Bermuda grass
Summary
Bermuda grass is a perennial grass that grows in the warm-season and used globally as lawn grass, forage grass, and for soil protection. It has scaly rhizomes and flat stolon, cultivated mainly in the tropical and subtropical regions of the world. It can grow on any type of soil such as alkaline soils, in drought conditions, high temperature, intensive sunlight. It belongs to the genus Cynodon, family Poaceae, and subfamily Chloridoideae. The pollen grain is round to oval.
It can be found throughout the world. A Filipino study reported Bermuda grass to be one of the most common outdoor allergens.
A study in Kuwait reported a prevalence rate of 55% sensitization to Bermuda grass by allergic rhinitis patients aged 6-64 years.
The Bermuda grass allergen is associated with asthma, allergic rhinitis, atopic dermatitis, sinusitis and conjunctivitis. A study conducted at Saudi University reported the prevalence of physician-diagnosed allergic diseases to be asthma (27%), atopic dermatitis (13.1%), and allergic rhinitis (5%).
Bermuda grass pollen has seven allergens and the major allergen is Cyn d 1. Mutual sensitization to profilin and/or polcalcin is linked with a longer duration of allergic disease and to co-sensitization to a larger number of species-specific allergen molecules. Partial cross reactivity with Bermuda grass can be seen with Timothy grass and maize species.
Epidemiology
Worldwide distribution
Bermuda grass is a native perennial turfgrass of the Mediterranean region but can be found throughout the world. A recent study conducted on adult Filipinos with allergic rhinitis (AR) stated Bermuda grass (67%) to be the most common allergen present in the environment. Similarly, Botswana also has the highest aeroallergen sensitization of Bermuda grass (41%) and it was found to be significantly associated with conjunctivitis (p< 0.001) and rhinitis (p< 0.01).
It is practically suitable for every ecosystem due to its eco-physiological and genetic traits coupled with both forms of propagation. Ontario had also reported high sensitization rates for Bermuda grass. It has also contributed to pollen rain in the semiarid environment of Tucson, Arizona and increases the risk of AR and asthma. A survey conducted by the Orange County Society of Allergy and Clinical Immunology in California also reported the highest (85%) positive rate for Bermuda pollens.
Risk factors
Pollens from flowers of Bermuda grass are hardly visible to the naked eye but can lead to major allergenic blow. Grass pollen sensitization imposes a threat of allergic respiratory diseases across the world due to its airborne nature. This pollen accounts for almost half of immunoglobulin E (IgE) related to allergies. Primarily, it leads to high production of allergen-specific IgE and on secondary sensitization, it leads to cross-reactivity.
Pediatric issues
Taiwan reported Bermuda grass to be highly sensitized pollen among asthmatic children (8%). Bermuda grass was also considered to be the most prevalent allergen among children with AR.
Route Of Exposure
Main
Exposure to Bermuda grass pollens may occur due to inhalation or skin contact. Changes in climatic conditions, pollen-derived nanovesicles and other small components have an explicit role in the development of allergic diseases.
Clinical Relevance
Allergic rhinitis
Allergic rhinitis (AR) is a heterogeneous disease. A 19-year retrospective study conducted between 1998 and 2017, reported a statistically significant increase in Bermuda pollen sensitization from 13.64% in 1998 to 30.9% in 2017.
A study conducted on 706 AR patients aged 6-64 years in Kuwait, reported a prevalence rate of 55% sensitization to Bermuda grass. Similarly, a study conducted among the Thai population (n=100) suffering from AR, reported 17% of the patients sensitive to Bermuda pollens.
In northern New South Wales, Australia, an association between Bermuda grass and AR in adults was reported.
Asthma
Pollens of different subtropical grasses, including Bermuda grass causes AR and worsens the condition of asthma. AR and asthma patients show higher allergic sensitivity and subfamily-specific immunoreactivity with pollen of subtropical grasses, compared with temperate grass pollens. This can be found across subtropical regions including Australia, Africa, India, and the USA.
In Southern China, positive tests for Bermuda grass specific IgE (sIgE) was reported for 21.8% of patients with AR, 22.8% for patients with asthma, and 22.7% for patients with both AR and asthma
In Spain from 101 patients with asthma, 70% were found to be highly sensitive to Bermuda grass.
Atopic Dermatitis
Students at Saudi University reported the prevalence of physician-diagnosed allergic diseases to be 27% for asthma, 13.1% for atopic dermatitis, and 5% for AR. The skin prick test was found to be positive for 20.8% of students for Bermuda grass.
Other diseases
Bermuda pollens that come along with sandstorm dust also triggers various airway illnesses. Reports also stated that Bermuda pollens are associated with sinusitis.
Prevention And Therapy
Allergen immunotherapy
Allergen immunotherapy may recover the patient from symptoms, reduce medication requirements, and improve quality of life. It can be of two types- Subcutaneous injection immunotherapy (SCIT) and Sublingual Immunotherapy (SLIT).
Prevention strategies
Avoidance
Avoidance from allergen is highly effective in seasonal allergic rhino-conjunctivitis patients. Aggravators such as smoke, traffic pollution should also be avoided as it may worsen AR. Other preventive strategies include wearing sunglasses, nasal filters, usage of balms and ointments for the nose. Minimizing outdoor activity when pollen is highest, avoiding going out during/after thunderstorms, pollen season, shutting windows and showering after high exposure.
Cross-Reactivity
Bermuda grass is comprised of widely distributed and genetically diverse nine species. The strongest genetic similarities were found between species C. aethiopicus, C. arcuatus, C. transvaalensis, C. plectostachyus, C. incompletus and C. nlemfuensis. Cross-reactivity had been reported between giant Bermuda grass extract and Bermuda grass.
There is a cross-reactivity between Phl p 1 of timothy grass and Cyn d 1 of Bermuda grass that may obstruct the identification of the allergen source causing sensitization.
The close allergenic relationship is reported between Salt and Grama to Bermuda grass. Johnson grass also shares some allergenicity with Bermuda grass.
A study conducted to determine the degree of antigenic and IgE-binding similarities found the lowest similarity between Maize (23%) and Bermuda (10%).
Bermuda grass has a low degree of cross-reactivity with Kikuyu grass.
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