Forage Poisoning in Horses

PetMD Editorial
Written by:
PetMD Editorial
Published: August 8, 2008



Botulism is a serious paralytic illness caused by toxins released by the bacterium Clostridium botulinum. It is normally associated with the ingestion of spoiled vegetative matter while grazing, and is sometimes referred to as forage poisoning. It takes about four to five days after eating the spoiled forage for symptoms to appear, but once they begin, neurological symptoms such as eating and swallowing difficulties are recognizable. This disease can occur in both adult horses and in foals. In foals, the disease is usually seen in animals less than four weeks old and is called "shaker foal syndrome."

Botulism is very serious and if left untreated, the disease is usually fatal. Unfortunately, even when treatment is initiated, the disease may still result in death.

Symptoms and Types

The toxin produced by this Clostridial bacterium causes motor paralysis which means that any nerves that function in the movement of the horse can be paralyzed. The signs for botulism in adult horses include:

  • Difficulty eating
  • Difficulty swallowing
  • Difficulty breathing
  • Food and saliva in the nose
  • Trouble walking
  • Head low to the ground
  • Recumbency
  • Generalized progressive weakness
  • Death

Signs of shaker foal syndrome include:

  • Foal found dead
  • Stilted gait
  • Muscle tremors
  • Inability to stand for lengthy periods of time
  • Inability to eat

There are seven distinct forms of botulism: designated types A through G. Those associated with horses include:

  • Type A: This form has been seen in several horse outbreaks in the northwestern United States (Washington, Idaho, Montana, Oregon)
  • Type B: Predominately referred to as forage botulism because of its association with contaminated forage
  • Type C: Known as carrion botulism because of the association with the ingestion of feed containing a decomposing carcass (e.g., rodent, cat, dog, bird) or from eating the bones of dead animals


Botulism occurs when a horse eats spoiled forage that contains Clostridium botulinum spores. These spores are found naturally in the environment. When these bacterial spores are ingested, they begin to reproduce and release their deadly toxin. As the toxin travels through the body, it prevents the passage of impulses from nerve to nerve, thereby causing progressive paralysis. 


Only your veterinarian can diagnose botulism, and it is important for the horse to be seen as early as possible after clinical signs appear for any chance of survival. Botulism can be difficult to diagnose based on laboratory tests on feces or stomach content assessments of the contaminated forage. Usually, diagnosis is made when other causes for paralysis are ruled out. Clinical signs can aid in this diagnosis and location can help as well, if there is history of other cases of botulism in the area.


Botulinum antitoxin is available at certain equine clinics, although it has been used with varying degrees of success. Usually, treatment centers primarily on supportive care. IV fluid therapy is required since the horse is unable to eat or drink. If the horse is unable to stand, physical therapy and other methods of maintaining circulation and preventing bedsores must be employed. Antibiotics are usually given as well, since the horse is at increased risk of aspiration pneumonia due to the inability to properly swallow. The same treatment plans are also used in foals. Treatment can be extremely prolonged and difficult both for the horse and the caretakers. Prognosis is extremely guarded.

Living and Management

Very few cases survive botulism, and this is because their respiratory muscles become paralyzed or due to secondary health problems attributed to generalized paralysis.


There is a botulism vaccine that can be sought by horse owners if they live in an endemic area. Pregnant mares in high-risk areas should be vaccinated to protect their foals.

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