Bahia grass
Summary
Pollen from bahia grass (Paspalum notatum) is an important allergen, especially in the warm subtropical regions. Bahia grass belongs to the tribe Paspaleae of the genus Paspalum. Originally found in Central and South America, it was later introduced to other regions of the world, including North America, Africa, Asia, India, and Australia. Bahia grass is wind-pollinated and presents a prolonged pollination period, from spring through autumn in Australia. Grass pollens of Bahia grass can trigger IgE-mediated allergenic responses like allergic rhinitis and asthma in sensitized individuals. The major allergen identified is Pas n 1 which belongs to group 1 family of allergens. Another allergen identified in Bahia grass is group 13 allergen termed as Pan n 13. Further, Bahia grass shows clinically relevant cross-reactivity with pollens of other temperate grasses.
Epidemiology
Worldwide distribution
The Pooideae grasses are abundantly found in parts of Africa, India, Asia, Australia, and America and the pollen of Bahia grass is prominent in some regions, like subtropical Australia, Gulf states, and Cuba. It is considered as one of the major allergies (rhinitis) causing pollens in these regions.
Sensitization to Bahia grass pollen also seems to be a major problem in the United States (US) pediatric population. With a sensitization rate of 38 % amongst military children with allergic rhinitis (AR), pollen from Bahia grass was shown to be the most frequently affecting grass pollen.
Also, in other parts of the world, sensitization to Bahia grass pollen has been reported to be of significance. A retrospective cross-sectional study in Argentina found that 69% of 894 patients with seasonal AR were sensitized to extracts of Bahia grass along with Sorghum halepense.
A study conducted in New South Wales, Australia found a 31.6% prevalence of sensitization for Bahia grass among 206 residents. Another study in Australia found that 85% of 34 grass pollen allergic patients with seasonal rhinitis were sensitive to Bahia grass pollen. Further, a retrospective (2001-2014), cross-sectional multicenter study in Sydney, Australia reported 43.8% of 755 patients sensitized to Bahia grass aeroallergen based on positive skin prick test (SPT). In Victoria, a temperate region of Australia, 78% of patients with seasonal rhinitis, with or without asthma, who were allergic to grass pollens, tested positive for the SPT to Bahia grass. In a subtropical region in Brisbane, Australia, SPT, and plasma IgE reactivity in the 233 patients included in a study were significantly higher (p<0.0001) with Bahia grass pollens than the Johnson grass, Bermuda grass or Ryegrass pollen.
Route Of Exposure
Main
Pollens from Bahia grass can enter the respiratory tract through inhalation. They can induce allergic reactions like allergic rhinitis and asthma, either by encountering the nasal mucosal surface or by inhalation of allergen particles (reduced size by hydration of pollen grain).
Clinical Relevance
The pollens of Bahia grass can provoke IgE-mediated type I hypersensitivity reactions in sensitized individuals.
Allergic rhinitis
Pollinosis, commonly known as seasonal AR, is caused by Bahia grass allergens in individuals who are sensitive to its pollens. AR or allergic rhino-conjunctivitis symptoms are seen once the allergen comes in contact with the mucosa of the airway and the conjunctiva in patients who were previously sensitized to these allergens. It is also considered a major trigger for AR in Australia, especially in subtropical regions like Brisbane where this grass is dominant.
It was found to be statistically associated [Odds ratio (OR)-14.47, Confidence Interval (CI) – 1.99-298.21] in a study conducted on 153 atopic patients sensitive to one or more aeroallergens in New South Wales, Australia. In a study on 18 Bahia grass pollen allergic patients in Melbourne, Australia, all the patients were clinically diagnosed to have allergic rhinitis.
Asthma
A study in Australia found that an increase in the concentration of grass pollens including Bahia grass, in the atmosphere was linked with a considerable increase in the visits to the hospitals' emergency department and increased admissions for asthma. This was attributed to Bahia grass and Bermuda grass pollens especially in subtropical regions like Brisbane where these grasses are dominant.
A study in New South Wales, Australia conducted on 21 atopic children sensitive to one or more aeroallergens, observed a statistically significant association (OR – 12.00, CI – 1.06-315.54) between Bahia grass and asthma in children. Bahia grass pollen sensitization was found in 28% of 99 patients with acute asthma in Cartagena, Colombia. Further, another study on 31 Bahia grass pollen allergic patients in Australia found that 17% were showing symptoms of asthma. Similarly, another study in Melbourne Australia reported only 33% of 18 Bahia grass pollen allergic patients suffering from asthma.
Cross-Reactivity
Cross-reactivity between Chloridoid (such as Bahia grass and Bermuda grass) and Pooid (such as perennial ryegrass, timothy grass, and Kentucky bluegrass) members have been observed. Part of the cross-reactivity may be attributed to the major allergen Pas n 1 that has been shown to cross-react, at the T-cell level, to another group 1 grass pollen allergens like Lol p 1 (ryegrass) and Cyn d 1 (Bermuda grass). Pas n 1 has shown homology to other group 1 allergens like maize (85%), ryegrass or timothy grass (64-66%), and Johnson grass (63%). However, since unique epitopes are present in subtropical grass pollen allergens that are not found in temperate grass pollen allergens, the overall cross-reactivity is usually considered to be limited. This was confirmed by a study that reported that extracts of temperate (Ryegrass and Timothy) grass pollens are unable to completely inhibit IgE reactivity of pollen extracts of subtropical species (Bahia, Johnson, and Bermuda grasses). It was found that IgE reactivity with Bahia grass pollen (Pas n 1) was also inhibited by Lol p (Lolium perenne), Phl p 1 (Phleum pratense), Sor h (Sorghum halepense) and Cyn d 1 (Cynodon dactylon).
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