Botulism in Horses: Causes, Signs, and Emergency Treatment

Botulism is a severe neurologic disease caused by toxins made by Clostridium botulinum. These toxins block normal nerve-to-muscle signaling, so affected horses develop flaccid paralysis rather than stiffness. Horses are especially sensitive to botulinum toxin, which is why even a small exposure can become life-threatening.

In adult horses, botulism most often happens after eating feed that already contains toxin, such as spoiled forage or feed contaminated by a carcass. Less commonly, the bacteria can grow in a wound and release toxin there. In foals, the bacteria may grow in the intestinal tract and produce toxin internally, a form often called toxicoinfectious botulism or "shaker foal syndrome."

The disease is an emergency because the same weakness that affects the tongue, eyelids, and limbs can also affect the muscles needed for swallowing and breathing. Some horses first look mildly weak or "off," then worsen over hours to a couple of days. Fast veterinary care gives your horse the best chance for stabilization and supportive treatment.

There are three main ways horses develop botulism. The first is forage poisoning, where the horse eats preformed toxin already present in spoiled feed. This can happen with decaying hay, haylage, silage, grain, or round bales contaminated by decomposing animal tissue. Even a small carcass in forage can create a dangerous exposure.

The second route is wound botulism, where Clostridium botulinum grows in a contaminated wound and produces toxin there. This is less common than feed-associated disease, but it is a recognized cause in adult horses. Deep punctures, neglected wounds, or areas of dead tissue can create the low-oxygen conditions the bacteria prefer.

The third route is toxicoinfectious botulism, most often seen in foals younger than about 4 weeks. In these cases, the bacteria colonize the intestinal tract and produce toxin inside the body. This form is often called shaker foal syndrome because affected foals may tremble and become progressively weak.

Risk is higher in areas where botulism is more common, in horses fed poorly preserved forage, and in unvaccinated horses exposed to type B botulism risk. Vaccination used in the United States targets type B botulism, which is important in endemic regions but does not cover every toxin type.

Diagnosis can be challenging, and your vet often has to make a presumptive diagnosis based on history, exam findings, and how the weakness is progressing. Clues that raise concern include poor tongue tone, dysphagia, weak eyelid or tail tone, decreased muscle tone overall, and a history of exposure to suspicious forage, a carcass-contaminated bale, or a wound.

There is no single perfect test that quickly rules botulism in or out in every horse. Your vet may collect serum, feces, gastrointestinal contents, or suspect feed for toxin testing, but these tests can be negative even in true cases. Because of that, botulism is often diagnosed by combining the clinical picture with exclusion of other causes of weakness or paralysis.

Other conditions your vet may consider include choke, equine protozoal myeloencephalitis, tetanus, toxicities, metabolic disease, and other neurologic or neuromuscular disorders. Hospital evaluation may include a full neurologic exam, bloodwork, endoscopy if swallowing problems are present, and monitoring of breathing and hydration. In many cases, treatment decisions must begin before definitive test results return.

Prevention starts with feed management. Do not feed spoiled hay, haylage, silage, or grain, and inspect round bales and stored forage carefully for mold, foul odor, wet pockets, or evidence of animal carcasses. Remove and discard any suspect feed right away. Good storage practices matter because Clostridium botulinum grows best in low-oxygen, decaying material.

Check pastures, paddocks, and feeding areas regularly for dead wildlife or other decomposing material. Clean water sources and feeders routinely, and address wounds promptly with veterinary guidance. Wound care lowers the risk of wound botulism, especially in deep punctures or contaminated injuries.

In areas where type B botulism is a known concern, your vet may recommend vaccination with type B toxoid for broodmares, foals, or other at-risk horses. This vaccine is used in the United States for prevention of type B disease, but it does not protect against every botulinum toxin type. Your vet can help decide whether vaccination fits your horse's geography, forage practices, and overall risk.

If one horse on the property is suspected to have botulism, stop feeding the same forage lot to other horses until your vet has reviewed the situation. Quick action may prevent additional cases.