Western ragweed
Summary
Western ragweed (Ambrosia psilostachya) is an herbaceous perennial weed, native to North America and introduced to many countries in Europe, Asia and Africa. Ragweed flowers in late summer to autumn; its pollen is associated with allergic rhinoconjunctivitis and asthma. Western ragweed shows high levels of cross-reactivity with other ragweeds (Ambrosia species) and mugwort (Artemisia species). Ragweeds also cross-react with other members of the Asteraceae family (sunflower and feverfew), Timothy grass and potentially other grass and tree pollens.
Epidemiology
Worldwide distribution
Ragweed pollen is an important allergen source: Ambrosia species are major elicitors of type I pollen allergy in North America, with an estimated prevalence of approximately 26%. A series of European studies recorded the prevalence of ragweed sensitization from 25% to over 80% in the late 1990s, with prevalence rates increasing constantly over time.
In a US national health survey, ragweed pollen was among the top three sensitizing allergens detected in participants aged 6 years and over; the prevalence of ragweed allergy increased rapidly from age 6 to young adulthood, then declined after age 30.
Risk factors
Elevated IgE activity has been demonstrated in response to ragweed pollen collected along busy roads, suggesting that environmental pollution may increase pollen allergenicity.
A study investigating the effect of environmental temperature on Ambrosia psilostachya pollen found that artificial warming increased the number of plant stems, leading to an 84% increase in total pollen production. Climate change modelling suggests a 27-89% increase in high allergy risk areas for Western ragweed by the year 2100, particularly in northern Europe.
Route Of Exposure
Main
The main route of exposure is inhalation.
Clinical Relevance
Ragweed pollen causes allergic rhinoconjunctivitis and asthma. In a Romanian study of 83 ragweed-allergic patients, 75% had moderate to severe rhinoconjunctivitis and 25% showed symptoms of allergic asthma. Most of these patients were polysensitized to other allergens including mugwort, other cereal/ grass pollens and other tree pollens. Some sources state that Western ragweed (A. psilostachya) is not significantly allergenic compared to Short/Common ragweed (A. artemisiifolia).
Asthma
Clinical experience suggests that ragweed pollen may induce asthma approximately twice as often as is found with other pollen allergies (i.e. in 40%–50% of ragweed-allergic patients).
Prevention And Therapy
Allergen immunotherapy
In vitro data show substantial allergenic cross-reactivity between Amb a (Short/Common ragweed), Amb p (Western ragweed) and Amb t (Giant ragweed), suggesting that an Amb a-based, single-species allergen immunotherapy may be successful in patients allergic to diverse ragweed pollens.
Cross-Reactivity
The presence of homologous allergens leads to extensive IgE cross-reactivity between the pollen extracts Amb p from Western ragweed (A. psilostachya), Amb a from Short/Common ragweed (A. artemisiifolia) and Amb t from Giant ragweed (A. trifida).
A high degree of IgE cross-reactivity is seen among different species of Ambrosia (ragweed) and Artemisia (mugwort) and to other allergenic plants of the Asteraceae family. There is moderate IgE cross-reactivity between the pectate lyases of ragweed and mugwort (Amb a 1 and Art v 6) and between the defensin-like proteins of ragweed, mugwort, feverfew and sunflower (Art v 1, Amb a 4, Par h 1, and Hel a 1).
A major Timothy grass pollen allergen (Phl p 4) has also demonstrated IgE cross-reactivity to the main ragweed allergen (Amb a 1), while another Timothy grass allergen (Phl p 7) is a broad cross-reacting allergen present in many different tree-, grass- and weed pollens, including ragweeds.
A recent Romanian study found cross-reactivity between Amb a 1 and Cry j 1, the main allergenic molecule from Japanese cedar (Cryptomeria japonica), a species not found in the local area. Although both are pectate lyase proteins, their sequence identity is only 46-49%.
References
- LOINC. Western Ragweed IgE Ab [Units/volume] in Serum 2021 [cited 2021 6.12.21]. Available from: https://loinc.org/6275-2/.
- CABI. Ambrosia psilostachya
- (perennial ragweed) Wallingford, UK2021 [cited 2021 6.12.21]. Available from: https://www.cabi.org/isc/datasheet/4692.
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