Equis ISSN 2398-2977

Toxicity: selenium

Synonym(s): Alkali disease

Contributor(s): Peter Aitken, Graham Munroe, Wilson Rumbeiha, Prof Alan Seawright, Rachael Conwell

Introduction

  • Cause: ingestion of selenium accumulator plants; accidental toxicity by excess selenium supplementation, either oral or parenteral.
  • Signs: chronic and acute syndromes; lameness, coronary exudate, abnormal hoof wall growth, pedal bone rotation, hair loss.
  • Diagnosis: diet analysis, hoof wall analysis for selenium levels; radiography.
  • Treatment: adjust diet accordingly.
  • Prognosis: depends on severity of injury to hoof wall, acute toxicity usually fatal.

Pathogenesis

Etiology

  • Selenium Selenium is an essential element but has a narrow therapeutic range.
  • As a component of gluthathione peroxidase, selenium acts as an antioxidant. Large amounts of selenium interfere with this mechanism and cause severe oxidative tissue damage.
  • Selenium replaces sulfur in sulfur-containing amino acids (methionine, cysteine and cystine) → weakened structure of the keratin in hoof and hair - cause of systemic toxicity not understood.

Predisposing factors

General

  • Alkaline soils in low rainfall.
  • Grazing animals.
  • Supplementation with feeds containing selenium (selenium is often combined with Vitamin E in supplements for horses with muscle disease).

Specific

  • Grazing pastures grown on soil with high selenium levels and/or grazing pastures containing selenium accumulator plants.
  • Periods of drought.

Pathophysiology

  • Chronic or acute ingestion of feed or forage containing in excess of dietary requirements:
    • Acute poisoning: single oral dose of 3.3-6 mg/kg, 115 ppm in feed for 5 weeks.
    • Chronic poisoning: >5 ppm dry matter; 10 ppb in water.
    • Selenium Selenium intake: 0.5-2.0 mg/kg bodyweight.

Timecourse

  • Acute toxicity can occur after a single dose, whereas chronic toxicity often develops over prolonged periods (>30 days).

Epidemiology

  • All animals exposed to high selenium at risk of development of clinical signs.

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.
  • O'Toole D et al (1996) Selenium-induced "blind staggers" and related myths. A commentary on the extent of historical livestock losses attributed to selenosis on western US rangelands. Vet Pathol 33 (1), 109-116 PubMed.
  • Raisbeck M F et al (1993) Naturally occurring selenosis in Wyoming. J Vet Diagn Invest 5 (1), 84-87 PubMed.
  • Witte S T & Will L T (1993) Investigation of selenium sources associated with chronic selenosis in horses of western Iowa. J Vet Diag Invest 5 (1), 128-131 PubMed.

Other sources of information

  • Walmsely J P (1999) Miscellaneous Hoof Conditions. In: Shoeing for Soundness. AEVA Seminar Proceedings.
  • Seawright A A et al (1983) Heavy Metal Intoxications in Horses. In: Veterinary Pharmacology and Toxicology. In: Proc 2nd Symposium EAVPT. Eds: Y Ruckebushet al. M T P Press Ltd, Lancaster, UK.
  • Van Gelder G A (1982) Selenium Toxicosis. In: Equine Medicine and Surgery. Eds: R A Mansmann & E S McAllister. American Veterinary Publications, California. Cat. No. 81-70196.


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