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Beating Botulism in HorsesBy Kentucky Equine Research Staff · October 16, 2017

Botulism, the bad boy of the equine toxin world, can kill horses and foals swiftly. As one of the most potent toxins known to affect horses (yes, even more toxic than snake and spider venom, arsenic, and mercury), botulism causes death almost undoubtedly unless affected animals receive the botulism endotoxin and aggressive supportive care. Want to do everything possible to prevent botulism and safeguard your steeds? Review this list to learn how, bearing in mind most of this particular article refers to forage poisoning rather than the less common shaker foal syndrome and wound contamination.

Feed and forage selection. Many cases of botulism occur after ingestion of the toxin from feed. The bacterium Clostridium botulinum (from decomposing small animal carcasses trapped in hay bales, for example) produces toxins, labeled A through H, which horses may ingest. Type B botulism occurs most frequently in adult horses, but horses and foals can also suffer from types A and C.

“Following ingestion, the toxin quickly blocks the junction between nerves and muscle. As a result, horses rapidly lose the ability to swallow, stand, and void their urinary bladder,” explained Kathleen Crandell, Ph.D., a Kentucky Equine Research (KER) nutritionist.

Rapid diagnosis. Did one or more of your horses suddenly begin to show neurological signs? Think botulism. Unfortunately, the available tests for botulism are time-consuming relative to the rapid timeline that botulism can kill. As such, botulism is usually a “diagnosis of exclusion,” meaning that if all other potential causes of malaise are ruled out, then botulism can confidently be ruled in. Unfortunately, the list of diseases that botulism mimics is rather extensive, starting with colic or choke as well as encompassing various neurological and muscular conditions such as rabies, the viral encephalomyelitis (Eastern, Western, and Venezuelan equine encephalomyelitis), equine protozoal myeloencephalitis, equine herpes encephalitis, West Nile virus, wobbler syndrome, white muscle disease, hyperkalemic periodic paralysis, and more.

Swift and aggressive treatment. “Once botulism is suspected, your horse’s best chance of survival is hospitalization, otherwise survival is poor,” recommended Crandell.

Indeed, one study* reported that survival rates for adult horses can be as poor as 10-50%. If a horse retains the ability to stand, survival can be higher. Foals typically have higher survival rates, up to 87-96%, even when mechanical ventilation is required. This likely could be attributable to the challenges associated with managing recumbent adult horses, such as pressure sores or muscle and nerve damage due to continual lying down. As one might suspect, treatment of affected horses is expensive, and time to complete recovery can be prolonged, sometimes up to 90 days.

According to the study, “owners of horses diagnosed with botulism can generally anticipate a two-week hospitalization period, with some stays shorter or longer. Nonsurviving horses were generally euthanized quickly, with a median duration of hospitalization of one day.”

Vaccination. Although the exact number of horses that suffer from botulism each year remains unknown, botulism toxins are clearly not to be trifled with. Only one vaccine licensed for the prevention of type B botulism exists. This vaccine does not protect against the other types of botulism, and is considered a risk-based vaccine by the American Association of Equine Practitioners (AAEP).

Alternate feeding strategies. Although many horsemen point their fingers at round bales as the penultimate cause of type B botulism in horses, any type of baled forage or feed can potentially be contaminated.

“One of the best ways to avoid botulism via nutrition management strategies is to break apart or unroll hay bales—square or round—as they are fed and spread the hay. This provides the opportunity to see decayed animal carcasses and sound the alarm that the botulism toxin could be present,” advised Crandell.

She added, “Don’t forget that small animals can also perish in tubs of grain or other concentrates or feedstuffs. Always tightly seal feed-storage containers to avoid problems.”

Do you have a question about feeding management? Contact a KER nutrition advisor for more information.

*Johnson, A.L., S.C. McAdams-Gallagher, H. Aceto. 2015. Outcome of adult horses with botulism treated at a veterinary hospital: 92 cases (1989-2013). Journal of Veterinary Internal Medicine. 29(1):311-9.