Rye grass
Summary
Grass pollens are the principal causes of respiratory allergic disease globally. The IgE reactivity to these allergens is manifested by about 40% of allergic patients and 20% of the general population. Perennial ryegrass (Lolium perenne) is a well-known allergenic member of the temperate grasses belonging to the tribe Poeae of the subfamily Pooideae. It is a cool-season forage, hay, lawn, and erosion-control grass, native to Europe but has now spread extensively to the temperate regions of Asia and North Africa, Northern and Southern America, Australia, and New Zealand. It is a potent seasonal grass pollen allergen in temperate regions like Australia, especially in the early summers. Molecular and biochemical characterization of ryegrass has revealed the presence of several allergen components, among which, Lol p 1 and Lol p 5 are the most dominant allergens responsible for triggering specific IgE reactions in more than 90% of sensitized individuals. It can induce allergic reactions affecting the upper and lower respiratory system like allergic rhinitis and epidemic thunderstorm asthma in sensitized individuals through inhalation. The allergen molecules of rye grass show limited cross-reactivity with Bermuda grass, Bahia grass, Timothy grass, Canary grass, Velvet grass, Maize, and Orchard.
Epidemiology
Worldwide distribution
Perennial rye grass pollen (RGP) is a potent seasonal aeroallergen in the temperate zones, especially in the early summers.
It is a major grass pollen allergen in Australia’s temperate regions like south-eastern Queensland. In Melbourne, exposure to perennial ryegrass is the primary reason for seasonal allergic rhinitis (AR) between late spring and early summer. A high prevalence of sensitization to perennial RGP is reported in Melbourne, Australia, which may be attributed to co-existing Bermuda grass sensitization.
Most patients with seasonal allergies in the Australian sub-continental region are sensitized to perennial ryegrass. A study on 330 grass-pollen allergic patients (35.3–46.6 years) with AR was conducted in subtropical (Queensland) and temperate (New South Wales, Western and South Australia) regions of Australia. The skin prick test (SPT) revealed that the sensitization to perennial ryegrass pollen and Lol p 1 was highest (amongst Johnson grass, Bermuda grass, and Bahia grass) in patients from temperate regions. The IgE test for Lol p 1 was significantly higher (p≤0.001) compared to other allergens in patients from South Australia. Further, a retrospective study conducted in Melbourne found 100% sensitization to perennial RGP in 85 asthmatic patients reporting to the emergency departments of a hospital during the thunderstorm asthma epidemic in November 2016.
A longitudinal asthma cohort study in Melbourne was carried out on 479 patients to understand the sensitization to rye grass. The patients were recruited at 7 years of age and underwent regular follow-up till the age of 50 years. Sensitization to rye grass was found to be the least in the non-asthmatic group (3%), increased in the viral-associated wheeze group (14%), and asthma (37%) groups. At 10 years of age, the prevalence in the Severe Asthma group was 45%. In the non-asthmatic group, the prevalence of rye grass sensitization increased from recruitment to 10 years of age (3% to 13%), decreased to 14 years of age (13% to 2%), and increased again to 21 years of age (2% to 12%). In the viral-associated wheeze group, there was an increase in the rate from the recruitment group to 10 years of age (14% to 22%) and from 14 to 21 years of age (23% to 35%). Sensitization prevalence to rye grass presented 2 peaks in the cohort, one from 7 to 10 years of age and the other from 14 to 21 years of age.
A study involving 66 children (5 months to 15 years) having Atopic Dermatitis (AD) evaluated the sensitization prevalence to perennial ryegrass, among other food and inhalant allergens, using IgE reactivity in Virginia, US. It was observed that sensitivity to perennial ryegrass first developed during the pre-school years and was seen to increase from infancy to adolescence. The sensitization prevalence was observed to be 17% in children less than 2 years of age that increased to 57% in 2-5-year-old children. However, a cross-sectional study in Mexico on 173 patients (2 to 64 years) with allergenic symptoms found the frequency of skin reactivity for perennial ryegrass to be 5.20%.
Further, a study in Korea on 2991 children and adolescents with AR found only 2.7% of individuals sensitized to perennial ryegrass.
Risk factors
The risk of rye grass pollen allergy is high during its pollination period. In Melbourne, the peak of atmospheric grass pollen count (APC) is around November end which is linked to the pollination season of perennial ryegrass. Also, the pollination season of perennial ryegrass coincides with the time of the year when thunderstorm asthma (TA) is most likely. The critical contributors to the risk of TA include a trifecta of RGP sensitization namely, serum RGP-specific IgE, seasonal allergic rhinitis (SAR), and thunderstorm exposure in the pollen season.
Pediatric issues
The International Study of Asthma and Allergies in Childhood (ISAAC) Phase Three, a cross-sectional survey, performed on 10,59,053 children [6-7 years (3,88,811 children) and 13-14 years (6,70,242 children)] in 98 countries found the average overall prevalence of rhino-conjunctivitis to be 14.6% and 8.5%, among the children aged 13-14 years and 6-7 years, respectively. It has been estimated that in the United States (US), around 13% to 17% of children live with ARC. Allergic rhino-conjunctivitis substantially affects the daily life activities of the school-going children causing disturbed sleep, limiting school, reduced school performance, and outdoor activities.
Route Of Exposure
Main
The significant route of exposure to RGP is through inhalation. It can lead to the development of allergic reactions affecting the upper and lower respiratory system like allergic rhinitis and epidemic thunderstorm asthma.
Clinical Relevance
Allergic rhinitis
Rye grass pollen is a significant source for AR during the springtime and early summers. Seasonal allergic rhinitis is also observed in individuals sensitized to RGP.
In patients sensitized to perennial ryegrass, typical ocular and nasal signs and symptoms are seen when intact grains of RGP are deposited in the upper respiratory tract through inhalation. A longitudinal asthma cohort study in Melbourne was carried out on 479 patients who were recruited at 7 years of age and underwent regular follow-up till the age of 50 years. It was found that the prevalence of AR increased in 1 year from 1% to 6% in the group without asthma and from 45% to 56% in the asthmatic group, in patients sensitized to RGP. This has important clinical implications as the adolescent period seems to be a significant phase wherein the prevalence of rye grass sensitization and rhinitis increased.
Asthma
A study evaluated patient (aged between 0 to more than 65 years) characteristics elicited during the pollination season for rye grass at New South Wales. Data from 9 inland base hospitals were collected from the past 13 years. It was observed that 53% of peak asthma count days spanned over the rye grass pollen season, establishing that the highest number of asthma cases occurred during the pollination season of rye grass. Patients more than 14 years of age signified 74% of the hospital visits on peak asthma count days during the pollen season while they accounted for 50% of the hospital visits on peak days at times other than the pollen season.
Thunderstorm Asthma (TA) occurs as a trifecta involving clinical AR, RGP sensitization, and outdoor exposure to RGP. TA is characterized by acute asthmatic attacks just after a thunderstorm. Clinical symptoms including breathlessness, wheezing, or cough develop suddenly in patients because of the bronchospasm. Also, recurrent asthmatic attacks are observed after periods of heavy rainfalls or thunderstorms. It may be due to the expulsion of huge amounts of respirable subcellular particles, containing allergens, from hydrated RGP.
The risk of asthma is strongly correlated with serum RGP–specific IgE levels. SAR patients who are sensitized to RGP may manifest bronchoconstriction, even though they may not have an established diagnosis of asthma. This condition is being termed Epidemic Thunderstorm Asthma (ETSA).
Atopic Dermatitis
A study on 479 patients in Melbourne, recruited at 7 years of age and regularly followed-up till the age of 50 years, observed a decline in the prevalence of eczema in a year from 21% to 8% in the viral-associated wheeze group, 45% to 18% in the asthmatic group, from 69% to 28% in severe asthma, from 14 to 21 years of age in patients sensitized to RGP. After 21 years of age, the prevalence of eczema neither increased nor decreased in any of the groups.
Prevention And Therapy
Prevention strategies
Since rye grass pollination may increase the burden of acute asthma, the residents must be warned at the earliest in case of a forecast warning for thunderstorms to prevent the risk of asthma during the ryegrass pollen season.
Avoidance
Individuals who are allergic to rye grass pollen can avoid its exposure by staying indoors and keeping all windows closed, especially in regions where perennial ryegrass is extensively found.
Cross-Reactivity
Cross-reactivity between the members of Pooids (example, Lolium perenne, Phleum pratense, and Poa pratensis) and Chloridoids (example, Cynodon dactylon, and Paspalum notatum) is well-known.
Similarity and sequence identity of nearly 90% has been observed between group I allergens of the subfamily Pooideae, including velvet grass (Hol l 1), timothy grass (Phl p 1), ryegrass (Lol p 1), and canary grass (Pha a 1), suggesting a close relationship between these grass species. IgE-binding (53%) and antigenic similarities (76%) have been noted between rye-grass pollens (Lol p 1) and orchard (Dactylis glomerata). Lol p 1 from ryegrass shares allergenic epitopes found on the timothy allergen Phl p 5. Cross-reactivity has also been noted between cocksfoot (Dac g I) and ryegrass (Lol p 1) and four identical epitopes have been identified between them.
Amino acid sequence homology of 70% is also reported between ryegrass (Lol p 1) and maize (Zea m 1) grass belonging to Panicoideae subfamily of Poaceae grasses.
Perennial ryegrass shows limited cross-reactivity with Bermuda grass. This is possibly due to the lack of group 5 allergen in Bermuda grass. However, Bermuda grass sensitized patients are often found to be sensitized to ryegrass in Melbourne. Lol p 1 and Lol p 2 were found to be cross-reactive with Cyn d 1 from Bermuda grass. Structural homology is found between Cyn d 1 and Lol p 1.
Only limited IgE cross-reactivity is observed between perennial ryegrass (Lol p 1) and Bahia grass Pas n 1. However, 64% sequence identity of Pas n 1 has been observed with Lol p 1.
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