Botulism is caused by a neurotoxin produced by the anaerobic bacterium Clostridium botulinum. The bacterium is found in the soil. It’s closely related to Clostridium tetani, the bacterium that causes tetanus. The toxins produced by C. Botulinum is one of the most potent poisons known to man. Horses are particularly sensitive to botulinum toxin; untreated foals can suffer up to 90 percent mortality. Mortality is also high in untreated adults.

Clinical Signs

The clinical signs of botulism can be confused with other conditions such as rabies, equine protozoal myelitis (EPM), tetanus, and azoturia (tying-up). Unlike most of these, however, botulism can progress very rapidly. The toxin works by inhibiting the release of acetylcholine (a neurotransmitter) at the neuromuscular junction, the point at which nerve endings meet muscle fibers. Without acetylcholine, muscles are not stimulated to contract and become progressively weaker and, finally, paralyzed. Affected horses lose the ability to swallow food or water, spilling grain and saliva from their lips, present ataxia, or incoordination, and can suffer other signs, such as depression, muscle tremors, a protruding tongue, dilated pupils, constipation, colic, shortness of breath, and violent spasms or seizures. Physical activity or transporting the horse may worsen clinical signs.

Within 48 hours, horses affected with botulism can be recumbent and unable to rise, typically with their chins resting on the ground. Respiratory paralysis usually forces euthanasia, or the horse will die spontaneously. The severity of signs, however, is largely dependent on the amount of toxin, and less severely affected horses might decline slowly, a characteristic that can confuse diagnosis.

In foals, botulism causes what is called shaker foal syndrome. The foals will be weak and unable to stand, will present impaired suckling, inability to swallow, decreased eyelid and tail tone, and dilated pupils. Ultimately, respiratory paralysis will cause death.

The most common way of a horse acquiring this disease is by ingestion of the toxin (not the bacterial spores themselves) in contaminated feed or water. Decomposing carcasses of rodents or birds in baled hay are often blamed, but it is also common for hay or silage to be contaminated through improper storage or poor fermentation. The risk is increased when horses are fed large round bales.


There is an antitoxin available, but unlike the tetanus antitoxin, the prices for the botulism antitoxin are most times prohibitive, with treatments reaching up to $7,000.


Botulism may be prevented through vaccination with BotVax B.

See Bacterial Diseases of the Horse for more information.