Botulism in horses is a severe and often fatal neurological disease caused by a toxin produced by the bacterium Clostridium botulinum. The bacteria can be ingested in contaminated feed (forage poisoning), can occur in an infected wound (wound botulism), or in the case of Shaker Foal Syndrome, spores can germinate and produce toxins in the gut of foals. 

 

CAUSES 

Clostridium botulinum is found ubiquitously in soil and leads to different variants of botulism labeled A through G; however, types A and B are the most common in horses. While there is some regionality regarding the strains, “horses anywhere could be at risk due to the environmental conditions associated with what the bacteria need to produce toxin,” explained Dr. Kirk Ramsey, DVM, Neogen Professional Services Veterinarian. “While botulism does seem to be endemic towards the warm, moist areas of the United States — the "Botulism Belt’ from Missouri to New York — it can occur anywhere.” The bacteria is typically found in decaying plant or animal matter. The toxins are produced when a small animal is trapped during the hay baling process and the body ends up tightly packed in the bale. The decomposing carcass creates an incubator for botulism spores which then contaminates hay. This means that any horse fed hay is potentially at risk as you don’t know exactly where the hay is grown, harvested, and baled. With hay being transported throughout the county, a horse in Southern California could be fed hay that was produced in an endemic region, like Kentucky. 

In the case of wound botulism, the bacteria infect a puncture wound, laceration, or castration site and the toxins multiply within the damaged tissue, leading to neurological deficits. 

 

SYMPTONS 

The major challenge of treating equine botulism is that it is difficult to diagnose based on physical symptoms, as it can present like other neurological diseases. Laboratory testing is either inconclusive or not time sensitive, and test results are often returned after it is too late. “Early treatment is essential, but often people don't know it's botulism,” said Scientific Consultant Gerri Brown, PhD.

The first clinical signs of botulism are weakness, exercise intolerance, and muscle tremors. “The botulism toxin inhibits the synaptic transfer of acetylchoine, which essentially paralyzes the nerve to muscle communication, leaving the animal paralyzed,” said Dr. Ramsey. “The physical symptoms begin with weakness that progresses to recumbency, meaning the horse can't stand.” Horses may have trouble keeping their eyes open, suffer a loss of tail tone, or their tongue will become flaccid, and hay will fall out of their mouth. “Owners often notice they are slobbering more and dropping food,” said Dr. Brown. By the time they go down and become recumbent, there is a very high mortality rate, she explained. While the horse maintains mental capacity, their body is shutting down. The most severe symptom is if the horse has ingested enough toxin to paralyze the diaphragm, and they are unable to breathe. “The botulism toxin is one of the most lethal substances known — even minute quantities can be incredibly devastating,” explained Dr. Ramsey.

 

Initial symptoms of toxicoinfectious botulism in foals is often characterized by an inability to swallow. Owners often notice foals with milk in their nostrils. As the paralysis progresses, foals develop muscle fasciculation (which causes twitching or shaking) and they become progressively weaker – these characteristic tremors are what gives the disease its name, Shaker Foal Syndrome. Foals typically have a better prognosis and chance of survival than adult horses, partially because it is easier to treat a smaller animal in a hospital setting than a full-grown horse. 

 

Unfortunately, in adult horses, botulism is often misdiagnosed as colic as the horse becomes stressed and anxious, which is typically perceived as pain. As initial clinical signs are vague, and symptoms develop rapidly, treatment of botulism can become costly, especially if the horse is hospitalized. The majority of recumbent horses are ultimately euthanized.  

 

TREATMENT 

If caught in time, there is an antitoxin that can be administered, which increases the survival rate up to 75 percent. The antitoxin helps to bind toxins that are floating in the bloodstream, so they can proceed to affect the neuromuscular junctions in the body. However, there will still be neurological damage even with the antitoxin as the nerves that were compromised will not recover.  

 

Once a horse becomes recumbent, they are often not able to stand on their own for four to six weeks, and it may be months before they can return to normal function if they survive the illness. Recumbency requires 24-hour care from equine nursing staff, who treat bed sores and other potential complications like pneumonia or urinary concerns, all of which take a large financial toll on the owner.  

 

PREVENTION  

“The best way to protect your horse is to prevent them from ingesting the Clostridium botulin through good working knowledge of your feed sources,” said Dr. Brown, but that is almost impossible to achieve. Even if you take meticulous care to bale your own hay, animal carcasses can still get rolled into it. Knowing where your hay is sourced and using a good quality production facility is the first step in prevention, however, while also observing that the hay you are feeding is of a high standard. Round bales are more likely to be contaminated than square bales because the way they are rolled captures more moisture. Remember, just because a bale looks good on the outside, there can still be danger lurking inside! 

 

“For wound care, it is critical that the area stays clean and does not close while dirty or infected,” Dr. Ramsey recommended. “This is why veterinarians try to keep their surgical sites as clean as possible, and recommend consistent monitoring of wound or surgical sites during the healing process,” he added. Treat all wounds thoroughly and ensure that they do not close prematurely as that can potentially exacerbate the condition. 

 

THE IMPORTANCE OF VACCINATION  

Vaccination is the most efficient weapon against botulism and is far easier and more cost effective than treatment. The BotVax® B vaccine from Neogen® specifically targets type B botulism, which makes up 85 percent of all equine botulism cases. The product was first developed in 1964, and it remains the only USDA-approved vaccine for botulism. “The vaccine is a toxoid, which primes the immune system to react to the toxin,” said Dr. Ramsey. “The vaccine stimulates the immune system to produce antibodies that can bind and neutralize the toxin when present. Having toxoid exposed antibodies in the immune system can reduce clinical signs, and prevent development of the disease,” he explained.  

 

The horse has an initial vaccination series of three injections once a month to bring it up to immunocompetence, then it receives an annual vaccine from then on. “Some owners opt to vaccinate a pregnant mare toward the end of gestation so that the foal is born with maternal antibodies that protect them,” Dr. Ramsey said. There are no reported adverse events of the vaccine, according to Dr. Ramsey, however, every individual’s response is unique. It is safe to administer to healthy adult horses, pregnant mares, and foals two weeks of age and older. For more information on the BotVax® B vaccine, please visit engage.neogen.com/equine-botulism.