Although traditionally thought to have low exposure rates outside of tropical and subtropical areas, Bermuda grass pollen has been shown by researchers to have high positive sensitization rates in different areas of Canada. “We tested study participants for Bermuda grass as part of a cross-border collaboration with another allergy research group in Texas, and we were surprised to see how many Canadians were sensitized to this non-native grass,” notes Anne K. Ellis, MD, MSc, FRCPC. Such sensitization may result from previous time spent in regions where the grass type is abundant or from a cross-reactive mechanism to local grass pollens. However, with sensitization not necessarily resulting in allergic rhinitis (AR) symptoms when exposed to Bermuda grass pollen, Dr. Ellis and colleagues sought to determine if Bermuda grass allergen extract could provoke AR symptoms in Canadian participants who were sensitized to Bermuda grass using the Allergic Rhinitis-Clinical Investigator Collaborative nasal allergen challenge (NAC). “We wanted to see if these study participants who had a positive skin test to Bermuda grass actually had evidence of clinical allergy/disease,” Dr. Ellis adds.
Getting to the Root Cause
For a study published in the Annals of Allergy, Asthma & Immunology, adults aged 18-65 who were sensitized to Bermuda grass or nonallergic completed a titrated allergen challenge during a screening visit, during which total nasal symptom score (TNSS) and peak nasal inspiratory flow (PNIF) were collected prior to allergen exposure. Those who were sensitized to Bermuda grass but did not have a hypersensitive response received a series of increasing concentrations of Bermuda grass pollen extract, with TNSS and PNIF measured 10 minutes following each. Upon reaching qualifying criteria (TNSS >8 and PNIF fall >50%), the titrated challenge was stopped and the last dilution of extract was deemed the qualifying concentration.
A cumulative single-dose NAC (sum of all allergen dilutions received at screening) was completed at a follow-up visit 21 to 28 days later. TNSS and PNIF were then recorded at 15 minutes, 30 minutes, 1 hour, and hourly until 12 hours after the NAC. Patient-reported symptoms were also collected at 24 hours post-NAC. Symptoms of postnasal drip, nasal congestion and/or stuffiness, nasal itching, sneezing, itchy and/or watery eyes, red and/or burning eyes, and itchy ears, palate and/or throat were self-rated by each participant on a 0-3 point scale.
Significant Differences in Sensitized Vs Nonallergic Participants
Among participants who completed the screening titrated allergen challenge, Bermuda grass wheal size, peak TNSS, and PNIF fall were both significantly greater in sensitized participants than in controls without allergy (Figure). Among sensitized participants, 90% achieved a clearly positive allergic response, according to European Academy of Allergy and Clinical Immunology guidelines, by reaching a TNSS of at least 5 when challenged, while 71% met the predetermined qualifying criteria (TNSS >8 and PNIF fall >50%). All sensitized patients who completed the screening visit also had positive skin prick test results for Timothy grass pollen extract.
During the NAC visit, when compared with nonallergic controls, those with Bermuda grass allergy had significantly greater mean TNSS at 15 minutes, 30 minutes, 1 hour, 2 hours, and 3 hours post-allergen challenge; significantly elevated TRSS at the same time points as well as at 4 hours; significantly greater Total Ocular Symptom Score at 15 minutes, 30 minutes, 1 hour, and 2 hours; and significantly greater mean PNIF fall at 15 minutes and 30 minutes.
Cross-Reactivity With Pasture Grasses?
“Participants with Bermuda grass sensitization do develop subjective and objective signs and symptoms following an NAC,” adds Dr. Ellis. “We believe, that due to the high degree of Bermuda grass sensitization seen and the lack of participants ever having lived in southern US, that patients have a higher degree of cross-reactivity to Bermuda grass if they are allergic to pasture grasses (eg, Rye, Orchard, Kentucky, Timothy) than previously believed, and treating these patients with standardized sublingual immunotherapy tablets that target pasture grasses should indeed be effective. I hope to conduct a pilot study to confirm this theory.”