Ryegrass staggers is a neurological condition that can be seen in most domestic grazing species, including horses. It occurs most commonly in late spring to early autumn and always in animals grazing perennial ryegrass pastures. Almost all varieties (cultivars) of perennial ryegrass are infected with an endophyte fungus, called Neotyphodium lolii, and this is because this fungus plays a protective role for the plant and enables it to persist over many seasons.
The condition of perennial ryegrass staggers is caused by ingestion of a tremorgenic neurotoxin (Lolitrum B) which is produced by this endophyte fungus. It is good to be aware of this condition, as horses grazing old dairy paddocks, or on sheep/beef properties are likely to be eating perennial ryegrass, a very common pasture type in southern Australia. It is important to note, that there are a range of ryegrass varieties that vary in the amount of endophyte that they are infected with, with newer varieties generally having lower amounts infected within the seed. This means that not every perennial ryegrass pasture will pose the same risk.
How does eating grass cause staggers?
During the warmer months, the amount of the neurotoxin (Lolitrum B) in infected plants may increase to toxic levels. This decreases as you get into the cooler parts of the year. This neurotoxin is thought to interfere with neuronal transmission in the cerebellum, a part of the brain that controls muscle movements and coordination. What are the signs of perennial ryegrass staggers? Signs generally develop over a few days, and may be as subtle as fine tremors of the head, or nodding movements. Noise, sudden exercise, or fright elicits more severe signs of head nodding with jerky movements and incoordination when they are forced to move. If an affected horse begins to run, it may develop stiff bounding movements with marked incoordination, which often results in collapse in lateral recumbency with stiffly extended limbs.
How do I treat it?
Treatment is simply aimed at removing the source of the neurotoxin, so removal off affected pastures. If a horse is displaying severe signs, it is important to not increase their stress or excitement levels as this will exacerbate the condition. Ensuring the horse remains calm is very important to ensure it doesn’t injure itself whilst suffering from incoordination. Left on their own, once removed from the pasture, horses will generally, over the course of a week or so, return to normal. Supplementation with magnesium and/or vitamin B1 may be beneficial in some cases, but are not a vital treatment. Providing an alternative foodstuff is the most important thing you can do for your horse, so ensuring they have access to good quality roughage and clean water is vital.