English
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]
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.
Complications
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)
- 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)
Causes
Horses can become infected with botulism in one of three ways: [1][2][3]