Botulism in Horses: How Infection Happens

Clostridium botulinum, the bacteria that produces the toxin responsible for botulism in horses, is no shrinking violet among microorganisms. The bacteria thrives in oxygen-free environments, including soil and water, and is an ever-present danger for horses.

While horses may become infected with botulism through an untended wound, ingestion of infected agricultural products, especially low-quality or poorly manufactured forages, is the primary route of transmission in horses.

Though many horsemen point to round-baled hay as a common denominator in botulism cases, such a leap is sometimes unfounded as it suggests that round-baled hay cannot be of sufficient quality for horses. On the contrary, harvesting methods have more to do with potential risk for botulism than bale shape or size. Contamination within hay bales is usually due to decomposition of animal carcasses or decaying plant material, possibly because of ill-timed harvesting.

While long-stem forage such as hay seems to be the primary conduit of botulism, other forage products have played host to Clostridium botulinum. On one occasion, an outbreak of botulism was traced to processed hay cubes contaminated with decaying rodents.

Risk of botulism is one reason why cattle are often fed hay that might prove dangerous to most horses. Cattle and horses have different sensitivities to toxins produced by the bacteria, and cattle possess a higher threshold for the toxin.

Adult horses are usually affected by botulism, though an unusual disease in foals called shaker foal syndrome has been attributed to a specific toxin produced by Clostridium botulinum.

Clinical signs of neurotoxicity typically arise in horses within 24 hours of exposure to the toxin, and signs include general weakness, inability to swallow, and poor muscle tone. Horses will generally become too weak to stand and will be unable to rise.

Prognosis for horses diagnosed with botulism is poor, though the outlook is more favorable for those that can remain standing.

Aside from vaccination, hay selection is key to avoiding botulism. Acquiring hay from a reputable grower is the best place to start. Even then, it is wise to inspect hay as thoroughly as possible before feeding. Feeding hay and feed off the ground is another potential safeguard. Various feeders are available that are manufactured specifically for horses with safety in mind.