealed that farmers exposed to grains from Sorghum grass species (like Johnson grass) were twice as likely to experience respiratory issues compared to farmers in contact with other varieties of grains. Some of the allergic symptoms seen in the impacted framers were coughing, tightness of the chest, and organic dust toxic syndrome (ODTS) (19). 

Prevention and Therapy

Prevention strategies

Avoidance

One of the main measures to be taken is to reduce the pollen amount in the respiratory system. During the pollen season, allergic patients are suggested to stay indoors or wear a mask outdoors to keep away from pollen allergens. In worsening conditions, patients might be advised to move to a pollen-free area (20).

Individuals working on farms have been advised to wear respirators as a preventive measure. (19). 

Molecular Aspects

Allergenic molecules

In Johnson grass pollen allergenic proteins, Sor h 1 and Sor h 13 have been recognized as significant allergens. The IgE-reactive elements of Johnson grass pollen, Sor h 1, Sor h 2, Sor h 13, and Sor h 23 isoforms have been identified and verified using mass spectrometry. The study revealed 76% (49 of 64) and 43.8% (28 of 64) of grass pollen-allergic patients’ sera showed IgE-reactivity to Sor h 1 and Sor h 13, respectively. (11). Sor h 7, a polcalcin allergen, has also been recognized (21). The following allergens in the table have been listed in the IUIS database (12). 

Allergen Biochemical name Molecular weight (kDa) Allergenicity
Sor h 1 Beta-expansin 30-35 49 patients of 64 showing positive results for SPT to Johnson grass, 76% were observed to have IgE reactivity with Sor h 1 (11).
Sor h 2 Expansin-like protein; grass pollen group 2 allergen 12 Serum from patients with positive SPT to Johnson grass showed IgE-reactivity to isoforms of Sor h 2 (Sor h 2.01 and Sor h 2.02) (11).
Sor h 13 Exopolygalacturonase (Glycosyl hydrolase 28) 54-55 (glycosylated protein) Sera from 28 (48.3%) out of 64 patients sensitive to Johnson grass pollen showed IgE-reactivity to Sor h 13 (11).

Biomarkers of severity

In a study, sera from 64 individuals allergic to grass pollen were examined using serum IgE reactivities with pollen and purified allergens. The study revealed purified Sor h 1 and Sor h 13 as clinically critical allergen elements of the Johnson grass pollen with serum IgE-reactivity of 76% and 43.8%, respectively (11). 

Cross-reactivity

Cross-reactivity is observed among the sub-species Panicoideae such as Johnson, Maize, and Bahia grass pollen (22-24). Johnson grass exhibits cross-allergenicity with grasses like timothy, sweet vernal, June, meadow fescue, ryegrass, red top, and Bermuda grass (25). As per a study, partial inhibition of IgE-reactivity towards sub-tropical Johnson, Bermuda, and Bahia grasses by temperate Ryegrass and timothy grass pollens was evident. This could be due to the unique epitopes in sub-tropical grass pollen allergens which are absent in temperate grasses (6). IgE cross-reactivity between subtropical grass (Johnson, Bermuda) and temperate grass (Ryegrass) may be responsible for inducing allergic reactions in individuals from either temperate or subtropical areas (26).

A study conducted on 133 patients in India exhibited various degrees of cross-reactivity within the investigated grass pollens (Bermuda grass, Cottonwool grass, Kikuyu, Johnson grass, and English bunch grass) as per ELISA inhibition experiments (27).

According to a study, high immunological cross-reactivity was evident among taxonomically related grass species, like Johnson grass (Sor h 1), ryegrass (Lol p 1), Bermuda grass (Cyn d 1), timothy grass (Phl p 1), and velvet grass (Hol l 1) (28). 

Compiled By

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Christian Fischer

 

Last reviewed: January 2021

References
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