Careless weed

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Code: w82
Latin name: Amaranthus palmeri
Source material: Pollen
Common names: Careless weed, Carelessweed, Palmer Amaranth, Palmer's pigweed

Allergen Exposure

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.

Careless weed is a species of flowering plant in the amaranth genus. It is native to most of the southern half of North America, and particularly, throughout the southern United States from southern California to Virginia. It has also been introduced to Europe, Australia, and other areas.

Careless weed closely resembles many other pigweed species. It is an erect summer annual that may reach 1.5-2m in height. It has one central stem from which several lateral branches arise. The leaves are alternate, glabrous (without hairs), and lance- or egg-shaped in outline. Leaf sizes are from 5 cm to 20 cm in length and 1 to 6 cm wide with prominent white veins on the under-surface. Leaves occur on relatively long petioles. (1)

The flowers are small, light-green, and inconspicuous produced in dense, compact, terminal panicles that are from 15 cm to 45 cm in length. Smaller lateral inflorescences also occur between the stem and the leaf petioles (leaf axils). Male and female flowers occur on separate plants. Each terminal panicle contains many densely packed branched spikes that have bracts that are 3 to 6 mm long. The plant blooms from June – November in the northern hemisphere. (1)

A small, dry, one seeded fruit is produced consisting of a single seeded utricle about 2 mm in size, which splits to show a glossy black to dark brown seed that is 1 to 1.2 mm long. The utricle is wrinkled when dry. (1)

Careless weed is often confused with other similar pigweed species, but differs in that no other pigweed species have terminal panicles that reach 45 cm in length, and that the terminal spike of is much smoother and narrower and less spike-like than either Common Pigweed (Redroot Pigweed) (A. retroflexus) or smooth pigweed (A. hybridus). The leaves of Careless weed are also without hairs and have prominent white veins on the under-surface unlike those of Common Pigweed. (1)

In Mexico, a candy is produced by drying the seeds, mixing with honey and baking.

Allergen Description

No allergens have been characterised.

Potential Cross-Reactivity

Cross-reactivity could be expected between species of the family Amaranthaceae, and in particular the genus Amaranthus.

Although no specific information on cross-reactivity of this plant with others exists, a high degree of cross reactivity has been reported to occur between Goosefoot (Chenopodium album) and Saltwort (Salsola kali), and other species taxonomically less related members of the Amaranthaceae family like Amaranthus retroflexus. Common allergenic determinants are present in these plants. (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. (3)

Clinical Experience

IgE-mediated reactions

Anecdotal evidence suggests that asthma, allergic rhinitis and allergic conjunctivitis are common following exposure to pollen from Careless weed; however, few specific studies have been reported to date. (4, 5, 6, 7, 8)

Sensitisation to Careless weed has been documented in Tucson, Arizona (4) and in Mexico. (5, 6, 7) In a Mexican study of allergic patients, sensitisation to Careless weed was demonstrated in 43.8%.(5)

In a study examining aeroallergen sensitization rates in military children in Texas undergoing skin testing for rhinitis, of 209 patients, 27% were sensitised to Common Pigweed or Careless weed. (8)

In a study in the Midwestern USA, evaluating the frequency of sensitisation to cannabis pollen, found that 61% were skin prick positive for cannabis and all subjects were also skin test positive to weeds pollinating during the same period: Ragweed, Pigweed, Cocklebur, Russian thistle, Marsh elder, and Kochia. (9)

In a study among Thai patients with vernal keratoconjunctivitis, positive results of skin prick testing to Acacia, Careless weed, mould, Johnson grass and Cow's milk were significantly more common in patients with palpebral VKC. (10)

Compiled by Dr Harris Steinman,


  1. Virginia Tech Weed Identification Guide. Palmer Amaranth: Amaranthus palmeri. Accessed: November 2011.
  2. Lombardero M, Duffort O, Selles JG, Hernandez J, Carreira J. Cross-reactivity among Chenopodiaceae and Amaranthaceae. Ann Allergy 1985;54(5):430-6.
  3. Perrick D, Stafford CT, Armstrong E, DuRant RH. Modification of the fluorescent allergosorbent test as an inhibition assay for determination of cross-reactivity among aeroallergens. J Allergy Clin Immunol 1991;87(1 Pt 1):98-103.
  4. Stern DA, Lohman IC, Wright AL, Taussig LM, Martinez FD, Halonen M. Dynamic changes in sensitization to specific aeroallergens in children raised in a desert environment. Clin Exp Allergy 2004;34(10):1563-669.
  5. Ortega EV, Vázquez MI, Tapia JG, Feria AJ. Most common allergens in allergic patients admitted into a third-level hospital. [Spanish] Rev Alerg Mex 2004;51(4):145-50.
  6. Gómez Castillo CA, Martínez Cairo Cueto S. Diagnostic use of enzymatic RAST skin tests and determination of eosinophils in nasal mucosa in allergic rhinitis. [Spanish] Rev Alerg Mex 1998;45(6):150-8.
  7. del Río Navarro BE, Mercado Ortíz V, Lerma Ortíz L, Montejo Bello M, Gazca Aguilar A, Sienra Monge JJ. Comparison of 2 skin test methods for the diagnosis of allergic diseases. [Spanish] Rev Alerg Mex 1996;43(4):100-3.
  8. Calabria CW, Dice J. Aeroallergen sensitization rates in military children with rhinitis symptoms. Ann Allergy Asthma Immunol 2007;99(2):161-9.
  9. Stokes JR, Hartel R, Ford LB, Casale TB. Cannabis (hemp) positive skin tests and respiratory symptoms. Ann Allergy Asthma Immunol 2000;85(3):238-40.
  10. Kosrirukvongs P, Vichyanond P, Wongsawad W. Vernal keratoconjunctivitis in Thailand. Asian Pac J Allergy Immunol 2003;21(1):25-30.


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