Bit & Bridle

Equine Botulism

Other Names

  • Shaker Foal Syndrome (a form of botulism occurring in young foals)
  • Forage poisoning
  • Limberneck

What is Botulism in Horses?

Botulism is a potentially fatal neuroparalytic disease caused by the ingestion of botulinum toxin, a potent neurotoxin produced by the anaerobic bacterium Clostridium botulinum. This toxin blocks the release of acetylcholine at neuromuscular junctions, preventing nerve impulses from reaching muscles and causing progressive, generalised muscle weakness and paralysis.

Horses are considered one of the most susceptible species to botulinum toxin, requiring only very small quantities to cause serious or fatal disease. The condition can affect horses of any age, breed, or sex, though foals between two and eight weeks of age are particularly vulnerable to a form known as “Shaker Foal Syndrome.” Adult horses most commonly acquire botulism through contaminated forage, particularly big-bale silage or haylage that has been improperly fermented.

Botulism is rare in the UK compared to North America but does occur, and the consequences of an outbreak are severe. Awareness of the risk — particularly regarding forage management — is the most important preventive tool available to horse owners.

Causes

Horses can develop botulism through three main routes:

  • Forage poisoning (toxico-infectious): The most common route in adult horses. The horse ingests preformed botulinum toxin present in contaminated feed — typically poorly fermented haylage, silage, or forage into which a dead animal (rodent, bird) has decomposed during baling.
  • Wound botulism: C. botulinum spores germinate within anaerobic wounds (deep punctures, lacerations with necrotic tissue) and produce toxin locally. Less common but possible in horses with wound infections.
  • Toxicoinfectious botulism (Shaker Foal Syndrome): Spores are ingested and germinate within the foal’s gut, producing toxin in situ. Occurs in foals under two months old, as their immature gut microbiome allows spore germination in a way that the adult equine gut does not.

Symptoms and Signs

Signs of botulism reflect progressive motor paralysis and typically worsen over 12-72 hours. The clinical picture depends on the dose of toxin ingested:

  • Generalised muscle weakness and trembling
  • Difficulty swallowing (dysphagia) — evidenced by dropping food from the mouth, difficulty drinking, saliva drooling
  • Weakness of the tongue — the tongue may protrude or be easy to pull from the mouth without resistance
  • Progressive inability to hold up the head
  • Stumbling, incoordination, and eventually recumbency
  • Mydriasis (dilated pupils) in some cases
  • Decreased eyelid and tail tone
  • Constipation and reduced gut sounds
  • Respiratory distress as paralysis advances to the diaphragm — a pre-terminal sign

In foals (Shaker Foal Syndrome): Trembling (hence the name), inability to stand, dysphagia, and generalised flaccid paralysis. Progression can be rapid.

Diagnosis

Diagnosing botulism is challenging because there is no simple, rapid field test. Diagnosis is typically based on:

  • Clinical signs: Progressive flaccid paralysis with intact consciousness in a horse with access to haylage or suspected contaminated feed
  • Mouse bioassay: The definitive test — serum, gastric contents, or suspected feed is injected into mice to detect the presence of toxin. Results take several days.
  • Feed investigation: Suspect forage should be tested and removed immediately. Look for evidence of dead animals incorporated into bales.
  • ELISA tests: Enzyme-linked immunosorbent assays are increasingly available for toxin detection in clinical samples
  • Post-mortem: Unfortunately, diagnosis is often confirmed post-mortem given the rapid progression of the disease

Rule-out differentials include Wobbler Syndrome, equine protozoal myeloencephalitis (EPM), and other causes of progressive weakness.

Treatment

Treatment is supportive and must be initiated rapidly. There is no practical antidote widely available in field conditions:

  • Remove contaminated feed: Immediately withdraw and isolate the suspected source of toxin
  • Antitoxin: Polyvalent botulinum antitoxin (Types A, B, C) may be available through specialist equine hospitals. It neutralises circulating toxin but cannot reverse established paralysis — early administration is critical.
  • Intensive supportive care: Hospitalisation with nursing care, assisted feeding (nasogastric tube), and intravenous fluids
  • Mechanical ventilation: In cases progressing to respiratory paralysis, mechanical ventilation may extend survival until the toxin effect wanes — this is expensive and resource-intensive
  • Antibiotics: For wound botulism, penicillin is indicated. Avoid aminoglycosides (such as gentamicin) as they can potentiate neuromuscular blockade.
  • Deep bedding and turning: Recumbent horses require frequent repositioning to prevent pressure sores and myopathy

Recovery, if it occurs, may take weeks to months as nerve terminals regenerate. Horses that survive are generally not left with permanent neurological deficits.

Prevention

Prevention centres on forage quality and, in high-risk regions, vaccination:

  • Forage management: Do not feed haylage or silage with a pH above 4.5, with visible mould, or where the bale was damaged prior to use. Any bale containing a dead animal should be discarded entirely.
  • Vaccination: A licensed botulism vaccine (Toxoid Type B) is available. Vaccination is recommended in regions with a higher incidence — notably parts of the USA — and for foals in endemic areas. Discuss with your vet whether vaccination is appropriate for your location and management system.
  • Rodent control: Minimise rodent activity around hay and forage storage to reduce the risk of carcass contamination
  • Avoid high-risk feeds: Be cautious with round-bale haylage, particularly from farms without strict quality control

Prognosis

The prognosis for equine botulism is poor to guarded, particularly in adult horses that present with advanced signs. Mortality rates in untreated or late-presenting cases can exceed 70-90%. With intensive hospital care and antitoxin, survival rates improve but remain heavily dependent on the dose of toxin absorbed. Foals with Shaker Foal Syndrome that receive intensive treatment have a somewhat better prognosis than adult horses. Early identification and rapid veterinary intervention are the most important prognostic factors.

When to Call the Vet

  • Any horse showing sudden onset generalised weakness, especially if accompanied by difficulty swallowing
  • A horse that was normal yesterday and is now stumbling or unable to hold its head up
  • A young foal (under two months old) showing muscle trembling and weakness
  • Multiple horses on the same premises showing similar signs — a shared forage source should be investigated immediately
  • Any suspected botulism is a veterinary emergency — call immediately and remove all feed from that batch

Frequently Asked Questions

How dangerous is haylage compared to hay for botulism risk?

Haylage and big-bale silage carry a higher botulism risk than dry hay because the moist, anaerobic conditions within the bale are ideal for C. botulinum spore germination — particularly if fermentation has been incomplete. Properly fermented haylage with a pH below 4.5 is considered safe. Any bale that smells off, contains visible mould, or has been damaged before feeding should be discarded. Always source forage from reputable suppliers with good quality control.

Can humans catch botulism from an affected horse?

The botulinum toxin itself is not contagious from horse to human in normal handling situations. However, contaminated feed could theoretically pose a risk if handled carelessly. Standard hygiene precautions when handling suspected contaminated forage are sensible. If you suspect a feed source is contaminated, handle it with gloves and avoid consuming it yourself.

Is botulism vaccination available in the UK?

Botulism vaccination is not routinely used in the UK as incidence is low compared to parts of North America. However, a Type B toxoid vaccine does exist and can be obtained through veterinary prescription in higher-risk situations. Speak to your vet about risk assessment for your specific management setup.

What should I do if I suspect contaminated haylage has been fed to my horses?

Remove the suspect forage immediately, bag it, and label it for testing. Contact your vet straight away, even if none of the horses are yet showing symptoms. Monitor all horses on the affected batch closely for the following 24-72 hours. Early antitoxin administration, before clinical signs are advanced, offers the best chance of a positive outcome.