Botulism is a fatal neuroparalytic disease that affects horses, humans and other animals worldwide. Botulism interferes with nerve signalling, weakening and often paralyzing the horse.

This disease is caused by the ingestion of botulinum toxin produced by Clostridium botulinum bacteria. Horses are particularly susceptible to botulism and only need to consume a small amount of botulism neurotoxin to become seriously infected.

When botulism occurs in foals, it is known as Shaker Foal Syndrome or toxico-infectious botulism. [1]

Botulism outbreaks occur sporadically but are fortunately rare. Feed or forage can be contaminated with botulism and infect multiple horses with access to that feed. Silage and haylage are the most common sources of botulism in equine diets.

Botulism in Horses

Clostridium botulinum is a gram-positive, spore-forming bacterium that produces the neurotoxin botulinum, which causes botulism.

Botulinum is one of the deadliest known neurotoxins. [2] This chemical causes weakness and paralysis in horses by interfering with neuromuscular signalling. [2]

Neurotransmitters carry messages from neurons to muscle cells to enable muscle function, including coordinating movement and other bodily functions. The botulism neurotoxin blocks the release of these neurotransmitters, resulting in muscle weakness. [3]

Muscle weakness due to botulism is symmetrical, affecting both sides of the horse’s body and can progress to paralysis. The onset and severity of symptoms are determined by the amount of neurotoxin the horse is exposed to.

There are seven serotypes of the botulism neurotoxin with different levels of toxicity. Serotypes A, B, C, and D have been identified in horses. [2]

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Clinical Signs

Clinical signs of botulism appear anywhere from several hours to 7 days after the horse is exposed to botulinum toxin. [1][4] Weakness and paralysis are usually most noticeable in the tongue, tail, legs, and neck. [3][5]

Common symptoms reported in horses with botulism include: [2][3][4][5][6]

  • Delayed pupillary response to light (the eyes are slow to adjust to changes in lighting or brightness)
  • Drooping upper eyelid
  • Reduced tongue, lip, throat, tail, and/or anal tone
  • Thrashing, especially while recumbent
  • Difficulty eating, chewing and swallowing
  • Salivation/drooling
  • Muscle tremors or muscle twitching
  • Frequent urination or dribbling
  • Recumbency (lying down)
  • Loss of appetite
  • Fast heart rate
  • Holding the head and neck in an extended or lowered position
  • Abnormal gait and/or stumbling

Other symptoms of botulism in horses include: [2][4][5][6]

  • Anxious behaviour
  • Lethargy
  • Depression
  • Dilated pupil
  • Nasal discharge
  • Reduced gut sounds
  • Constipation
  • Stiffness
  • Urinary retention
  • Shortness of breath
  • Facial edema
  • Displaced soft palate
  • Weight loss
  • Fast respiratory rate

As the disease progresses, horses will alternate between standing and lying down. Botulism is called shaker foal syndrome in foals because when the foals try to stand, they tremble and return to recumbency. [7]

Death occurs if the horse’s respiratory system becomes paralyzed. [2] Once horses can no longer stand, they usually do not recover.


Botulism can lead to further complications in the horse, including: [5][7]

  • Colic
  • Aspiration pneumonia (difficulty swallowing causes food or water to enter the lungs resulting in pneumonia)
  • Decubital ulcers (ulcers that form on recumbent horses because of the constant pressure on their skin)
  • Ulcers on the cornea of the eye
  • Gastric ulcers, especially in foals

Other complications include: [5][6]

  • Diarrhea
  • Cellulitis (a skin infection)
  • Salmonella infection
  • Hyperlipidemia (high cholesterol)
  • Thrombophlebitis (blood clots, especially in the legs)
  • Muscle abscesses
  • Facial paralysis
  • Abortion (not in all cases; botulism-infected mares have given birth to healthy foals)
  • Myonecrosis (serious muscle infection)
  • Tissue hypoxia (inadequate oxygen)
  • Neuropraxia (damaged peripheral nerves)


Horses can become infected with botulism in one of three ways: [1][2][3][5]

  1. Adult horses may ingest the C. botulinum neurotoxin and absorb it from their digestive tract into the bloodstream.
  2. Foals may ingest C. botulinum spores which germinate in the digestive tract and produce neurotoxin. The neurotoxin is then absorbed into the bloodstream. This form of botulism is known as toxico-infectious botulism or shaker foal syndrome.
  3. Horses may get wound botulism if an open wound is contaminated with C. botulinum spores. The spores germinate and release the neurotoxin into the horse’s bloodstream. Some potential causes of open wounds are injury, injections, castration, hernia repair, or other procedures. Wound botulism is rare in horses.

There are also reports of the botulinum neurotoxin becoming airborne and poisoning horses via inhalation. [2]

Once the neurotoxin enters the bloodstream, it is carried to the nervous system, where it interferes with neuromuscular signalling and leads to paralysis.

Horses only absorb a small amount of botulinum from the digestive tract, but because this toxin is so potent, it can have devastating effects. [10]

Sources of Botulism

C. botulinum can be found in soil, decaying carcasses, rotting or molding forage, and improperly fermented haylage or silage. [2][9] Improper fermentation or storage of forage provide conditions that allow C. botulinum to multiply. [6]

Other feeds, such as grain or concentrates, can also be sources of botulism, although this is less common. [1] Oats, potatoes, hay cubes, and chaff are other potential sources.

Once C. botulinum is present on a farm, the bacteria can enter the soil and remain there, causing future outbreaks. [1] Avoid raking soil into hay and keep animal carcasses away from hay to prevent contamination. [6]


Botulism is difficult to diagnose because horses can be infected by such small amounts of neurotoxin that are not detected by standard tests. The neurotoxin may only reach a level high enough to be detected for a few hours after the onset of severe symptoms. [1]

Mouse Bioassay

The mouse bioassay is the standard test used to diagnose botulism in horses. Serum from the affected horse’s blood is injected into mice. Some of the mice are also injected with botulism antitoxin.

If the horse is positive for botulism, the mice injected with serum will show botulism symptoms, while the mice injected with both serum and antitoxin will not. [1][8]