Equine Botulism at a Glance
Botulism Source Distribution
Comparison
| Feature | Feed-Borne | Wound Botulism | Shaker Foal |
|---|---|---|---|
| Cause | Ingest preformed toxin in feed | Toxin from wound infection | Toxin from gut colonization |
| Onset | 12–48 hours after ingestion | 3–14 days after wound | 2–4 weeks old |
| Signs | Dysphagia, tremors, recumbency | Stiff gait, weakness, paralysis | Muscle tremors, difficulty eating |
| Mortality | 50–70% if untreated | 30–50% | 70–90% if untreated |
Botulism Emergency Response
Botulism Prevention Checklist
| Vaccinate broodmares before foaling | |
| Never feed moldy or spoiled hay/silage | |
| Discard round bales with visible decay | |
| Keep feed bins clean and dry | |
| Avoid feeding hay that contains carcasses | |
| Vaccinate foals in endemic areas | |
| Clean and disinfect wounds thoroughly | |
| Report suspected cases to vet immediately |
Botulism in horses is caused by botulinum neurotoxin (BoNT) produced by Clostridium botulinum bacteria. The toxin blocks acetylcholine release at neuromuscular junctions, causing progressive motor paralysis that typically begins with difficulty swallowing, progresses to generalized weakness, and ends in respiratory muscle paralysis and death without intensive care. Horses are among the most sensitive species to BoNT -- the lethal dose is approximately 1 ng/kg body weight. Mortality without treatment approaches 90%; even with intensive supportive care, many affected horses die. Vaccination with the botulinum toxoid (particularly against type B, the most common equine strain) is the primary prevention tool and strongly recommended for horses in endemic areas and breeding farms.
| Exposure Route | Primary Risk Group | Geographic Focus |
|---|---|---|
| Forage-associated (shunt) | Adult horses — contaminated hay / haylage | Nationwide; round bale feeding especially high-risk |
| Shaker foal syndrome | Foals under 6 months | Mid-Atlantic endemic (VA, MD, KY) — Type B most common |
| Wound botulism | Any horse with deep puncture wounds | Uncommon; requires anaerobic wound conditions |
Sources of Exposure
Equine botulism occurs through several routes, each with different risk profiles:
Forage-associated botulism (shunt botulism): The most common form in adult horses. Decaying plant matter, dead animals (mice, birds, rabbits) contaminating hay bales or haylage, and improperly fermented silage or haylage contain C. botulinum that multiply and produce toxin. A single contaminated section of a hay bale can cause botulism in multiple horses; round bale hay fed on pasture is particularly implicated because decomposing organic matter, moisture accumulation, and dead animals are commonly incorporated. Haylage and silage (fermented forage) are very high-risk if fermentation pH is insufficient (above 4.5) to inhibit C. botulinum growth.
"Shaker foal" syndrome (toxicoinfectious botulism): Affects foals under 6 months old. Clostridium botulinum spores germinate in the intestinal tract of young foals (who lack the mature competing gut flora of adult horses) and produce toxin in situ. Type B is most common in the eastern US; Type C is associated with foals in the western US. The classic presentation is a nursing foal that progressively loses the ability to suckle, then cannot stand, develops the characteristic "shaking" from muscle weakness, and dies from respiratory failure without treatment. Regional endemic areas: the mid-Atlantic states (Virginia, Maryland, Kentucky) have particularly high C. botulinum type B spore concentrations in soil.
Wound botulism: C. botulinum infects contaminated wounds (particularly puncture wounds in anaerobic tissue) and produces toxin locally. Less common than forage-associated botulism but possible in horses with soil-contaminated wounds.
Clinical Signs: Recognizing the Progression
Botulism causes flaccid (limp, not rigid) paralysis progressing from the head downward. Signs appear 12 hours to 7 days after toxin exposure, depending on dose:
Early/mild: Difficulty chewing or swallowing (dysphagia) -- hay drops from the mouth half-chewed ("quidding"); water drips from the muzzle after drinking; slow eating; muscle trembling, particularly of the shoulders and flanks. Pupillary light response may be reduced (pupillary dilation from autonomic blockade).
Moderate: Generalized weakness causing stumbling, reluctance to move; inability to lift the head normally; salivation; constipation from GI dysmotility; decreased urination from bladder dysfunction; progressive respiratory effort increase.
Severe/terminal: Recumbency (cannot stand); open-mouth breathing; respiratory failure. A horse that cannot rise has a very poor prognosis. Hypoxemia from inadequate ventilation, aspiration pneumonia from dysphagia, and urinary bladder rupture from retention are the primary secondary complications.
Treatment
Botulism antitoxin: if available and administered early (before toxin is fully bound to nerve terminals), equine botulinum antitoxin neutralizes circulating toxin. Equine origin antitoxin (commercial product) is the standard of care; human heptavalent antitoxin is not licensed for equine use in the US. Cost: substantial ($500-$1,500 per dose; multiple doses may be required). Efficacy is greatest when given before clinical signs are severe -- another argument for recognizing early signs.
Supportive care: mechanical ventilation for horses in respiratory distress (available only at large referral centers with equine ventilator capability); nasogastric feeding for horses unable to swallow; IV fluids; bladder catheterization; turning recumbent horses every 4-6 hours to prevent decubital ulcers and aspiration; antimicrobials for secondary bacterial complications (avoid aminoglycosides, which potentiate neuromuscular blockade).
Recovery takes weeks to months as new nerve terminal sprouts grow to replace blocked junctions -- horses that survive the acute phase improve gradually.
Vaccination Protocol
The BoNT type B toxoid (Neogen Calvenza, Clostridium botulinum type B toxoid) is approved for horses and provides protection against type B botulism -- the most common equine form in endemic US regions. AAEP recommends vaccination for: horses in endemic regions (mid-Atlantic states, particularly Virginia, Maryland, and Kentucky where type B spore concentrations are high); breeding farms (where shaker foal syndrome risk is significant); any horse regularly eating haylage or silage. Primary series: 3 doses at 4-week intervals; annual booster. Pregnant mares: booster at 4-6 weeks pre-foaling to transfer passive immunity to foals via colostrum. Foals on endemic farms: begin primary series at 2-3 months of age (earlier if dam not vaccinated and farm is endemic).
Sources: AAEP Botulism Vaccination Guidelines; Wilkins (2003) botulism review, Veterinary Clinics Equine Practice; Whitlock & Buckley (1997) shaker foal syndrome; Merck Veterinary Manual equine botulism; USDA NRCS C. botulinum spore distribution data.