Introduction
South Africa has one of the largest floral kingdoms in the world, with over 8000 species of flora found in the Cape Peninsula alone. In Southern Africa there are 947 indigenous grass species and 115 naturalised grass species. All of these produce pollen at different times of the year and the levels of pollen in the atmosphere are variable, depending on the season, the presence of wind or thermal currents.
Although abundant pollen is produced everyday, not all pollens cause allergic symptoms. The pollens that are wind dispersed are often allergenic whereas the sticky heavier pollens, produced by brightly coloured flowers and dispersed by insects or birds, are often not allergenic
How do grass pollens cause allergy?
Pollen grasses are carriers of protein substances which when inhaled release these proteins when in contact with a moist surface. These proteins are often enzymes. The pollen proteins bind to IgE antibodies on mast cells lining the airways and this results in the release of substances, e.g. histamine and other mediators which cause swelling and constriction of the airways.
Are pollen grains an important cause to asthma?
Viral infections are the commonest precipitants of acute asthmatic attacks in allergic subjects. However, exposure to pollens in pollen sensitive individuals increases the irritability of the airways in asthmatic subjects to other triggers of asthma, e.g. cold air, exercise and emotional stimuli. In some patients, direct exposure to pollens alone will trigger an asthma attack. These patients typically present with seasonal asthma, particularly in Spring, when a particular grass, weed or tree is flowering. Pollen induced asthma is more common in Europe than in South Africa, since the seasons are more well defined. In South Africa pollens more commonly cause seasonal hay fever than asthma.
Which pollens are important?
The grass pollens are by far the most abundant of the pollens and because they are small they are able to be inhaled and enter the airways of the lungs. Rye grass, Bermuda grass, Kikuyu grass and Eragrostis pollens are abundant, but because the grass flowering seasons in South Africa are very long (from August to April) well defined seasonal symptoms are not always recognised.
Tree pollens, by contrast, are released for short periods (usually only a few weeks in the year in early Spring i.e. August-September) and typically cause well defined seasonal asthma. Important tree pollens include Acacia, Willow, Oak, Plane and Popular. Abundant pollen is also produced by Jacaranda and Port Jackson trees but the relationship between exposure to these pollens and asthma symptoms is not clear at present.
Weed pollens are an important cause of asthma in Europe and America (e.g. ragweed) but are not believed to be important triggers of asthma attacks in Southern Africa. Weeds such as English plantain are commonly found and some individuals are allergic to English plantain.
Flower pollens of the compositae family, e.g. daisies, chrysanthemums and cosmos are unusual causes of Asthma.
What is the value of radio pollen broadcasts?
Pollen forecasting has been found to be useful in Europe where flowering seasons are well studied, clearly defined and pollen release predictable. In South Africa, we have very little data on the value of pollen forecasting. There is a need for detailed local studies of our allergenic flora and the levels of pollen which cause symptoms. Because of the very long grass pollen seasons in South Africa, it is very difficult to “avoid” the pollen season by going away on vacation (as is done in other parts of the world). It is important for patients with tree allergies to be aware of the flowering times of trees. These are more well defined than the grasses and tree pollen is more easy to avoid.
How does one test for pollen allergy?
Pollen allergy can be diagnosed by either skin prick tests or by laboratory tests on a sample of the patients blood. These tests are very reliable for the identification of the grass, tree or weed sensitive patient. It is important, however, that antihistamines should be discontinued a few days before skin tests are performed since antihistamines can block the results of skin tests. Cross reactivity to grasses means that when you react to one grass you are often reacting to another grass.
Cross reactivity between grass pollens is very common, but your doctor should be able to determine whether you are allergic to one or both of the major grass sub-families by screening tests with Bermuda grass and Rye grass. Cross reactivity is less common with tree allergens and individual tests can be conducted for specific trees.
What is the treatment for pollen allergy in asthmatics?
Avoidance is the most effective treatment strategy for pollen allergy. Pollen allergens are difficult to avoid completely, but simple measures such as keeping windows closed, avoiding grass cuttings and avoiding direct exposure to flowering trees or flowers should be attempted.
Pollen exposure is an uncommon precipitant of an acute asthmatic attack, but if asthma is induced, the treatment is the same as for any other asthma attack.
Desensitisation to pollen allergens is safe, successful and recommended if symptoms are predominantly those of rhinoconjunctivitis. Pollen desensitisation for asthmatic subjects is a risky procedure and could induce a severe asthmatic attack. It is therefore not recommended as a routine treatment at present, although further research studies on the effectiveness and safety of this procedure are currently in progress.
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Copyright: Allergy Society of South Africa.
Written by Prof. Eugene Weinberg, Dr Gloria Davis and Prof. Paul Potter
Endorsed by ALLSA.
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