Introduction

Ambrosia (Ambrosia spp.) is a genus of flowering plants in the Asteraceae family that comprises approximately 50 species. The most common include giant ragweed (Ambrosia trifida), western ragweed (Ambrosia psilostachya), and common ragweed (Ambrosia artemisiifolia) [1–3]. Because common ragweed is by far the most widespread species, the terms ragweed and common ragweed are used interchangeably throughout this article. Ragweed species are native to North and South America [4], but as invasive plants they have spread to Europe, Asia, and Australia [1–4]. They were first recorded in Europe in France in 1846 [4]. Today, their main European distribution is found in Russia, Ukraine, Hungary, Poland, France, and Italy [2, 3]. Their rapid spread is of growing concern because ragweed pollen is among the world's most potent airborne allergens, posing a major health risk to people with pollen allergies.

Ragweed in Lithuania

Ragweed was first identified in Lithuania in 1884 near Klaipėda [4]. Although the plant is not widespread in the country, isolated specimens are recorded every year. Ragweed flowers relatively late, typically from late July through August and September. Consequently, ragweed pollen is detected throughout Lithuania during August and September. Changes in agricultural land use and climate warming, particularly warmer and longer autumns, have significantly contributed to the spread of ragweed [4]. Because ragweed pollen is extremely light, it can be transported over long distances and reach Lithuania from distant parts of Europe. Airborne pollen concentrations are monitored and studied by researchers at Šiauliai University.

Ragweed Pollen

Ragweed is an annual, wind-pollinated herbaceous plant that typically grows between 20 and 180 cm in height and produces large quantities of pollen. Its fruits are single-seeded achenes measuring 4–5 mm in length. The pollen (Fig. 2) is nearly spherical, with an average equatorial diameter of 17.4 µm, or slightly oval near the equator, with an average diameter of 18.8 µm. The pollen grains are small, lightweight, thin-walled, and have a non-sticky surface [4]. The threshold concentration capable of triggering allergic symptoms is approximately 10 pollen grains per cubic meter of air. To date, 22 ragweed allergens have been identified and characterized [3]. Amb a 1 is the major allergen, recognized by approximately 95% of individuals allergic to ragweed [2, 5]. Amb a 11 is considered the second major allergen, eliciting reactions in approximately 66% of sensitized patients [2, 6]. Some ragweed allergens are species- or genus-specific, whereas others are panallergens capable of causing cross-reactivity within the Asteraceae family. Ragweed and mugwort (Artemisia vulgaris) both belong to the Asteroideae subfamily of Asteraceae. Nearly all patients sensitized to mugwort pollen also react to ragweed pollen, whereas most individuals sensitized to ragweed do not react to mugwort allergens [2, 7]. The mugwort allergen Art v 1 is homologous to the ragweed allergen Amb a 4, while Art v 6 shares homology with Amb a 1, making cross-reactive allergic responses possible [7, 9].

Clinic

In 1875, the American physician Morrill Wyman first described ragweed pollen allergy as autumnal catarrh [2]. Ragweed pollen-induced allergic rhinoconjunctivitis presents with symptoms that are clinically indistinguishable from rhinoconjunctivitis caused by other airborne allergens. Asthma commonly develops in individuals with ragweed-induced allergic rhinitis. Although asthma symptoms initially follow a seasonal pattern, they may progress to persistent asthma after several years if appropriate treatment is not provided. The rate of progression depends on individual factors, including smoking and genetic predisposition [2]. Oral Allergy Syndrome (OAS), also known as pollen-food allergy syndrome, develops because of cross-reactivity between pollen allergens and proteins found in certain fruits, vegetables, and other plant-derived foods. Symptoms usually affect the mouth and throat, develop rapidly, and typically last 5–30 minutes. They are caused by heat-labile proteins present in fresh fruits and vegetables that cross-react with pollen allergens [10, 11]. For example, individuals allergic to ragweed pollen may develop symptoms after eating melon, watermelon, banana, zucchini, or cucumber [12]. Because ragweed belongs to the group of sesquiterpene-containing plants, direct skin contact with the plant may also cause contact dermatitis [2].

Summary

Over the past decade, Lithuania has increasingly been affected by air masses originating from regions where ragweed has become established and formed extensive populations. As a result, the number of days with detectable ragweed pollen in the air is expected to increase. Current projections suggest that by 2050, airborne ragweed pollen concentrations across Europe could increase fourfold. It is important to recognize that allergic symptoms in Lithuania may also be caused by relatively uncommon plants such as ragweed, despite its limited local distribution.

Dr. Miglė Kuprienė

Vilnius City Clinical Hospital Allergology Center

 

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Read more in the "Internistas" magazine supplement "Allergy. Asthma. Immunology", 2018.