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What is Annual Ryegrass Toxicity?

The parasitic nematode Anguina funesta carries the bacterium Rathayibacter toxicus into the ryegrass seed heads. The bacteria colonizes and multiplies inside the galls (knots on the plant). Rathayibacter toxicus produces an orange-yellow toxic secretion and the toxicity develops at flowering.

Ryegrass that has been infected remains toxic even after the plant has dried up. Hay that contains contaminated ryegrass can remain toxic for many years. Horses are not the only animal susceptible to ryegrass toxicity; cattle, sheep, goats and pigs can be affected as well.

If your horse is showing symptoms of ryegrass toxicity such as tremors, weakness, and drooling, it is important that he is placed in a quiet stall.  A quiet stall will keep him from getting overly excited or stressed, which can trigger the symptoms to worsen. Consult a veterinarian as soon as possible.  Ryegrass toxicity can be fatal.

Ryegrass is not a toxic plant, it becomes toxic when the bacterium Rathayibacter toxicus enters the seed heads.  The bacteria produces the poisonous chemical called corynetoxin.  Once the contaminated ryegrass is ingested, it begins to affect the horse’s central nervous system.

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Symptoms of Annual Ryegrass Toxicity in Horses

After the toxic ryegrass is ingested by the horse, it may take 4-5 days before clinical signs develop.  Symptoms may include:

  • Lack of coordination
  • Falling
  • Convulsions
  • Stiff legs
  • Tremors
  • Pregnant mares may abort their foal
  • The horse becomes anxious, is spooked easily
  • Dragging of back limbs
  • Drooling
  • Muscle twitching
  • Wide-based stance
  • Exercise or excitement may trigger reoccurrence of the symptoms
  • Staggering
  • Weakness
  • Death

Causes of Annual Ryegrass Toxicity in Horses

  • Parasites carry bacteria into the seed heads
  • The horse ingests contaminated ryegrass while foraging
  • The horse is fed hay or grain that has contaminated ryegrass
  • Hay is prepared in contaminated machinery, then fed to the horse
  • The central nervous system is affected by corynetoxin

Diagnosis of Annual Ryegrass Toxicity in Horses

The veterinarian may want to go over the horse’s medical history.  He needs to know what symptoms you have observed and when they started.  The veterinarian may want to walk your horse on a lead before performing a physical exam.  The gait and stance of your horse will be evaluated as will the pulse, blood pressure, and heart rate.  The veterinarian may listen to the horse’s heart, lungs and gastrointestinal intestinal tract with a stethoscope.  The horse’s reflexes will be tested. The veterinarian may also want to see the color of your horse’s gums and his stomach; limbs and muscle tones may be palpated.  If the veterinarian suspects an annual ryegrass toxicity, he will most likely want your horse’s feed and/or the ryegrass in the pasture to be tested for toxins.

Treatment of Annual Ryegrass Toxicity in Horses

Once the veterinarian diagnoses ryegrass toxicity he will place your horse on stall rest. Stall rest will help the horse remain calm and it will keep him safe.  Horses with ryegrass toxicity can hurt themselves by falling; documentation shows many horses with this toxicity have been found drowned in bodies of water after they attempted to cool themselves off or quench their thirst.  

When annual ryegrass toxicity is the case, affected horses need to be fed safe feed and kept away from the pasture and paddocks until the forage has been examined. The veterinarian may recommend a broad spectrum mycotoxin which helps bind all the toxins in the horse’s gastrointestinal tract, so they can be excreted. The faster the toxins are removed, the less damage there is the central nervous system.

Recovery of Annual Ryegrass Toxicity in Horses

Early detection of annual ryegrass toxicity in horses has a better recovery prognosis.  Horses that eat toxic ryegrass for an extended period of time have a guarded prognosis.

Your horse may continue to show symptoms up to 10 days after the toxins are removed from his diet.  Follow up visits are necessary to monitor the horse’s progress. It is important to prevent annual ryegrass toxicity from reoccurring.  If the pasture has ryegrass, it should be inspected for the orange-yellow secretion typically found on the grass seed heads.  

An agricultural agent can recommend where to have the ryegrass tested for Rathayibacter toxicus. The “pre-flowering ryegrass test” can determine if there are any bacterium in the ryegrass.  The horse’s feed and hay should only be bought from a reputable distributor.  It is a good idea to ask to see the vendor’s declarations.  This will let you know if the hay has been tested for toxins.  

Regular spraying of an herbicide can help prevent the parasitic nematode Anguina funesta from contaminating the ryegrass. Proper pasture management is very important for the safety of the horse’s health.