Equis ISSN 2398-2977

Toxicity: locoweed

Synonym(s): Locoism

Contributor(s): Graham Munroe

Introduction

  • Cause: prolonged ingestion of certain species ofAstragalusandOxytropislegumes (locoweeds)   Locoweed (Astragalus / Oxytropis spp)  : contain alkaloids that damage lysosomal enzyme essential in oligosaccharide cellular metabolizm   →   cellular vacuolization.
  • Incidence: commonly found in western North America where horses will ingest locoweeds in early spring or at other times of forage scarcity.
  • Signs: wide range - depend on chronicity of ingestion; particularly neurologic and general loss of condition.
  • Diagnosis: intracytoplasmic vacuoles evident on histopathology.
  • Treatment: mild cases may respond to preventing locoweed ingestion; chronic cases often die or are killed.
  • Prognosis: guarded to poor.

Pathogenesis

Predisposing factors

General
  • Areas of Western, North America where locoweeds can occur.

Specific

  • Scarcity of normal forage:
    • Overgrazing.
    • Early spring.
    • Drought.

Pathophysiology

  • Prolonged ingestion of certain species ofAstragalusandOxytropislegumes (locoweeds): contain alkaloids that damage lysosomal enzyme essential in oligosaccharide cellular metabolism   →   cellular vacuolization.
  • Indolizidine alkaloids swainsonine and swainsonine N-oxide are the toxic principles in locoweeds (similar to Darling Pea (Swainsomaspp) poisoning in Australia).
  • These alkaloids inhibit alpha-mannosidase and lysosomal enzymes esential in cellular metabolism of oligosaccharides.
  • Mannose-rich oligosaccharides accumulate in lysosomes   →   vacuolation and disruption of cellular function.
  • Vacuolation occurs in renal cortical tubular cells as early as 4 days and in the CNS neurons by 8 days.
  • If consumption of the plant stops, the vacuoles will disappear in the early stages of the disease, but if grazing is prolonged, permanent cellular damage occurs.
  • By 30 days most tissues of the body, except muscle, are affected.
  • Vacuolization of axons, glial cells and Purkinje cells of the cerebellum and cerebral cortex   →   neurologic signs.
  • Damage to cells of the liver, pancreas, thyroid and parathyroid glands   →   weight loss and emaciation.
  • Retinal and lacrimal cells can be damaged   →   impaired vision.

Timecourse

  • First vacuolation: 4 days.
  • Most body tissues affected: 30 days.

Epidemiology

  • Locoweeds usually emerge in the early spring before other forage plants are available.
  • Other peak periods are associated with periods of forage scarcity, eg droughts.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Knight A P (1987) Locoweed poisoning. Comp Cont Educ Pract.9, F418 VetMedResource.
  • McIlwraith C W and James L F (1982) Limb deformities in foals associated with ingestion of locoweed by mares. JAVMA 181, 255 PubMed.


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