Birmingham Medical News Blog: Ambrosia artemisiifolia "This Ambrosia was definitely not the food or drink of the Greek gods"

By: H. Wayne Shew, Ph.D. Birmingham-Southern College, Department of Biology in conjunction with Alabama Allergy and Asthma Centers

A. artemisiifolia is the scientific name of common ragweed, the bane of many hayfever sufferers. Ragweed is the leading cause of allergic rhinitis in the U.S., affecting some 75% of allergy sufferers. It is a fall flowering weed that is a particular problem in the Eastern and Midwestern regions of the country where it can be found in abundance in rural areas, and in locations of soil disturbance due to construction, gardening, land clearing, etc. There are 17 species of Ambrosia, all of which trigger allergic rhinitis, but two species are of most importance in triggering allergy symptoms due to their widespread distribution throughout the eastern half of the country.

Common ragweed is not recognized by most people since it is typically small (height of about six inches to two feet, but may reach six feet) with green, unimpressive flowers. As a result, you may have the plant growing close to where you live and work and never realize it may be the trigger of your allergic rhinitis attack. Giant ragweed, A. trifida is more likely to be noticed because of its size, typically 1-4 meters (3 to 12 feet) in height, and having 3- lobed but occasionally 5- lobed leaves, which are 3-12 inches long. The flowers of giant ragweed are like those of common ragweed however, green and relatively inconspicuous. The pollen production by ragweed flowers is anything but unimpressive or modest. Some estimates are that a single plant can produce one million pollen grains in a single day, and some estimates put the number of grains produced at close to a billion over the growing season of a single plant.

You should to learn to recognize and avoid ragweed plants, but if you have this allergy it is impossible to escape contacting the pollen during ragweed season. The pollen produced by ragweed is small, light, and easily transported in the air. Ragweed pollen has been collected as far as 400 miles out-to-sea, and at altitudes of two miles. Ragweed plants typically begin flowering in the northern part of Alabama in late August and continue until sometime in October, usually till frost. Since the pollen travels great distances as part of the aerospora, you might find yourself experiencing symptoms in early to mid-August due to pollen produced in southern areas of the state or Florida. If you suffer from ragweed allergy, you should try and avoid outside activities as much as possible on warm, dry days during the late summer and early fall. Most of the pollen is shed in the morning to early afternoon, but during the peak of the season (mid- to late-September) you will potentially find that there are high levels of pollen present in the air throughout the day.

Ragweed is one of the plants that make up the flora in early successional stages following clearing of land, and in fields following cultivation, if the field is left unattended or if the edges aren’t treated with herbicides or mowed routinely. Hence, if you live near a site of disturbed soil due to land clearing, construction, or cultivation, you may find a lot of ragweed plants growing nearby. This could result in a higher ragweed pollen count in your local vicinity. If you suffer from ragweed allergy, you should avoid areas where large numbers of ragweed plants are growing which could help reduce the likelihood of an allergy attack. However, as mentioned above once ragweed begins flowering in the fall the pollen counts will increase everywhere and it will be impossible to avoid breathing in some of the pollen. Follow your physician’s advice regarding methods to reduce your allergy symptoms.

One additional note, ragweed pollen is also associated with oral allergy syndrome. This is a condition in which proteins in certain foods that resemble the proteins in the ragweed pollen grain lead to the immune system treating the food as a foreign invader, that is, the food proteins show cross-reactivity with the pollen proteins. "Ragweed, in theory, cross-reacts with bananas and melons, so people with ragweed allergies may react to honeydew, cantaloupe, and watermelons, or tomatoes," says Warren V. Filley, MD, from the Oklahoma Allergy & Asthma Clinic in Oklahoma City. You can add zucchini, sunflower seeds, dandelions, chamomile tea, and echinacea to the list of plants that can show cross-reactivity with ragweed allergens.

(Photos below, except for pollen grain, from USDA Plant Database, http://plants.usda.gov)