Common Pigweed

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Code: w14
Latin name: Amaranthus retroflexus
Source material: Pollen
Family: Amaranthaceae
Common names: Common Pigweed, Redroot Pigweed
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.

Allergen Exposure

Geographical distribution
The Amaranth family comprises about 40 genera and 475 species. They are mostly weedy herbs though some genera are low or climbing shrubs. The flowers of all are characterised by extreme simplicity. Some species are wind pollinated whereas others are insect pollinated. The flowers may be monoecious or dioecious, but they are always small, often greenish or yellowish (1).

The Amaranthus genus is annual herbs. The genus comprises about 50 species of which about 35 are native to North America (1). The genus includes 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 2 m 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 incon-spicuous, and are produced in dense, compact, terminal panicles that are approximately 2 cm wide and from 5 to 20 cm 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.

Careless weed (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.

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.

A 14 kDa and a 35 kDa allergen have been identified, but the allergens have not yet been fully characterised (2-3).

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 (4) 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 (5).

Clinical Experience

IgE-mediated reactions
Common pigweed pollen commonly induces asthma, allergic rhinitis and allergic conjunctivitis (6-9).

Pigweed pollen has been shown to be a common aeroallergen in the Midwestern USA (10), the Tampa Bay area, Florida (11), St. Louis, Missouri (12), and in Lincoln, Nebraska, USA (13). In a study examining aeroallergen sensitisation rates in military children in Texas undergoing skin testing for rhinitis, of 209 patients, 27% were sensitised to Common pigweed or the closely related Careless weed (A. palmeri) (9).

Pigweed pollen is also an important aeroallergen in Salamanca, Spain (14). In the central region of Coahuila, Spain, 5.4% of allergic individuals were sensitised to this pollen (6). Common pigweed pollen has been reported in Burgos, Spain (15).

In Mexico (16), Israel (17) and North China (7), Pigweed is a major contributor to the aeroallergen load. In Thailand, 16% of patients with allergic rhinitis were Pigweed-sensitised (5). In an assessment of allergic diseases and sensitisation in people aged 65 and over in Mexico, 3.6% were sensitised to Common pigweed (18).

Compiled by Dr Harris Steinman,


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As in all diagnostic testing, the diagnosis is made by the physican based on both test results and the patient history.