Botulism is a disease that affects not only horses, but also a wide variety of animals including humans. It is closely related to the organism that causes tetanus, but is more deadly. Clostridium botulinum is always present in the environment. Eight distinct toxins of botulism have been identified.
Once the toxins enter the horse's system, the incubation period is from 24 hours to several days. Once within the system, the toxins reproduce repeatedly and rapidly in the gut of the animal. These toxins block the connection between the nerves and the muscles.
Reports of botulism in horses date back to the early 1980's in England and Australia. In recent years, outbreaks have been reported in the United States and Germany.
The ingestion of pre-formed toxins is the usual cause of infection in adult horses. This normally occurs when the animal eats bits of decayed or decaying material in their haylage, silage or hay. When hay is baled at about 45-50% moisture and wrapped in plastic or placed in a bag that reduces the presence of oxygen, it sets the stage for the development of Clostridium botulism.
Hay often becomes contaminated with the botulism bacterium during the raking and baling process. Rabbits, rodents and birds are killed during the mowing process and become incorporated in the hay. The smallest amount of decaying or decayed carcass may be distributed into many bags or bails and may not be detected because of the quantity and way of making the large bales or the haylage/silage.
A second common mode of contracting botulism by horses is through growth of the agent in the gastrointestinal system, also known as toxicoinfectious botulism. In cases where toxicoinfection is the route of the attack, the horse ingests spores that then germinate and produce toxins within the gastrointestinal tract.
Shaker Foal Syndrome is an example of this mode of contracting botulism. It occurs at approximately three to eight weeks of age, causing muscle tremors resulting in the name.
Wound infection is the third mode of contracting botulism. Although it is less common, toxins are produced at the site of the wound and absorbed into the horses system. Sometime seen in foals, the umbilical stump becomes infected and the toxins enter the system resulting in the classic symptoms of the disease.
The best methods of prevention include making sure that all horse feed is clean, harvested and prepared properly and are not allowed to become contaminated. Horse troughs and feed bins should be cleaned on a regular basis and water should be fresh each day to preclude becoming contaminated by rodents or bird droppings. A vaccine is available but is usually recommended only in endemic areas.
For treatment to work, the distinct toxin needs to be identified and the particular antiserum given as soon as possible. Anti-toxin treatments are expensive and may run as high as $3,000 per horse without any guarantee that the treatment will work. In suspected cases of botulism, work closely with your veterinarian to achieve the best possible outcome.