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.

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)
  • 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)

Causes

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]

Diagnosis

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]

Antibodies

A diagnosis can also be made if botulism antibodies are detected in the blood of a horse showing botulism symptoms. However, this method is not effective for vaccinated horses. [4]

Laboratory Tests

Your veterinarian may diagnose your horse with botulism if the neurotoxin or C. botulinum spores are found in gut contents or feces. C. botulinum is not a natural member of the horse’s gut microbiome, so any trace of the bacteria in the horse’s gut would indicate a botulism infection. [7][11]

Another diagnostic method is testing the horse’s feed and forage for C. botulinum spores or neurotoxin.

These tests often come back negative because the amount of toxin needed to infect a horse is so small that it may not be picked up by laboratory analysis.

Differential Diagnosis

Because lab tests often report false negatives, your veterinarian may diagnose botulism in horses showing symptoms by ruling out other possible diseases. [7][11]

Needle Electromyography

Needle electromyography (EMG) can also support a botulism diagnosis by identifying if there are problems with neuromuscular signalling. [4] In this procedure, your veterinarian inserts a needle electrode directly into your horse’s muscle tissue to record the electrical activity.

Treatment

If your horse shows any symptoms of botulism, contact your veterinarian immediately to obtain a diagnosis and determine the best treatment plan for your horse.

Antitoxin

Horses should receive antitoxin for their specific serotype of botulism as soon as possible. Early intervention is critical because once the neurotoxin binds to the horse’s neurons, its effects cannot be reversed. [9]

Some horses with mild botulism infections may survive without antitoxin. However, administering antitoxin will improve the horse’s likelihood of recovery. [9]

Antibiotics

Some research reports on the use of antibiotics to treat equine botulism, but this is not well-studied. [8][9]

Studies in humans and mice show that antibiotics can cause C. botulinum to release more neurotoxin. In humans, antibiotics are only used to treat some cases of wound botulism and never in young babies. [12][13]

Oral administration of penicillin can increase the release of neurotoxin by C. botulinum in foals. Other antibiotics, such as metronidazole, may increase the risk of botulism in other species and do not kill C. botulinum. [1]

Caution should be taken when considering the use of antibiotics to treat horses with botulism. Follow your veterinarian’s directions on whether to use antibiotics for your horse’s case.

Supportive Care

During treatment and recovery, supportive care may need to be provided to improve outcomes and keep the horse comfortable.

Nutritive Support

Horses that struggle to eat or drink should be given fluids intravenously or via a nasogastric tube, as well as enteral (liquid) or parenteral (non-oral) nutrition. [3][9]

When feeding through a nasogastric tube, horses that cannot stand should be kept in a sternal position, so they are upright rather than lying on one side. [7]

Mechanical Ventilation

Mechanical ventilation can be given to help horses breathe. Mechanical ventilation greatly enhances a foal’s likelihood of recovery. [8]

Catheterization

Male horses that are fully recumbent may need to be catheterized to allow them to urinate. [7]

Physical therapy

Horses may also require physical therapy to support recovery and limit further health complications. Recumbent horses that lay on one side for too long are at greater risk of ulcers.

Recumbent horses should be kept in a sternal laying position if possible or flipped from one side to the other every few hours. A sling can be used to help flip the horse or to help get the horse standing. [8][9]

When horses are mostly or fully recumbent, keeping the horse on a mattress or waterbed may help reduce the formation of decubital ulcers. [8]

Additional Supportive Care

The following forms of supportive care can also be administered: [1][6][7]

  • Anti-inflammatory medications
  • Activated charcoal
  • Mineral oil to prevent constipation and impaction colic
  • Histamine receptor blockers and sucralfate to prevent gastric ulcers
  • Eye ointments to protect the eye from the weak, drooping eyelid
  • Treatment for complications such as pneumonia or gastric ulcers

Recovery

If treatment is successful, horses and foals usually regain the ability to eat, drink, and stand 7 – 10 days after beginning treatment. Full recovery takes several weeks because of the time needed for muscle tone to return. [3][4][7]

Horses that can no longer stand have a low chance of recovery. Complete recumbency can happen within hours of clinical signs onset or may take several days.

Death occurs when horses develop respiratory paralysis or are euthanized. Euthanasia is usually recommended for horses that have lost the ability to stand.

Prognosis

Survival rates reported in botulism outbreaks range from 10% – 48% of affected horses and 87 – 96% in foals who receive mechanical ventilation or are hospitalized. [5][8]

Survival rates are higher in horses that are diagnosed and hospitalized early on, quickly treated with antitoxin, or minimally exposed to botulism toxin. Horses that are treated before they become recumbent are more likely to recover. [5][9]

Horses exposed to larger doses of the botulism neurotoxin are less likely to survive. The chance of recovery is also determined by the neurotoxin serotype the horse is affected by. [5]

Prevention

Botulism is fortunately very rare. However, if you live in a region with a history of outbreaks, taking the following preventative measures can help to reduce your horse’s risk of contracting botulism.

Vaccination

Vaccination is effective for preventing botulism in horses. The botulism vaccine is a risk-based vaccine, meaning horses should be vaccinated if they are at a heightened risk of being infected. This includes horses that are fed silage or haylage on a regular basis.

The botulism vaccine is a toxoid vaccine made using an inactivated form of the botulism neurotoxin. When administered to a horse, it stimulates antibody production against the botulinum toxin. [2]

Vaccines only provide protection against one botulism serotype. A horse vaccinated for type B botulism that is later infected with type A botulism will not be protected. [14]

Mares can be vaccinated while pregnant to prevent their foals from contracting botulism. Mares that are vaccinated 4 – 6 weeks before giving birth pass the antibodies to their foals in colostrum when the foal nurses. [15]

When first vaccinating a horse against botulism, three doses are administered four weeks apart. A booster can be administered annually. [15]

Feed Management

Improper storage or fermentation of silage or haylage can increase the risk of botulism. Since horses are very sensitive to botulinum neurotoxin, avoiding fermented forage can eliminate a potential source of botulism. [6]

Silage or haylage can provide optimal conditions for Clostridia to grow, particularly if the pH is above 4.5. Proper silage and haylage preparation ensures the pH is lowered quickly to inhibit bacterial growth. [16][17]

Some silage and haylage manufacturing conditions can result in higher pH and increased risk of botulism including: [17]

  • Slow or poor packing
  • Drier silage or haylage
  • Low initial carbohydrate content of the forage

Ryelage is associated with more botulism cases than other types of fermented forage. Consider feeding other forage sources or ensure your horse is vaccinated if you do feed ryelage or fermented feeds. [1]

If you are feeding silage or haylage, be sure to submit a sample for analysis that includes a pH measurement. Avoid feeding these if the pH is above 4.5.

Clostridia is found in soil so avoid raking soil into hay and dispose of any hay that comes into contact with small animal carcasses. Cover hay or forage and store it off the ground on skids to reduce moisture exposure and prevent spoilage. [6]

Discard rotting or moldy forage, and do not allow your horse to consume it. [11]

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References

  1. Whitlock, R.H. et al. Botulism. Vet Clin North Am Equine Pract. 1997.
  2. van der Kolk, J.H. et al. Bacterial diseases. Infectious Diseases of the Horse (2nd Ed). 2022.
  3. Frey, J. et al. Alternative vaccination against equine botulism (BoNT/C). Equine Vet J. 2007.
  4. Jahn, P. et al. Botulism in horses: a case report. Vet Med (Praha). 2008.
  5. Johnson, A.L. et al. Outcome of adult horses with botulism treated at a veterinary hospital: 92 cases (1989–2013). J Vet Intern Med. 2014.
  6. Shnaiderman-Torban, A. et al. An outbreak of equine botulism type D in Israel. Equine Vet Educ. 2017.
  7. Whitlock, R.H. et al. Equine botulism. Clin Techniques Equine Pract. 2006.
  8. Taylor, S.D. et al. Mechanical ventilation and management of an adult horse with presumptive botulism. J Vet Emerg Crit Care (San Antonio). 2014.
  9. Johnson, A.L. Botulism: knowledge and preparation improve outcome for horses. Equine Vet Educ. 2017.
  10. Galey, F.D. Botulism in the horse. Vet Clin North Am Equine Pract. 2001.
  11. Hunter, J.M. et al. Round bale grass hay: a risk factor for botulism in horses. Compendium on Continuing Education for the Practising Veterinarian -North American Edition. 2002.
  12. Fenicia, L. et al. Considering the antimicrobial sensitivity of the intestinal botulism agent Clostridium butyricum when treating concomitant infections. Eur J Epidemiol. 2003.
  13. Santos, J.I. et al. Potentiation of Clostridium botulinum toxin by aminoglycoside antibiotics: clinical and laboratory observations. Pediatrics. 1981.
  14. American Association of Equine Practitioners Botulism. AAEP. 2019.
  15. American Association of Equine Practitioners Infectious Disease Committee Vaccinations for adult horses. AAEP. 2020.
  16. Driehuis, F. et al. Silage review: Animal and human health risks from silage. J Dairy Sci. 2018.
  17. Kung Jr, L. Understanding The Biology Of Silage Preservation To Maximize Quality And Protect The Environment . UC Davis. 2010.