Equis ISSN 2398-2977

Toxicity: sorghum

Synonym(s): Cyanogenic plant poisoning

Contributor(s): Gigi Kay, Vetstream Ltd

Introduction

Print off the Owner factsheet on Pasture management and Poisonous plants - what to look out for to give to your clients.

Pathogenesis

Etiology

  • Consumption of hybrid crosses of sorghum and sudan grass most commonly implicated. Other species of grass implicated less frequently.
  • Sorghum vulgare(sorghum grass).
  • Sorghum almum.
  • S. bicolor.
  • S. sudanense(sudan grass).
  • S. halapense(Johnson grass). 
  • Toxic principles believed to be cyanogenic and lathyrogenic glycosides.
  • Well dried hay made from sudan or Johnson grass does not seem to have toxic effects.

Predisposing factors

General
  • Environmental factors that increase cyanogenic glycoside content of specific plants:
    • Periods of rapid lush growth.
    • Seasons of medium/high rainfall.
  • Lack of good quality alternative forage.

Specific

  • Chronic exposure and ingestion ofSorghumcrops.
  • Young crops (<45 cm high) contain highest levels of cyanide.

Pathophysiology

  • Chronic ingestion ofSorghumspp (also known as Sudan grass).
  • Young and stressed plants have highest content of cyanogenic glycosides.
  • Up to 25% morbidity in groups of grazing horses.
  • Ingestion of glycoside-containing plants   →   digestive hydrolysis by beta-glycosidase in large intestine   →   hydrocyanic acid (cyanide, HCN or prussic acid)   →   cyanide absorbed from the gut   →   forms stable intracellular cyanide-cytochrome oxidase complex   →   inhibition of cellular respiration.
  • Chronic ingestion of cyanogenic glycosides   →   cyanide released into cells of CNS   →   degeneration of cauda equina segments/nerve roots   →   ataxia and bladder paralysis   →   incontinence and predisposition to cystitis.
  • Cauda equina =
    • Terminal portion of the spinal cord.
    • Begins at approximately lumbosacral vertebral junction.
    • Includes terminal segments of the spinal cord and the nerve roots of all five sacral and all coccygeal spinal cord segments.
    • The last two sacral segments (S4 and S5) and the first few coccygeal spinal segments lie in the first sacral vertebra.
    • Nerve roots lie within the sacrocaudal vertebral canal then exit via intervertebral foramina.
  • Sacrococcygeal nerve roots   →  
    • Sciatic and gluteal nerves .
    • Pudendal and rectal nerves   →   external genitalia, motor to perineal muscles, sensory to rectum, anus, perianal and perineal skin.
    • Pelvic nerves   →   parasympathetic innervation to bladder, urethra, distal small colon, rectum, anus and genital erectile tissue.
    • Coccygeal nerves   →   skin, muscles of the tail.
  • Brain and heart cells have high level of cytochrome oxidase activity   →   these tissues severely affected in acute cyanide intoxication   →   cerebral anoxia   →   seizures   →   coma   →   death.

Timecourse

  • Clinical signs can occur within 1 week to 6 months of grazing affected pasture.

Epidemiology

  • Horses grazing pasture containing sorghum species, particularly during seasons of high rainfall.

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.
  • Hahn C (2006) The wobbly horse: differential diagnoses. In Pract 28 (1), 8-13 PubMed.
  • Fordyce P S et al (1987) Use of an ELISA in the differential diagnosis of cauda equina neuritis and other equine neuropathies. Equine Vet J 19 (1), 55-59 PubMed.
  • De Lahunta A (1978) Diagnosis of equine neurologic problems. Cornell Vet 68 Suppl 7, 122-132 PubMed.
  • Knight P R (1968) Equine cystitis and ataxia associated with grazing of pastures dominated by sorghum species. Aust Vet J 44 (5), 257 PubMed.

Other sources of information

  • Furr M (2008) Equine Neurotoxic Agents and Conditions. In: Equine Neurology. Eds: Furr M & Reed S. Blackwell Publishing Ltd. ISBN: 0-8138-2519-9.
  • Van Gelder G A (1982) Poisoning by Cyanogenic Plants. In: Equine Medicine and Surgery. Eds: Mansmann R A, McAllister E S & Pratt P W. 3rd edn. Vol 1. American Veterinary Publications. Catalog Card No. 81-70196.
  • McKenzie R A (1994) Poisonings Through Pasture Grasses or Feedstuffs. In: Proc 16th Bain-Fallon Memorial Lectures. Australian Equine Veterinary Association. ISBN: 0-646-1834-2.
  • Mackay R J (1997) Cauda Equina Syndrome. In: Current Therapy in Equine Medicine. Ed: Robinson N E. W B Saunders. ISBN: 0-7216-2633-5.


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