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Key takeaways

Ryegrass staggers occurs when horses ingest a toxic dose of ryegrass contaminated by fungal or bacterial toxins. Lolium, or ryegrass, is a type of grass found worldwide, commonly used in pastures, lawns, and for soil erosion control. 

  • Lolium grasses include a wide array of species, including annual and perennial varieties
  • When contaminated, both varieties can cause life-threatening staggers; symptoms include muscle tremors, incoordination, and recumbency
  • Perennial ryegrass toxicosis can cause heat stress, as well as severe reproductive issues in pregnant mares, including decreased milk production and difficulty delivering
  • Diagnosis of both conditions is based on history of exposure, feed testing, and symptoms
  • There is no antidote or specific treatment for the two syndromes
  • Prognosis for annual ryegrass toxicosis is poor, and the majority of severely affected animals die
  • Prognosis for perennial ryegrass toxicosis prognosis is guarded
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A closer look: Ryegrass Staggers in Horses

Ryegrass is not toxic in and of itself. Plants become toxic when they are contaminated by fungal or bacterial toxins.

Poisoning in horses can be caused by both annual and perennial ryegrass; even though symptoms are similar in each case, the mechanism of the toxicosis differs greatly depending on which variety was ingested.

No antidote or specific treatment is available; as such, prevention is paramount.

Early medical attention and removal of infested ryegrass are the best ways to ensure the animal’s survival.

Risk factors

Ryegrass staggers is uncommon in horses. Ryegrass toxicosis is potentially life-threatening and must be treated as an emergency. Ingestion of both annual and perennial infested ryegrass can lead to severe neurological symptoms.

Perennial ryegrass is more likely to be infested with fungal toxins during spring and summer months. Annual ryegrass has the highest concentration of toxin in the summer, particularly as the seeds begin to ripen.

Possible causes

Annual ryegrass staggers is caused by ingestion of Lolium rigidum infested with bacterial toxins. The ingestion of the bacteria causes severe and frequently fatal neurological symptoms.

Perennial ryegrass staggers: is caused by ingestion of fungal toxins produced by the Neotyphodium lolii fungi on perennial varieties of ryegrass. This form of staggers carries additional risks specific to pregnant mares.

Main symptoms

Perennial ryegrass toxicosis can cause additional symptoms, such as

  • Heat stress
  • Decreased milk production (pregnant mares)
  • Difficulty giving birth (pregnant mares)

Testing and diagnosis

Diagnosis of ryegrass staggers is primarily based on symptoms, history of exposure, and identification of the bacteria or the fungus in forage through feed testing.

Steps to Recovery

Once diagnosed, treatment options include:

  • Removal of ryegrass source
  • Providing adequate amounts of uncontaminated feed and water
  • Stall rest in a safe environment to prevent injury during collapse or spasms

Specific treatment for annual ryegrass staggers includes:

  • Anti-seizure medication

Specific treatment for perennial ryegrass staggers includes:

  • Heat mitigation measures (e.g., fans, water sprinkles, proximity to freshwater)
  • Domperidone: ingestion of annual ryegrass can cause affected mares to produce little to no milk. Domperidone can be used to counteract the toxin’s effects on milk production

Prognosis is dependent on the kind of infected ryegrass ingested.

Annual ryegrass toxicosis carries a poor prognosis and severity; affected animals often die shortly after developing symptoms.

Perennial ryegrass toxicosis carries a guarded prognosis. If diagnosed early, horses can recover from both neurological and reproductive issues days after the removal of the infected ryegrass. Mildly affected animals are able to recover.


Ryegrass staggers is not contagious.

The most effective method of preventing ryegrass staggers is ensuring that horses do not feed on infested ryegrass. Strategies include:

  • Inspecting hay or other forage sources before feeding
  • Removing infected ryegrass from pastures
  • Preventing overgrazing of pastures
  • Planting endophyte-free ryegrass seed

Is Ryegrass Staggers in Horses common?

Both conditions are uncommon in horses.

Typical Treatment

  • Supportive care
  • Removal of ryegrass source
  • Seizure medication
  • Domperidone


Megan C. Romano , DVM, DABVT - Writing for MSD Veterinary Manual
Megan C. Romano , DVM, DABVT - Writing for MSD Veterinary Manual
Wayne Simpson , MSc (Microbiology), BHort Sc - Writing for MSD Veterinary Manual
University of Kentucky College of Agriculture, Food, and Environment - Writing for The Horse
Erica Larson - Writing for The Horse
Janice L. Holland, PhD - Writing for The Horse
Christopher Brown DVM BVSc PhD DipACVIM; Wilson Rumbeiha BVM PhD DipABT DipABVT - Writing for Vetlexicon

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