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- Material and methods
Background: Sensitization to pollen and spores of the Southeast Asian tropical region is not well documented. This study evaluated the allergenicity of the tropical airspora in Singapore.
Methods: On the basis of the results of an aerobiologic survey of the airspora profile of Singapore, crude extracts of 23 main spore (fungal and fern) and pollen types were prepared. A total of 231 patients with asthma and/or allergic rhinitis and 76 healthy controls were evaluated by skin prick test (SPT). Total and specific IgE levels were also quantified by the fluorescence allergosorbent test (FAST).
Results: All 23 allergenic extracts tested elicited positive SPT responses. Among the patients with atopic diseases, extracts of oil-palm pollen (Elaeis guineensis) were observed to have the highest frequency of positive reactions (40%), followed by extracts of resam-fern spores (Dicranopteris linearis) (34%) and sea-teak pollen (Podocarpus polystachyus) (33.8%). Fungal spores with the highest SPT responses were Curvularia spp. (26–32%) and Drechslera-like spores (31%). Positive responses to these extracts correlated with total serum IgE levels of the subjects and were significantly associated with the presence of atopic disease.
Conclusions: We have documented sensitization to tropical pollen and spores in our population. Its association with atopy suggests that it has a role in allergic diseases in the tropics.
Allergic respiratory diseases such as asthma and rhinoconjunctivitis are common disorders ( 1). Asthma is a common cause of childhood morbidity in Singapore and is a growing concern there ( 2, 3). Rhino-conjunctivitis is also a major cause of morbidity. The prevalence of rhinitis is reported to be 44% among children ( 2) and 25–33% in adults ( 4, 5).
Airborne pollen and spore allergens have been implicated as one of the main causes of allergic respiratory problems in temperate countries ( 6, 7). In tropical Asia, little information on pollen or spore allergens is available. Aerobiologic surveys in Singapore show an abundance of outdoor airborne spores and pollen year round ( 8). Recent studies have also suggested that the trends for acute asthma exacerbation were associated with variations in the local airspora profile ( 9, 10). In this study, we set out to evaluate the allergenicity of the airspora of Singapore.
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- Material and methods
The aerobiologic survey of the Singapore environment showed the presence of a spectrum of fungi and fern spores and pollen that are unique to this region ( 16, 17). This is not surprising as Singapore is a tropical island city-state with climatic conditions that favor vegetation growth all year round and a characteristically diverse local flora. The use of commercial temperate pollen and fungal extracts for the evaluation of immediate hypersensitivity in our population has a very low frequency of positive response ( 19, 20). The need for a panel of allergens for the diagnosis of inhalant allergies in this tropical region is thus highlighted. This study has thus provided the basis for the delineation of the environmental triggers of allergy in Singapore and the region.
The choice of pollen and spore allergens evaluated in this study was based on the frequency and abundance of each individual type found in our aerobiology survey. Out of more than 100 species of airborne spores and pollen, 23 of the most frequent and abundant were tested. SPT was used as the method of choice in this study as its results have been shown to correlate closely with respiratory allergies ( 13). Nevertheless, in vitro detection and quantification of specific IgE were also carried out to verify these results.
The overall prevalences of skin reactivity among individuals with atopic diseases to at least one of these allergens were 82.7%, 64.9%, and 68.0%, for fungi spores, fern spores and pollen, respectively. The higher number of fungal extracts used (12 out of the 23 extracts) may explain the higher frequency of responses to fungal spore extracts. However, a similar frequency of fungal spore responses among patients with atopic diseases of different age groups was also observed. The presence of these fungal spores, particularly the Curvularia spp. and Drechslera-like spores, in the indoor environment may result in significant exposure from a young age. Our data on the indoor mycologic environment in Singapore showed that the fungal spores present outdoors were also found indoors (unpublished data). In other studies, sensitization to fungal spores has also been found to occur as early as the age of 4 years ( 21).
Although a high proportion of the patients with atopic diseases were sensitized to the fungal spores, most of the reactions were weak. Despite their greater abundance, the lower frequencies and weaker responses to mold spore extracts than to pollen have also been reported elsewhere ( 22). In Singapore, fungal spores were found in the atmosphere perennially, with maximum spore loads reaching concentrations far above 1000 spores per cubic meter of air per day. Elsewhere, others have reported concentrations of over 100000 spores per cubic meter of air even under “normal” conditions ( 22). Nevertheless, most mold-sensitive individuals have frequently been reported to have perennial symptoms of rhinitis and conjunctivitis ( 22), as in the pattern of allergic symptoms in Singapore ( 2).
Very little is known of the allergenicity of tropical pollen allergens, although studies have suggested that they are allergenic ( 23). The finding of a high frequency of sensitization to oil-palm pollen is unique to this study, and is likely to establish recognition of a novel pollen allergen. It is of interest to note that the atmospheric concentration of oil-palm pollen is positively correlated with wind speed and direction ( 17). It is likely that this pollen was blown in by the monsoon from Malaysia, where large plantations of oil palm are found. Our preliminary work on the characterization of the allergenic components of the local airborne spores, such as the oil-palm pollen, revealed multiple allergenic components ( 24). Chakraborty et al. ( 23) also demonstrated the allergenicity of another species of palm pollen in Calcutta, India.
Another pollen, Acacia auriculiformis, had a relatively high frequency of sensitization. Other species of Acacia around the world have also been found to be allergenic. Acacia species in the southwestern USA are predominantly ornamental plants, which shed sufficient airborne pollen to sensitize patients suffering from pollinosis ( 25). In Israel, Acacia is also considered strongly allergenic ( 26), and it may be an important occupational allergen in Mediterranean regions ( 27).
Fern spores represent about 7% of the total outdoor airborne spores in Singapore and are present year-round, appearing to be common in the atmosphere of Southeast Asia ( 28). Fern-spore allergy has rarely been documented, and ferns have not been regarded as an important source of allergens, as their allergens are not readily extracted in solution. However, as shown in this study, sensitization to fern spores does occur and may be related to their abundance in the tropics. In Thailand, Bunnag et al. ( 28) showed that more than 70% of the atopic patients tested responded to extracts of fern spores, and allergy was substantiated by nasal provocation tests. In Israel, sensitization to spore extracts of ornamental ferns has been documented ( 29).
The frequency of sensitization to spore and pollen extracts was significantly associated with the presence of atopic disease and serum total IgE levels of the subjects studied. These findings support the importance of these inhalant allergens in this region. However, sensitization rates to these allergens among young children (3–5 years) with atopic conditions were not significantly different from those of healthy controls. Moreover, the degree of reactivity among these children was observed to be generally weaker than for adults with atopic diseases. In contrast to indoor allergens such as dust mites (or even fungi), the frequency of sensitization to pollen and fern spores increased from childhood to adulthood. These results suggest that the development of pollen or fern spore sensitivity may require longer periods of exposure.
Although dust mites are the predominant sensitizing allergens, this study has identified the pollen and spore allergens of the Singapore environment. The association of these allergens with atopy suggests a role in the pathogenesis of allergic diseases in the tropics. Nevertheless, we are aware that a positive SPT response or the presence of circulating specific IgE does not necessarily lead to or indicate atopic disease. However, our current data showed that we were able to evoke allergic symptoms and responses in sensitized patients by nasal provocation challenge with extracts of these pollen and spore allergens (unpublished). Further characterization of their allergenic components is necessary for a better understanding of these allergens, and to assist in the standardization of allergenic extracts for diagnostic and therapeutic purposes.