g2 Bermuda grass

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. 

 Nature

Bermuda grass is a perennial grass having horizontal shoots and the leaves coming off the stem, interchangeably. The bud contains 3-7 purple colored flowering spikes. It is planted in lawns, but the runners are very robust and are hard to manage and invade flowerbeds (1). The flowering period is usually between May and September (2).

Habitat

Bermuda grass grows in warmer regions. Although, it is immune to very high temperatures, yet is prone to hard or persistent frost. It is exclusively prevalent as a weed in subtropical conditions in both annual and perennial crops and in pastures, fallows and waste areas. It occurs in a wide range of soil types under semi-arid and irrigated conditions with varying pH and salinity. It can be easily found in waste places, agriculture fields, irrigated ditches, streambeds and also on the sides of the roads (1, 3).

Taxonomy

Figure 1: Phylogenetic tree of Bermuda grass (C. dactylon) (6) 

Tissue

The Bermuda grass pollen grain has a round to oval shape with a diameter of 33 µm. The pollen grain is operculate (2)

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 (7). 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) (8).

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 (9). It has also contributed to pollen rain in the semiarid environment of Tucson, Arizona and increases the risk of AR and asthma (10). 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 (11). 

Risk factors

Pollens from flowers of Bermuda grass are hardly visible to the naked eye but can lead to major allergenic blow (12). 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 (13). Primarily, it leads to high production of allergen-specific IgE and on secondary sensitization, it leads to cross-reactivity (14). 

Pediatric issues

Taiwan reported Bermuda grass to be highly sensitized pollen among asthmatic children (8%) (15). Bermuda grass was also considered to be the most prevalent allergen among children with AR (16).

Worldwide distribution

It is found in all of Europe except Scandinavia, in the USA, South Africa, Australia, India, and Japan. The origin is Africa but it is found throughout the tropical and subtropical regions including Asia, North, Central and South America, the Caribbean, and islands in the Pacific Ocean (5). It is the most common tropical lawn grass and is sensitive to extended frost. It is mostly used as fodder in Brazil. In the Southern US, it is considered as an imperative pasture grass. Commercial cultivations are done for several genotypes and hybrids of Bermuda grass and over one thousand accessions are now offered at the U. S. National Plant Germplasm System (3). 

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 (17).

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 (18).

A study conducted on 706 AR patients aged 6-64 years in Kuwait, reported a prevalence rate of 55% sensitization to Bermuda grass (19). Similarly, a study conducted among the Thai population (n=100) suffering from AR, reported 17% of the patients sensitive to Bermuda pollens (20).

In northern New South Wales, Australia, an association between Bermuda grass and AR in adults was reported (21). 

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 (22).

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 (23)

In Spain from 101 patients with asthma, 70% were found to be highly sensitive to Bermuda grass (24). 

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 (25). 

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 (26). 

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) (27). 

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 (27). 

Allergenic molecules

Bermuda pollens contain seven allergens. It also contains species-specific allergen. Among the following, Cyn d 1 is the major allergen as its reactivity with IgE is the highest (22, 28).

• Cyn d 1 (a major Group 1 allergen, consists of 11 isoforms; Beta-expansin; 32 kDa),
• Cyn d 7 (Polcalcin, calcium binding protein; 12 kDa),
• Cyn d 12 (Profilin; 14.5 kDa),
• Cyn d 15, (9 kDa),
• Cyn d 22 (enolase),
• Cyn d 23 (9 kDa),
• Cyn d 24 (Pathogenesis-related protein PR-1; 21 kDa),

The major allergen found in Bermuda pollen is Cyn d 1. It can be found in 76-100% of patients allergic to grass pollens. Increased levels of IgE to Cyn d 1 can be found at primary sensitization (17).

Cyn d 1 allergen may occur as a single 34 kDa protein or as a mixture of 34 and 29 kDa proteins. It contains two isoallergens known as Cyn d 1a and b. Cyn d 1a is a 32 kDa protein, and Cyn d 1b is a 31 kDa protein. Around 50% of Bermuda sIgE shows reactivity to Cyn d 1 whereas only 10% shows to Cyn d 7 (22, 29).

Mutual sensitization to profilin and/or polcalcin is linked with a longer duration of allergic disease and co-sensitization to a larger number of species-specific allergen molecules (30). 

Biomarkers of severity

Serum specific IgE antibodies to nCyn d 1 is a molecular specific biomarker of genuine sensitization to Chloridoideae grass pollen i.e. Bermuda grass (30). Cyn d 1 is the most abundant pollen in Bermuda grass, concluding to 15% of the whole-pollen extract. The frequency of sensitization is from 76-100% (30). As it is immunologically different from Phl p 1 of Timothy grass, it is therefore a suitable marker for sensitization to Cynodon dactylon (30). Moderately higher levels of IgE specific to nCyn d 1 is considered as a marker of sensitization to Bermuda grass (30).

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 (31).

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 (30).

The close allergenic relationship is reported between Salt and Grama to Bermuda grass. Johnson grass also shares some allergenicity with Bermuda grass (32).

A study conducted to determine the degree of antigenic and IgE-binding similarities found the lowest similarity between Maize (23%) and Bermuda (10%) (33).

Bermuda grass has a low degree of cross-reactivity with Kikuyu grass (34). 

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Fabio Iachetti

Last reviewed: October  2020