Johnson grass
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.
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.
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.
In Thailand, a study conducted on 100 children aged 0-16 years suffering from asthma revealed 14% were sensitized to Johnson grass. 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.
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.
Route Of Exposure
Main
The main route of exposure for Johnson grass pollen is through inhalation (airway).
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.
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.
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.
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).
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.
Individuals working on farms have been advised to wear respirators as a preventive measure.
Cross-Reactivity
Cross-reactivity is observed among the sub-species Panicoideae such as Johnson, Maize, and Bahia grass pollen. Johnson grass exhibits cross-allergenicity with grasses like timothy, sweet vernal, June, meadow fescue, ryegrass, red top, and Bermuda grass. 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. 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.
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.
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).
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