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LOCOWEED - Astragalus species





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Distinguishing features

Locoweed is a perennial herb that grows in clumps with pinnately compound leaves and lacking tendrils. It has blue, white, or purple pea-like flowers and short or long, often inflated pods. Seeds are kidney-shaped and retain vitality for 40+ years. Taxonomy and proper identification are difficult since at least 372 species of Astragalus exist, most being toxic. Astragalus is the largest of the legume forms in the U.S.. 

Text Box: Factoid:  A. membranaceus (milk-vetch) root is used in China as a medicine, A. boetica seeds are roasted by the Swedes and used as a coffee substitute and A. cicer, A. falcatus. A. meliotoides and A. adsurgens are used as forages and ground covers. Description. As many as 300 species of locoweeds are recognized in North America although not all are toxic.  Species identification usually requires a trained specialist.  Astragalus sp. are mostly perennial, stemmed or stemless herbs.  Leaves are alternate and pinnately compound.  Flowers are leguimous.  The fruit is a legume pod of various sizes and shapes.  The seeds are kidney-shaped.  Oyxtropis sp. should be considered with the Astragalus since they contain the same toxins and cause identical adverse effects.  Not all Astragalus species are toxic.
Geographic range.  Locoweeds are found from the Great Plains to the Rocky Mountains, primarily in dry, alkaline soils.

Toxic principle.  Locoweeds contain one of three toxic fractions.

      Miserotoxin, a glycoside, is hydrolyzed in ruminants to a toxic form known as 3-nitropropanol.  Toxicity may be due, in part, to nitrite released from the 3-nitropropanol or to an unknown neurotoxic principle.  A. emoryanus and A. miser are species with high concentrations of miserotoxin.

     Swainsonine, an alkaloid found in many locoweeds, inhibits a-mannosidase.  Inhibition  leads to increased lysosomal accumulation of oligosaccharides, resulting in neuronal swelling and vacuolization.  Examples of swainsonine-containing locoweeds include A. lambertii, A. mollisimus, A. lentigenosus, and Oxytropis.

     Selenium.  Some species of Astragalus and Oxytropis are selenium accumulators

Toxicity.  Fresh plants are toxic.  The toxicity is gradually lost as the plant dries, except for that of selenium accumulators, which are not affected by drying.  Swainsonine poisoning occurs after 2 weeks or more of ingestion.  Cattle and horses may become habituated to locoweed and seek it out, even when good forages are available.  Swainsonine may be passed in the milk.


Clinical signs


      Neurologic signs.  Demyelination of the posterior spinal cord causes incoordination, hypermetria, ataxia and clicking of the dewclaws (a condition known as "cracker heels").

      Respiratory signs include emphysema (primarily in sheep), dyspnea and cyanosis.  Sudden collapse and death may occur within 4-24 hours in animals poisoned by large doses


      Neurologic signs.  Affected animals exhibit ataxia, emaciation, staggering, and proprioceptive deficits.  Horses may become belligerent or startle violently as a result of mild stimuli


      Vision may be impaired by decreased lacrimation and retinal degeneration

     Immunosuppression of the cellular immune system can occur.

      Abortions are common in mid to late pregnancy, and teratogenic effects (e.g., contracted tendon) also occur.

      Male infertility with reduced spermatogenesis is reported.

Selenium accumulators are discussed in the metals lectures.

Laboratory diagnosis.

Miserotoxin:  non-specific

Swainsonine:  decreased serum a-mannosidase activity combined with alkaline phosphatase activity and  albumin and T4 and T3.


Misertoxin:  gross lesions are not dramatic.  There may be congestion of the liver with some hepatomegaly, pulmonary emphysema and pneumonia and increased amounts of CSF.  Microscopically, lesions include pulmonary alvelolar emphysema, bronchiolar constriction, interlobular edema and fibrosis, Wallerian degeneration in the spinal cord and peripheral nerves.

Swainsonine:  gross lesions are not specific.  However, microscopic lesions are rather dramatic and can be described as neurovisceral cytoplasmic vacuolation.  Vacuoles are observed in organs such as the thyroid gland, pancreas, lymph nodes, lymphocytes and liver.


  • Often unrewarding

  • Remove from source of exposure

  • Depending on the length of exposure, damage can be irreversible

  • Horses recovered from locoism should be considered unsound

Cattle, especially calves, grazing on high mountain ranges (~2000 to 3000 m) have an increased incidence of congestive right heart failure. This is called high mountain disease or HMD.  The incidence and severity of HMD and the time required for it to occur is decreased when cattle ingest locoweeds.  Several hypotheses have been proposed to explain why locoweeds enhance the hypoxic effects of high elevations, thus increasing the incidence and severity of HMD.  Changes in vascular integrity of the cardiopulmonary system, vasoconstriction of the pulmonary vascular bed or depression of the central respiratory centers may be involved.


Copyright 2002

University of Pennsylvania
Created by:    Alexander Chan (2003), Daphne Downs (2002), Chris Tsai (2001), Brett Begley (2000), Janet Triplett (1997)
Faculty Advisor:  Dr. Robert Poppenga