Locoweed - Taxa Producing Swainsonine

Taxa Producing Swainsonine

Swainsonine is produced by a small number of species, including species in several genera of plants and two genera of fungi.

Oxytropis is distributed throughout western North America, particularly in the Great Plains and Rocky Mountains. However, most species of Oxytropis have narrow habitat requirements and within those habitats are abundant only in unusually wet years. The species most frequently encountered by livestock are O. lambertii (Lambert locoweed, purple locoweed, woolly locoweed) and especially Oxytropis sericea (white locoweed, white point locoweed, white point loco). Swainsonine has also been found in O. campestris (in Canada).

Some species of Astragalus (milkvetch) are also referred to as locoweed. These are primarily species which grow in areas with high selenium content in the soil. Swainsonine has been found in:

• A. earlei (Big Bend loco)
• A. mollissimus (purple woolly loco)
• A. pubentissimus (green river milkvetch)
• A. lentiginosis (spotted locoweed, freckled milkvetch)
• A. wootoni (garbancillo)
• A. nothoxys (beaked milkvetch)
• A. tephrodes (ashen milkvetch)
• A. humistratus (ground cover milkvetch)

In Argentina, locoism (locoismo) was first reported in 2000. A flock of sheep grazing a pasture with Astragalus pehuenches was poisoned and 220 sheep (73%) died. Although this was the first report of locoism in South America, swainsonine had been isolated previously from A. pehuenches and several other species in Argentina and Peru.

In the Old World, native plants causing locoism have not been reported. Astragalus lusitanicus in Morocco was suspected, but has been shown be neither a producer of swainsonine nor an accumulator of selenium. Its toxicity is suspected to be due to a novel alkaloid. In Spanish, common names of this species include altramuz loco, chochos locos, garbanzo del diablo, and garbancillo.

In Australia, species of Swainsona (Darling pea) that cause pea struck include:

• S. luteola
• S. greyana
• S. galegifolia (smooth Darling pea)

Astragalus and Oxytropis are 2 of 20 genera (and 78 names of genera) in the tribe Galegeae, subtribe Astragalinae. Some authorities include Swainsona in the subtribe. Formerly, Swainsona was in another subtribe, Coluteinae, that has been combined into Astragalinae.

Swainsonine has also been isolated from Sida carpinifolia and Ipomea carnea, and both species have been reported to cause locoism.

Embellisia, a fungus isolated from Oxytropis lambertii, has also been shown to produce swainsonine and to cause locoism in rats. Rhizoctonia leguminicola, a fungal plant pathogen that may occur on red clover, also produces swainsonine. Although intoxication due to this fungus resembles locoism, it has additional signs and symptoms due to the production of other toxins.