Throughout the year and over a horse's lifetime, horse owners need to be aware of a deadly neurological disease that can have fatal consequences if the affected horse is not treated in a timely manner.
Equine botulism is a primary neurological disease that can manifest over hours to days. The horse is clinically affected with the disease most commonly by eating feedstuffs that are not stored appropriately. The horse ingests the spores from this bacteria and these spores affect the nervous system in the horse, making it difficult for the horse to stand, prehend food and swallow appropriately.
Every year we have horses present to the emergency service or to the daytime medicine service at Galbreath Equine Center displaying signs of neurologic disease that are most consistent with equine botulism. The classic signs of botulism that horses display are muscle weakness, trembling, weak tail movement and eyelid tone, and diminished swallowing. A tongue stress test may also show decreased tone and difficulty eating grain.
"Clinical cases of botulism may often present with dysphagia (inability to prehend, chew and swallow feedstuffs and water normally due to ingestion of the spores found in decaying plant material or feedstuffs (hay)," said Dr. Eric Schroeder, associate professor. "Usually horses are being fed hay this time of year because they're not out to pasture due to the change in the weather and the lead up to the cold weather of winter. However, if not kept dry and off the ground, hay becomes a perfect medium for spores to multiply. Once ingested, the spores spread throughout the body and bind to neurotransmitters on the nerves controlling muscle tone, blocking muscles from being able to contract normally."
"Botulism is thought to be present only in Ohio, western Pennsylvania, southeast Indiana and Kentucky, western New York, northern Tennessee and parts of Virginia and West Virginia,” Dr. Schroeder said. It is not yet known why only these U.S. regions are affected, but it is presumed to be related to climate and soil type.
"This disease is one of the more common neurologic diseases that we are faced with on the emergency service," Dr. Schroeder said. "Case prevalence can be as high as 15-20 cases per year."
The hay fed to the horse prior to showing any clinical disease usually looks completely normal. With this disease you cannot visualize the spores within the hay, therefore it makes it very difficult to predict. It is vital though that if any horse is showing clinical signs or presumed clinical signs, that the hay is not fed to any other horse. In this part of the United States, we also instruct owners to have their horse vaccinated for botulism. This is one way to help prevent the disease, as is making sure the hay that is fed off the ground does not come into contact with water and the hay is baled appropriately dry.
In most cases of horses thought to be affected with botulism, if they remain standing, their prognosis for survival is better than if the horse loses the ability to stand. Horses that are still able to stand and walk (a positive sign) despite weakness and swallowing difficulty are much easier to treat than horses that are recumbent. One of the common treatments that we prescribe to a horse showing clinical disease of botulism is hyperimmunized plasma, which helps eliminate the toxins before binding to the muscle neurotransmitters. An indwelling feeding tube is commonly placed as well because these horses cannot swallow normally -- therefore we have to provide food to them in the form of a liquid-based diet. Oftentimes, the affected horse is at an increased risk of aspiration of saliva and food, therefore secondary pneumonia is a significant complication. The affected horse is often started on antibiotics to help prevent pneumonia from the increased risk of aspiration.
Dr. Schroeder said most horses can recover within seven to 14 days depending on the severity of the clinical signs. In rare cases (approximately 10%), the horses may require up to 30 days of hospital care, with the most severely affected case requiring extensive nursing care. The Galbreath Equine Center at Ohio State University is well-equipped to handle these situations.
Early diagnosis is key to providing appropriate and often lifesaving treatment with hyperimmunized plasma being the mainstay therapy, Dr. Schroeder said.
"Survival is much less among horses that are recumbent and have shallow difficulty in breathing," Dr. Schroeder said. "Standing horses, however, have greater than an 80% chance of survival."
The key message: botulism is preventable. Veterinarians can purchase a vaccine that requires two subsequent boosters but provides adequate protection to the horse. This vaccine needs to be administered at least annually in this part of the United States and, ideally, biannually. Dr. Schroeder also encourages practitioners to ask about horse feed (types of feed and how the feed is stored and fed).
"These measures go a long way toward preventing heartbreak as well," Dr. Schroeder said.
For equine emergencies, call (614) 292-6661. See vet.osu.edu/vmc/equine for more information.
