Ryegrass staggers is a neuromuscular disorder caused by ingestion of perennial ryegrass (Lolium perenne) containing the endophyte fungus Epichloe festucae (Neotyphodium lolii). E. festucae produces several ergot alkaloids and indole-diterpene alkaloids, with Lolitrem B as the main toxin responsible for onset of clinical signs. Ryegrass staggers was first recognized in 1906, involving the muscular incoordination associated with horses and cattle grazing on ryegrass pastures.

The severity and morbidity of staggers varies among horses, with 5 to 75% of other horses in the same pasture, affected. Clinical signs vary from slight trembling in the neck following hard exercise to severe tetanic spasms and collapse.

Most outbreaks of ryegrass staggers occur in Australia and New Zealand. Sporadic cases have occurred in livestock in Europe and the Americas, most of them involving the consumption of contaminated hay. In Japan, there were several cases of ryegrass staggers associated with feeding cattle and horses with contaminated ryegrass straw that had been imported from Oregon.