Department of Agriculture and Forestry, State of Louisiana - Mike Strain DVM, Commissioner

Botulism is a deadly disease caused by the toxins produced by the bacterium Clostridium botulinum. The botulinum toxin is a potent neurotoxin that impairs nerve function, including those of the diaphragm, leading to paralysis. When the nerves to the diaphragm are paralyzed, the affected animal stops breathing and will die as a result.

There are seven types of botulism recognized (A, B, Ca, Cb, D, E, F, and G), based on the antigenic specificity of the toxin produced by each strain. Types A, B, E, and F cause human botulism. Types A, B, C, and D cause most cases of botulism in animals. In horses, type B botulism is responsible for more than 80% of the cases.

The bacterium and its spores are widely distributed in nature. They are found in soil, sediments of streams and lakes, and in the intestinal tracts of fish and mammals. The bacteria will produce toxins under the conditions of decaying plants and animals.

A horse can get botulism in three ways:
• By consuming forage or feed containing the bacteria, which will then produce the toxins in the intestinal tract (more commonly found in foals, known as shaker foal syndrome, or toxicoinfectious botulism).
• By consuming feed or forage containing the pre-formed toxins of C. botulinum (known as forage poisoning).
• Through wounds contaminated with the bacteria, generally puncture wounds. Wound borders will close, providing an anaerobic environment, which is a favorable condition for the bacteria to produce the toxins.

Although the incidence of the disease is low, it is of considerable concern because of its high mortality rate if not addressed and treated immediately and properly. The mid-Atlantic region of the eastern United States, and especially Kentucky, is where botulism is most commonly found, although the disease is reported worldwide. The spores of C. botulinum Type B can be found in the soil of most regions of the United States, although they are more frequently found in the northeastern and Appalachian regions. The western region is more abundant with C. botulinum type A, and type C occurs mainly in Florida.

The frequency of occurrence of foodborne botulism in humans and in horses correlates with the distribution of the types of spores in the soil.

Treatment
Treating a horse with botulism can be very costly, difficult, and often too late. It is better to prevent the disease than to treat it. Recommended treatment for botulism includes early administration of hyperimmune plasma containing antitoxin. The antitoxin binds to the toxin molecules that are free-floating in the bloodstream and neutralizes them before they bind to nerve cells, but they cannot reverse the effects of bound toxin. The bond that forms between the toxin and the nerve cell is irreversible.

The horse’s body can make new neuromuscular junctions to replace the ones that are affected by the toxins; however, this process requires 7 to 10 days. It is a challenge to keep a horse alive that is recumbent and cannot eat or drink. In adult horses, being recumbent for a few days poses a problem in itself. They can develop pressure sores, colic, muscle damage, etc. Moreover, the horse will need to be mechanically ventilated and administered supportive therapy. However, it is very difficult to keep an adult horse on a ventilator for days, as the available machines are not designed to support this workload. If the paralysis has extended to the breathing muscles of an adult horse, it is humane to euthanize it.

Prevention
Horse owners should be cautious about feeding hay that has been rained on during the harvesting phases. Roundbaled hay is particularly a risk factor when baled at excessive moisture content. Any hay with rotten or decaying material should not be fed to horses. Since the spoiled material is most likely to be internal in round hay bales, it may be impossible to visually determine this condition unless the bales are opened. If the exterior of the bale is rotten with dark discoloration and moldy or if the bales feel warm, they should not be fed to horses. An unspoiled round bale, put out for a group of horses, is generally not a problem.

There is also a risk for botulism if horses are being fed silage or haylage, especially if the fermentation process was
inadequate to lower the pH to inhibit the growth of the bacteria and toxin production. Haylage, silage, and high-moisture hay are more prone to spoilage. For people who own horses and cattle, and thus feed silage to all their animals, it is important to mention that cattle are not as sensitive to botulism as horses, but they do die from this disease.

Vaccination
There is a USDA-approved vaccine available to prevent botulism. The vaccine can be purchased from your veterinarian. Talk with your veterinarian about the best vaccination schedule for your herd. You can find a proposed vaccination schedule for foals and adult horses from the American Association of Equine Practitioners (AAEP).

Source: University of Kentucky – College of Agriculture’s Cooperative Extension Service – Botulism: A Deadly Disease That Can Affect Your Horse