Ambrosia, a highly allergenic plant, is related to mugwort. Even in small quantities, Ambrosia pollen can trigger severe allergic reactions such as asthma. Spread of this weed in Switzerland was successfully halted in good time thanks to the official obligation to report its presence and to combat the plant.n.

“Aggressive” pollen

Except in regions of western Switzerland and Ticino, the concentrations of Ambrosia pollen in the rest of Switzerland are still too low at present to trigger symptoms in humans. A minimum quantity of pollen is necessary for an allergic reaction. For instance, it takes 50 grass pollen grains/m³ to trigger moderate to severe allergic symptoms. In the case of Ambrosia, however, just 11 pollen grains/m³ are sufficient. In addition, they are very small pollen grains which easily get into the upper airways and lead to asthma symptoms. This is why experts describe Ambrosia pollen as particularly aggressive.

A substantial sensitisation rate

Studies by the allergy ward of Zürich University Hospital and studies on more than 5800 people (SAPALDIA Phase II) show that currently in Switzerland 8% of the population studied is sensitised to  Ambrosia pollen. The rate is actually 21% in patients with existing pollen allergy. If common ragweed were to spread more widely, it is highly likely that a substantial proportion of these people would quickly be suffering from allergic symptoms due to Ambrosia. Their pollen allergy symptoms would probably intensify considerably and their symptoms would last a few weeks longer, deep into the autumn.

Mugwort and Ambrosia interaction

Ambrosia is often confused with mugwort. Both plants are composite flowers (Asteraceae). There are cross-allergies between them because of the close relationship of the pollen. Mugwort has been found in large quantities in Switzerland for years and also triggers corresponding allergies. As a result of cross-reactions, people allergic to mugwort are nearly always predisposed to having an allergic reaction to Ambrosia with corresponding symptoms. If the concentrations of Ambrosia pollen were to increase, many of these people might develop allergic symptoms to the “new” pollen in a very short time.

Economic consequences

As well as the health aspects, doctors also point to the economic consequences.

In Canada and the USA, where common ragweed has spread massively in the last two decades and where today it is the most significant allergen, real hay fever and asthma epidemics are recorded during the relevant pollen seasons. They cause additional costs running into double to triple-digit millions. Professor Jäger from Vienna, one of the world’s most noted pollen allergy experts, estimates that in Austria additional costs of up to 600 euro per year and per allergy sufferer are caused by Ambrosia pollen. 30% of all pollen allergies are attributable to Ambrosia in our neighbouring country.

Facts and figures

In Switzerland around 1.2 million people, roughly 15–20% of the population, are affected. The flower of a single blade of grass contains around 4 million flower pollen grains. Conifers are among the flower pollens that do not trigger any allergy. Visible as a yellow dust-fall in springtime, at most they cause irritation of the conjunctiva.

Editors: aha! Swiss Allergy Centre in co-operation with the Scientific Advisory Board.

aha! Swiss Allergy Centre helps

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