ISSN 2398-2977      

Grass sickness


Synonym(s): Grass disease, Equine dysautonomia


  • A frequently fatal equine dysautonomia/polyneuropathy of central and peripheral nervous systems, with acute, subacute or chronic presentations. Predominant signs results from dysfunction of the gastrointestinal tract.
  • Cause: unknown; strong evidence for toxicoinfectious form of botulism; many other potential causes have been investigated.
  • Signs: acute colic, gastric reflux, dysphagia, severe weight loss, muscle tremors, tachycardia, patch sweating.
  • Diagnosis: often presumptive based on season and previous history of grass sickness on the premises, clinical signs, rectal exam, intestinal (ileal) biopsy. Results of phenylephrine eyedrops contribute. Histopathology of autonomic neurons for chromolytic changes definitive.
  • Treatment: acute - euthanasia; chronic - supportive, eg housing, feed palatable concentrates; analgesia.
  • Prognosis: grave - acute cases; guarded - chronic cases.
Print off the Owner factsheet on Equine grass sickness to give to your clients.



  • No evidence for contagious agent; environmental factor highly suggestive.
  • The role of toxic plants, bacteria, viruses, fungi, molds, parasites and insects have all been investigated.
  • Current favored hypothesis for toxicoinfectious form of Clostridium botulinum type C and local production of neurotoxins.
  • Role of Fusarium verticillioides (formerly Fusarium moniliforme) mold on pasture has been implicated in cases of grass sickness in Scotland.

Predisposing factors


  • Access to grazing.
  • Season: cases occur predominantly in the spring, with smaller peak in the autumn.
  • Good body condition.
  • Recent stress.


  • Premises where disease has occurred previously.
  • Change of pasture in past 2 months.
  • Recent period of cool, dry weather.


  • Proposed ingestion of neurotoxin at pasture → absorption via gut mucosa, particularly ileum → damage to enteric nervous system.
  • Dysfunction of autonomic system → gut stasis, inhibition of gland secretions, sweating, Horner's syndrome Neurology: Horner's syndrome.


  • Acute: 3-4 days.
  • Intermediate: 7-14 days.
  • Chronic: months-years.


  • Young horses in good to fat body condition kept in at risk geographical areas. Affected horses have been shown to have low serum levels of antibodies to Clostridium botulinum.
  • Seasonal risk factors include spring (April to July, with peak in May), and autumn; cold, dry weather with overnight frosts; prolonged dry spells (10 days at temperatures 7-11°C).
  • Premises risk factors include sand or loam soils; high soil nitrogen content; large number of horses on the premises (studs, livery yards); rearing of domesticated birds on the premises.
  • Management risk factors include grazing or feeding of freshly cut grass, recent stressful procedures, recent administration of ivermectin, mechanical removal of feces, frequent chain harrowing, soil disturbance, lack of supplementary feeding with hay or haylage, and recent changes in feed.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Pirie R S, Jago R C & Hudson N P H (2014) Equine grass sickness. Equine Vet J 46 (5), 545-553 PubMed.
  • Wylie C E (2014) Equine grass sickness in Scotland: A case-control study of signalment- and meterology-related risk factors. Equine Vet J 46 (1), 64-71 PubMed.
  • Mellor N E et al (2013) Successful treatment of chronic grass sickness in a donkey. Equine Vet Educ 25 (12), 628-632 VetMedResource
  • Pirie R S (2013) Equine grass sickness in a donkey. Equine Vet Educ 25 (12), 633-635 VetMedResource.
  • Schwarz B (2013) Equine grass sickness: what's new? Vet Rec 172 (15), 393-394 PubMed.
  • Copas V E N et al (2013) In equine grass sickness, serum amyloid A and fibrinogen are elevated, and can aid differential diagnosis from non-inflammatory causes of colic. Vet Rec 172 (15), 395 PubMed.
  • Schwarz B, Brunthaler R, Hahn C & van den Hoven R (2012) Outbreaks of equine grass sickness in Hungary. Vet Rec 170 (3), 75 PubMed.
  • Whlie C E, Proudman C J, McGorum B C & Newton J R (2011) A nationwide surveillance scheme for equine grass sickness in Great Britain: Results for the period 2000-2009. Equine Vet J 43 (5), 571-579 PubMed.
  • Mair T S, Kelley A M & Pearson G R (2011) Comparison of ileal and rectal biopsies in the diagnosis of equine grass sickness. Vet Rec 168 (10), 267-276 PubMed.
  • Ireland J L & Newton J R (2011) Improving antemortem diagnosis of equine grass sickness. Vet Rec 168 (10), 261-262 PubMed.
  • Waggett B E et al (2010) Prevalence of Clostridium perfringens in faeces and ileal contents from grass sickness affected horses: Comparisons with 3 control populations. Equine Vet J 42 (6), 494-499 PubMed.
  • Newton J R et al (2010) Equine grass sickness: Are we any nearer to answers on cause and prevention after a century of research? Equine Vet J 42 (6), 477-481 PubMed.
  • Lyle C & Pirie R S (2009) Equine grass sickness. Equine Pract 31 (1), 26-32 VetMedResource.
  • Wylie C C & Proudman C J (2009) Equine grass sickness: epidemiology, diagnosis and global distribution. Vet Clin North Am Equine Pract 25 (2), 381-399 PubMed.
  • Nunn F G, Pirie R S, McGorum B et al (2007) Preliminary study of mucosal IgA in the equine small intestine: specific IgA in cases of acute grass sickness and controls. Equine Vet J 39 (5), 457-460 PubMed.
  • Wijnberg I E et al (2006) The role of quantitative electromyography (EMG) in horses suspected of acute and chronic grass sickness. Equine Vet J 38 (3), 230-237 PubMed.
  • Wales A D & Whitwell K E (2006) Potential role of multiple rectal biopsies in the diagnosis of equine grass sickness. Vet Rec 158 (11), 372-377 PubMed.
  • Proudman C J (2005) Equine grass sickness. Equine Vet Educ 17 (1), 25-26 VetMedResource.
  • Hudson N P H & Pirie R S (2005) Four cases of equine grass sickness: acute, subacute, chronic and surviving chronic grass sickness. Equine Vet J 17 (1), 19-25 VetMedResource.
  • Newton J R et al (2004) An epidemiological study of risk factors associated with the recurrence of equine grass sickness (dysautonomia) on previously affected premises. Equine Vet J 36 (2), 105-112 PubMed.
  • McCarthy H E et al (2004) Grass sickness is associated with low antibody levels to Clotridium botulinum: a matched case-control study. Equine Vet J 36 (2), 123-129 PubMed.
  • McCarthy H E et al (2004) Why are certain premises at increased risk of equine grass sickness? A matched case-control study. Equine Vet J 36 (2), 130-134 PubMed.
  • Hahn C N, Mayhew I G & de Lahunta A (2001) Central neuropathology of equine grass sickness. Acta Neuropathol 102, 153-159 PubMed.
  • Hunter L C & Poxton I R (2001) Systemic antibodies to Clostridium botulinum type C - do they protect horses from grass sickness (dysautonomia)? Equine Vet J 33 (6), 547-553 PubMed.
  • Collier J, Collier S & Rossdale P D (2001) Grass sickness - the same old suspects but still no convictions! Equine Vet J 33 (6), 540-542 PubMed.
  • John H A, Creighton A J & Baird A (2001) Thoracic sympathetic chain ganglion neuronal abnormalities that may explain some of the clinical signs of grass sickness. Vet Rec 148, 180-182 PubMed.
  • McCarthy H E, Proudman C J & French N P (2001) Epidemiology of equine grass sickness - a literature review (1909-1999). Vet Rec 149, 293-300 PubMed.
  • Hahn C N & Meyhew I G (2000) Phenylephrine eyedrops as a diagnostic test in equine grass sickness. Vet Rec 147, 603-606 PubMed.
  • Perkins J D et al (2000) Functional and histopathological evidence of cardiac parasympathetic dysautonomia in equine grass sickness. Vet Rec 146 (9), 246-250 PubMed.
  • Hunter L C et al (1999) The association of Clostridium botulinum type C with equine grass sickness - a toxicoinfection? Equine Vet J 31 (6), 492-499 PubMed.
  • Wood J L N et al (1999) Equine dysautonomia - has grass been blamed unfairly all this time? Equine Vet J 31 (6), 451-452 PubMed.
  • Singer E R (1998) Gastric reflux - what does it mean? Equine Vet Educ 10 (4), 191-197 VetMedResource.
  • Milne E (1997) Grass sickness - an update. In Practice 3, 128-133 VetMedResource.
  • Milne E M, Doxey D L, Woodman M P et al (1996) An evaluation of the use of cisapride in horses with chronic grass sickness (equine dysautonomia)Br Vet J 152 (5), 537-549 PubMed.
  • Doxey D L, Tothill S, Milne E M & Davies Z (1995) Patterns of feeding and behaviour in horses recovering from dysautonomia (grass sickness). Vet Rec 137, 181-183 PubMed.
  • Doxey D L, Milne E M, Woodman M P et al (1995) Small intestine and small colon neuropathy in equine dysautonomia (grass sickness). Vet Res Commun 19 (6), 529-543 PubMed.
  • Milne E & Wallis N (1994) Nursing the chronic grass sickness patient. Equine Vet Educ 6, 217-219 VetMedResource.
  • Milne E (1991) Grass sickness. Equine Vet Educ 3, 196-199 Wiley Online Library.
  • Ricketts S W (1991) Diagnosis of grass sickness at autopsy. Equine Vet Educ 3, 221-223 (detailed description of collection technique for celiacomesenteric ganglion on post mortem) Wiley Online Library.

Other sources of information

  • Pirie S (2004) Current Perspectives on Prevention and Management of Equine Grass Sickness. In: Proc 43rd BEVA Congress. Equine Vet J Ltd, UK. pp 60.
  • Pirie R S (1999) Grass Sickness - Current Status. In: Proc 38th BEVA Congress. pp 135-136.
  • Davis Z J (1999) Nursing Horses with Chronic Grass Sickness. In: Proc 38th BEVA Congress. pp 41.



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