Annual Rye Grass Toxicity in Horses


Annual ryegrass toxicity (ARGT) is a serious and increasingly more common toxicity of horses that consume annual ryegrass infected by the bacterium Rathayibacter toxicus.

The bacterium is carried into the ryegrass by a nematode, Anguina funesta, and produces toxins within seed galls from the end of flowering, through seedset, to seed maturity. Toxicity develops at flowering and seedset.

Infected ryegrass remains toxic even when it has dried off. Hay made from toxic ryegrass will also be toxic. All suspected cases we have seen have been from horses consuming meadow hay infected with toxic ryegrass.

Clinical Signs

Signs can occur days after consuming infected hay, however it can also take weeks to develop. Sudden death can occur. All cases of rye grass toxicity we have seen at our practice have been from horses consuming meadow hay infected with ARGT. However, ARGT may be seen in horses consuming any hay or chaff infected with the toxin.

Clinical signs are usually neurologic and include :
– Ataxia (wobbliness)
– Seizures
– Muscle tremors
– Hypersensitivity/hyper-reactivity to stimulus such as noise, movement, touch etc
– Disorientation

ARGT horses are not usually weak. Most are able to stand. However, they can be extremely dangerous to handle and caution must be exercised when treating these horses. Veterinary attention should be sought immediately.

Horses affected with ARGT are at risk for sudden death in the 7-10 days following the initial clinical signs.

Some make a full recovery, whilst others remain with permanent neurologic deficits such as ataxia.


Largely based on clinical signs, feeding history and demonstration of infected rye grass seeds in hay from laboratory testing. To learn more about testing your hay go to the DPIRD website. Intestinal contents from deceased horses can also be tested for the presence of the toxigenic bacterium.


There is no specific treatment for ARGT in horses. Veterinary attention should be sought at the first signs that “something isnt right”. Controlling of seizures is important and drugs may be used to counteract hyper-reactivity. Once initial veterinary first-aid is perfomed, affected horses should be moved to a safe and quiet environment. They should be closely monitored. As previously mentioned, affected horses are at risk for sudden death 7-10 days following initial clinical signs. Some may make a full recovery, however others are left with permanent neurological deficits.


Keys to prevent ARGT in horses include :

– feeding only hay which has been tested ARGT-free (however this does not completely guarantee some rolls/bales will not contain the toxin)

– purchase hay from reputable suppliers

– do not continue to feed hay which affected horses have been grazing. Remove these from paddocks immediately and dispose appropriately


Contact us on 0427 072 095 if your horse is showing any signs or you would like more information.