Botulism Shaker (Foal Syndrome or Forage Poisoning) in Horses

Key Takeaways

Botulism is poisoning due to botulinum toxin, produced by the bacterium Clostridium botulinum

• In most equine cases, the toxin enters the body through spoiled food (generally hay or silage)

• Botulinum enters the bloodstream and blocks nerve stimulation of muscle contraction, impairing motion and muscle function

• Symptoms include general weakness, difficulty eating, progressive muscle paralysis, and loss of appetite

• A diagnosis is usually reached through the elimination of other possible underlying causes of paralysis

• Treatment consists of injecting the antitoxin into the bloodstream, followed by long-term supportive care

• The condition is fatal in about 50% of adult horses

• Prognosis is better in foals with a survival rate of about 96%

A Closer Look: What is Botulism in Horses?

Horses often contract botulism by eating food contaminated with the toxin produced by Clostridium botulinum bacteria. Botulinum toxin is typically found in spoiled, moldy hay or in hay that had a decomposing carcass baled with it. Silage, haylage, and fermented forages are also common sources of botulinum poisoning.  

Sometimes, especially in foals, bacterial spores are ingested and mature, producing the toxin directly in the gastrointestinal tract. Once produced, the toxin spreads through the bloodstream. 

Another possible route of exposure is wound contamination by bacterial spores. Deep penetrating wounds and castration sites are most at risk of becoming contaminated. 

Severe symptoms are related to the progressive paralysis of the heart and lungs. Death is the most likely prognosis when the paralysis reaches the cardiorespiratory system. In severe stages of illness, horses may show symptoms such as:

• Severe respiratory distress

• Collapse

• Sudden death

In foals, symptoms might also include:

• Stilted movement or uncoordinated movement (ataxia)

• Difficulty defecating 

• Large amounts of milk dripping from the mouth after nursing

Risk Factors

Botulism is an extremely dangerous condition. It must be considered a medical emergency as it can lead to paralysis, difficulty breathing, and death. Death can be rapid, occurring as soon as 1 or 2 days after contact with the bacteria. The survival rate is poor in adult horses with about 50% surviving. In foals, the prognosis is generally more positive with up to 96% surviving, however foals require intensive veterinary care including mechanical ventilation.

Possible Causes

Botulinum toxin disrupts the nervous system. Once the toxin is in the bloodstream, it prevents proper signaling between nerves and muscles, preventing muscle contraction, leading to paralysis. If there is sufficient toxin in the bloodstream, paralysis will affect the heart and breathing structures, which is usually fatal.

Main Symptoms

Botulism appears with many different symptoms caused by progressive muscle paralysis, including any or all of: 

• Weakness

• Recumbency

Lack of coordination

• Dilated pupils

• Lethargy

• Drooling

Nasal discharge

• Muscle tremors

• Loss of appetite

Colic-like symptoms

• Progressive paralysis

• Difficulty in chewing

• Difficulty swallowing

• Impaired vision

Testing and Diagnosis

Botulism is extremely complicated to diagnose.Since there is not a specific test, diagnosis often comes from eliminating other possible causes of paralysis. The symptoms of botulism are also similar to those of choke and colic — which are common in horses — adding to the complexity of determining the root cause. Diagnostic tests to rule out other conditions include:

• Physical examination

• Colic work-up including nasogastric intubation and rectal palpation

• Blood work

• Specific testing for infectious causes with similar symptoms

Note: nasogastric intubation should only be performed by a veterinarian. There is no safe way to intubate a horse at home.

Steps to Recovery

In many cases, botulism is fatal before a diagnosis is reached. 

Treatment must be immediate. If botulism is suspected, antitoxin is injected directly into the bloodstream. After treatment, most horses need hospitalization and intensive supportive care which includes:

• Feeding soft or sloppy feed or providing food through a nasogastric tube

• IV fluids

• Manual evacuation of feces and urinary catheterization 

• Supportive slings to help horses stand

• Turning or rotating recumbent horses to prevent pressure sores

• Mechanical ventilation in severe cases where respiratory function is affected

The survival rate for adult horses is about 50%. In foals, the prognosis is more positive with about 96% of foals surviving. When treatment is successful, horses need up to ten days to regain the ability to swallow. A full recovery is expected in about a month, but more severe cases might take up to 90 days.


Vaccination is recommended, especially in high risk areas. The vaccine effectively protects from the most common toxin causing botulism (C. botulinum type B) but does not protect from other toxins. Pregnant mares can be vaccinated before foaling to provide protection to the newborn foal. Avoiding feeding fermented forage products or spoiled hay can also help prevent botulism.

Is Botulism Common in Horses?

Horses are the most sensitive to botulism among domesticated animals, however cases are uncommon. There are areas where the risk is higher, such as the United States.

Typical Treatment

• Antitoxin

• Supportive care

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