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Pigweed Information

Amaranthus retroflexus

Family: Amaranthaceae

Common names: Common Pigweed, Redroot Pigweed

Source material: Pollen

Note: Lamb’s Quarters (Chenopodium album) is occasionally also called Pigweed or Smooth Pigweed but does not belong to the Amaranthaceae family. There is a particular resemblance in the cotyledon stage, but Lamb’s Quarters cotyledons often have a mealy grey cast and the first true leaves are alternate, unlike those of any of the Pigweed species.

Palmer Amaranth (Amaranthus palmeri) also resembles Common and Smooth Pigweed, but the terminal panicles of this species are much longer and narrower. Common Pigweed is also often confused with other Pigweed species.

A weed species producing pollen, which often induces hayfever, asthma and conjunctivitis in sensitised individuals

Allergen Exposure

Geographical distribution

The Pigweed family contains many genera and over 500 species, including Common Pigweed, Powell Amaranth, Prostrate Pigweed, and Tumble Pigweed, the most common of these being Common Pigweed. Pigweeds are annual plants that germinate from seeds from late winter through summer.

Common Pigweed is a common weed found throughout the world, in particular in Europe, the USA, Brazil, Korea, Spain, Mozambique, Mexico, Hungary, Germany, and Afghanistan.

Common Pigweed is an erect summer annual that may reach 2m in height. The stems are stout, erect, and branched, usually with short hairs, especially near the upper portions of the plant. The plant has a shallow taproot that is often reddish in colour.

The leaves are grey-green and oval-spearhead-shaped, and covered with dense, coarse hair. Red or light-green stripes run the length of the tall main stem. Seeds are in bushy spikes at the top of the plant and in the axils of the leaves. Although Pigweed is primarily an upright grower, it will lie near the ground with constant mowing.

The flowers are greenish-grey and inconspicuous, and are produced in dense, compact, terminal panicles that are approximately 2cm wide and from 5 to 20cm in length. They are mixed with bristle-like bracts. Smaller inflorescences also occur between the stem and the leaf axils. Male and female flowers occur on the same plant (i.e., the structure is monoecious). Common Pigweed flowers in high summer and fall, very shortly after germination, and deposits thousands of seeds during a single season, producing over 100,000 seeds per plant. The seeds are small, shiny, and black.

Environment

The Common Pigweed is found in horticultural, nursery, and agronomic crops, wild landscapes, roadsides, and also in pastures and forages.

Unexpected exposure

North American Indians used A. retroflexus for flour and warm drinks.

Pigweed contains a nephrotoxin that causes kidney failure. It also contains soluble oxalates and is capable of accumulating nitrates. Toxicity can be due to a combination of these causes.

Allergens

A 14-kDa and a 35-kDa allergen have been identified, but the allergens have not yet been fully characterised(1,2).

Potential Cross-Reactivity

An extensive cross-reactivity among the different individual species of the genus could be expected, as well as to a certain degree among members of the family Amaranthaceae(3) and Chenopodiaceae(2). Atriplex latifolia, Beta vulgaris, Salsola kali and Amaranthus retroflexus were compared with an extract from Chenopodium album by both in vivo and in vitro methods. The study’s results suggest that common allergenic determinants are present(2).

In a study using a fluorescent allergosorbent test, similar antigenic determinants were found between Short Ragweed and Giant Ragweed, Cocklebur, Lamb’s Quarters, Rough Pigweed, Marsh Elder, and Goldenrod. Cocklebur and Giant Ragweed were highly potent in competitively binding to short ragweed IgE. The other pollens demonstrated lower potency of cross-reacting antigens(4).