Pollen is divided into three main groups: grasses, weeds, and trees. Our study aimed to determine the most prevalent allergenic pollen among Egyptian patients with respiratory allergies (AR and/or BA). The study was conducted on 200 adult patients with newly diagnosed respiratory allergies.
Most of our patients with respiratory allergies were living in urban areas (65%), which agreed with a previous study that demonstrated that patients living in urban areas had a higher prevalence of pollen-induced AR than patients living in rural areas (23.1% versus 14.0%, P < 0.001) [20].
This observation was previously explained by David Strachan, who noticed a reduced prevalence of asthma among children living on farms as increased exposure to pathogens during childhood resulted in tolerance to different allergens. He concluded that living in urbanized areas leads to reduced pathogen exposure, with a subsequent high risk of allergy [21].
In a study on 25,393 women, residents of rural areas had a lower odds ratio of developing allergies than residents of urban areas, relying on the fact that the former lived in areas with low levels of pollution, thus avoiding traffic-related air pollution, which is the main environmental cause of the high prevalence of allergies in urban areas [22]. Another fact supporting our results was that carbon dioxide (released in urban areas) acts as an important nutrient to plants, which allows the growth of a greater number of plants producing different allergenic pollen in urban areas, resulting in an increased allergy prevalence [23].
21.5% of our patients complained of OAS symptoms. Two large Korean studies conducted on patients with respiratory allergy had results similar to ours, with the prevalence of OAS ranging from 20.0–41.7% [11, 24].
Regarding the SPT results, 186 patients had positive SPTs for different allergens. We detected a higher prevalence of people allergenic to HDM (D. pteronyssinus, 32%), followed by Timothy grass (28.5%), maize grass pollen (28.5%) and D. farinae (28%).
Previous studies reported that 45–85% of patients with respiratory allergies were SPT-positive to HDM [25]. Frankland and El-Hefny were the first to point out the high prevalence of people allergenic to HDM in Egypt, especially D. farinae, attributing Egypt’s dry climate as a major reason behind the high prevalence. Since then, many studies have confirmed the widespread presence of HDM and other mites in domestic settings across Egypt and in the Middle East [26].
The prevalence of pollen allergy differs globally by region, depending on the climatic conditions. For example, Middle Eastern countries are known to have a dry, desert-like climate. Moreover, countries such as Egypt also have coastal regions with a Mediterranean climate [27]. Besides the effects of climate on the high diversity of pollen worldwide, introducing invasive allergenic pollen increases pollen prevalence, with new pollen sensitization [28].
In this study, grass pollen was the most frequent pollen allergen that patients were sensitized to, with 28.5% of patients reacting to Timothy grass and maize grass pollen. This was followed by Russian thistle (24%) and cultivated rye (23%), which are in contrast to tree pollen that has less frequent sensitization (elder, 7%; birch, 6%). Our results agree with the fact that grass pollen is one of the most frequent reasons for plant-related allergenic reactions worldwide. It is thought that at least 40% of patients with respiratory allergies worldwide are sensitized to grass pollen allergens [29], which comprise 9,000 different grass species (Poaceae) [30]. Timo et al. demonstrated that grass was the most frequent cause of pollen allergy in Europe and the USA. This is because grasses grow on every continent and in almost all types of environments. Grasslands were estimated to constitute 20–30% of the vegetation covering Earth, which explains the large percentage of grass pollen allergy globally [31]. Comparative studies from Middle Eastern countries, which include Egypt, Jordan, Kuwait, and the Kingdom of Saudi Arabia (KSA), showed that grass pollen was the most frequent allergen responsible for respiratory allergies among patients, with Bermuda grass being the most frequent in Kuwait and KSA [32–35].
On the other hand, a Chinese study that included 6,043 patients with AR revealed that most of these patients were sensitized to weed pollen [20]. Another study by Sung et al. in Korea found that the sensitization rate to trees (25.2%) was the highest among pollen types, with birch pollen being the highest among tree pollen (10.3%), followed by weeds (19.9%) (mugwort and ragweed) [36].
We found that 66.8% of patients with AR had pollen allergy, whereas 33.2% of patients with BA were positive to pollen allergens. The most prevalent pollen in patients with BA was maize pollen (26.5%), whereas Timothy grass pollen was the most frequent (28.3%) in patients with AR. In our study, tree pollen (elder) was significantly more prevalent in patients with asthma than in patients with AR. This could be explained by the fact that grass pollen, because of its large size, has a low probability of entering the lower airways and triggering asthma [37]. In contrast, tree pollen allergens have been recorded in sizes that are small enough to enter the lower airways, which may cause asthmatic responses in susceptible people [38].
To the best of our knowledge, we are the first to report the relationship between the severity of AR and BA and the types of pollen allergens. Regarding AR, patients with moderate/severe intermittent AR showed the highest SPT positivity to maize pollen (29.2%). Meanwhile, patients with moderate/severe persistent AR showed the highest SPT positivity to Timothy grass pollen (32.4%). Patients with partially controlled asthma showed the highest positivity to maize pollen (30%), which was also true for patients with uncontrolled asthma (37.5%).
We compared the residency of the patients with the types of pollen and noticed that the most prevalent pollen types in rural and urban areas were Timothy grass (34.3%) and maize (29.2%), respectively, which means that grass pollen as a whole is prevalent throughout urban and rural areas. The high prevalence of grass pollen allergy in urban areas can be explained by the elevated levels of CO₂ that have been shown to increase grass pollen production by 50% per flower [39]. Consistent with our results, a study conducted in Finland demonstrated that grass pollen exposure increased with the decreasing level of urbanization. This might be illustrated from the allergology point of view that the most urban environments, with small frequently managed patches of vegetation, have little potential to produce considerable amounts of pollen grains and to expose people to pollen. Thus, staying in urban environments would reduce the overall exposure to grass pollen, although occasionally higher levels of exposure may be encountered [31].
In addition, our study revealed that the prevalence of Bermuda grass pollen was higher in rural areas than in urban areas, and the difference was statistically significant (P value = 0.008). However, Jorge et al. in Colombia disagreed with this, and they aimed to explore whether the growth conditions of the Bermuda grass pollen modify the allergic response of patients with AR in rural versus urban areas. They concluded that Bermuda grass pollen was more prevalent in urban areas, and its growth conditions modify its allergenicity [40].
There were monthly variations in the frequency of exacerbations during the study period. Peaks of allergic respiratory diseases recorded in January, December, March, and June were 26.5%, 21.5%, 18%, and 16%, respectively. This could be explained by the higher rate of viral infections and allergen exposure during winter (December and January), pollination of the majority of pollen during spring (March), and when the grass pollen is at its peak (late spring) (June), with a lower level of exacerbation in summer when the climate becomes drier. Our results could be easily illustrated by research performed on the onset and duration of several allergenic pollen types in the USA, indicating that the start dates for the appearance of significant allergens is prevalent earlier in the year (January). Another fact is that grass pollen is present in the air during late spring and early summer (March and June) [22].
In this study, the most prevalent cross-reactive food with pollen, as evidenced by the SPTs, were oranges (12%), followed by tomatoes (11.5%) and soya (11%). We demonstrated that tomatoes were highly cross-reactive with Timothy grass (43.5%) and maize pollen (39.1%). Oranges were highly cross-reactive with cultivated rye and Russian thistle (33.3%). Soya was highly cross-reactive with cultivated rye (36.4%), Timothy grass (27.3%), and Russian thistle (27.3%). In line with our study, a paper published by the EAACI demonstrated that 60% of food allergies in adults are linked to an inhalant allergy [41]. Another study in Bosnia revealed that 50% of the study population was allergic to Timothy grass, which was highly correlated with tomatoes, melons, and peanuts [42]. Vieths et al. performed a study on the cross-reactivity between allergens and pollen, and they revealed that there was a significant cross-reaction between grass pollen and tomatoes and oranges. This may be attributed to the fact that profilins and cross-reactive carbohydrate determinants, which are present in grass pollen, are also present in different fruits and vegetables, such as oranges and tomatoes [43].
The limitations of this work are that it was conducted in a single center with a relatively small sample size. Moreover, the lack of molecular techniques to study the different allergens used in this study may add to our limitations.
In conclusion, pollen plays a substantial role in patients with respiratory allergies in Egypt. In addition, grass pollen is the most prevalent type of pollen, especially in urban areas.