Common ragweed (Ambrosia artemisiifolia) is an aggressive plant with its origin most probably from the regions of Northern America. It is recognized as a potential trigger for type I allergic reaction which generally occurs in the later parts of the summer and autumn seasons. It is recognized as a serious health issue in America and various countries in Europe. Allergic rhinitis (AR) due to hypersensitivity reaction to inhaled ragweed allergens, is a prevalent medical problem in all age groups including children, adolescents and adults. Numerous allergens of A. artemisiifolia are found out of which, 22 of them are already well recognized and 6 are tagged as major. Of all the allergens, Amb a 1 is considered as the allergen of paramount importance as 95% of ragweed sensitive individuals have increased levels of IgE and show positive findings to the skin tests. Allergen-specific immunotherapy (AIT), symptomatic medications and allergen avoidance are the main strategies for the prevention and treatment of allergic rhinitis and asthma due to A. artemisiifolia. Significant cross-reactivity exists between common ragweed and mugwort, marshelder or other grass pollens.

Nature

There exist more than 40 species of this genus and common and giant ragweed are the most common ones. Amongst all species, short ragweed is commonly found and an aggressive one which is a major contributor to allergic reactions particularly in the late summer season all around the world. This plant is recognized by its divided stem with leaves originating from either side of a common axis (1).

Habitat

Common ragweed is an aggressive plant with its origin most probably from the regions of Northern America and is now prevailing globally. Climate change, increased urbanization and long-distance transportation increased the ragweed pollen spread and domesticated ragweed in non-invaded areas (1).

Taxonomy

Common ragweed (Ambrosia artemisiifolia) hails from the Angiospermatophyta phylum to  the  Dicotyledonouses  class, to  the Asterales order , to the Asteraceae family, and to the Ambrosia genus. This genus comprises of 42 species (2).

Tissue

Ragweed pollen is tricolporate pollen with a granular surface. It has particle size of lesser than 10 µm and gets deposited in the peripheral parts of the respiratory tract leading to signs of lower respiratory tract infection (3).

Worldwide distribution

The wide penetration of ragweed into European countries has been found documented in the evidences. The Hungarian region of Carpathian Basin is the part with the maximum air pollution with common ragweed pollen in Europe, which ranges from 77 to 87% of the total pollen count. (4) The air pollution due to this pollen is also a burgeoning issue in the countries like Italy and France.(5) In the northern parts of Italy, there is an increasing trend of sensitization towards this particular aeroallergen. (6) In Switzerland also, there is an increasing trend towards measured ragweed pollen counts in Geneva since starting of sampling in 1979. An interesting finding from a survey conducted by National Health and Nutrition Examination, about 26% of the individuals were sensitized to the pollen from ragweed in the US. It is also a principal allergen found in the Canada. In another study done in atopic patients, 44.9% of the individuals were found to be sensitized to the ragweed pollen (3).

Worldwide distribution

Common ragweed, originating from North America, is spreading rapidly across the numerous European nations (8). Over the last decades, an increased occurrence of allergic reaction to common ragweed has been observed in the European countries like France, Austria, Italy, Hungary and a few others (7).  

Main

The ragweed pollens are the airborne and the main route of exposure is through inhalation (6, 9).

Allergic rhinitis with or without conjunctivitis

The pollen from short ragweed is a major reason behind severe type I respiratory allergies, with the incidence of IgE sensitization continuously increasing (10). It is one of the major reasons for seasonal allergic rhinitis (AR) and associated conjunctivitis (AR/C) in the Northern parts of America (11). In the year 1989, around 45 percent of the sensitized patients to common ragweed developed respiratory symptoms in the form of rhinitis and bronchial asthma in late summer. After a few years, an increase in the percentage was observed up to 90 percent (7).  Exposure to ragweed pollen majorly results in allergic diseases such as allergic rhinitis, asthma and allergic skin reactions (1). 

Asthma

AR and asthma are some of the leading allergic conditions which are commonly found to be related to sensitization to the common ragweed pollen. This pollen was recognized well in the early 19^th century as the major factor responsible for AR and asthma. Soon after that, the first eradication program was started in the 1940s by using herbicide (1). The asthma occurrence was observed in about 30 percent of the sensitized individuals which increased up to 40 percent in a short period of time (7).

There are main preventive strategies for asthma as well as rhinitis (allergic) such as allergen-specific immunotherapy (AIT), symptomatic medications and allergen avoidance. 

Avoidance

This strategy effectively controls symptoms of allergy. However, it is extremely difficult to achieve complete prevention. There are various strategies such as the use of air conditioners, closing windows particularly during the day-time, as well as avoidance of outdoor activities amid pollen season at its peak (1).

Allergen immunotherapy

In one randomized controlled trial, adult patients with allergic rhinitis with or without conjunctivitis (AR/C) were included. They were randomized to receive either daily self-administration of 1.5, 6, or 12 units of ragweed AIT or placebo for a period of 52 weeks. Patients who received AIT of 12 units were able to tolerate it without any side effects (12).

Allergenic molecules

Ragweed contains a variety of allergens. Among these, 22 allergens are already well established and 6 are considered major. Various ragweed pollen allergens have been characterized at the molecular level. Amb a 1 is considered as the most important allergen due to positive skin test results in around 95% of sensitive individuals and elevated levels of IgE antibody in serum (10). Ragweed contains allergens such as Amb a 6, Amb a 8, and Amb a 9, Amb a 10. Other allergens including Amb a 1, Amb a 3 to 5 and Amb a 7are also described. Amb a 1 is considered as the major allergen responsible for allergic reaction in most of the patients. Also recently another novel major allergen Amb a 11 was categorized which had cysteine protease activity (10).

Biomarkers of severity

Amb a 1 is a major allergen from Ambrosia artemisiifolia (ragweed pollen). Three different recombinant variants are recognized such as Amb a 1.1 to 1.3 and have been found in the pooled sera of the individuals with an allergy to ragweed. A small number of individuals with IgE reactivity is found primarily with Amb a 1.1, though the majority of them show reactivity to Amb a 1.1 to 1.3 allergens (13).

Cross-reactivity

The 4 major ragweeds including short, giant, western, and false strongly cross-react.  Due to strong cross-reactivity between short, giant, western, and false ragweed species, it is not required to do skin tests or treat with multiple members instead a single choice suffices. A strong cross-reactivity is seen with ragweed and mugwort, marshelder or cocklebur (14).

Cross-reactive allergens (major) are identified in the short as well as giant ragweed. These allergens are not considered identical as their major differences in allergenicity are amongst Amb a 1-2 and Amb t 1-2 and other minor allergens (6).

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Christian Fischer

Last reviewed: October  2020