This disease is relatively uncommon, but it’s important to recognize the signs.
With news headlines swirling around a recent batch of processed forage leading to cases of botulism in horses, the questions arise — what is botulism? How does it affect horses? What are the signs? Should we be worried? Veterinarian Dr. Rudy Madrigal, DVM, at Equine Sports Medicine and Surgery in Weatherford, Texas, shares his insights on this neurologic disease.
What is Botulism?
Botulism is a bacterial infection in which a toxin is produced and leads to clinical signs. Madrigal says there are seven types of toxins that stem from this infection, but only three are known to affect horses: clostridium botulinum types A, B and C.
“We see the B toxin most commonly on the East Coast, Kentucky, more the Northeastern states,” Madrigal said. “The A toxin is more of the Western states where you’ll see them. The C toxin is most commonly seen in Florida, and sporadically in other areas. That’s the one most often seen in animal carcasses.”
The toxin binds to neuroreceptors in the body and most commonly results in flaccid paralysis.
“This means the nerve conduction is unable to happen, and the result is the horses get weak,” Madrigal said. “It gets progressively worse, and the severity is dependent upon when you start treating them, and how much toxin they ingest.”
How Can a Horse Contract Botulism?
There are two main ways a horse can acquire botulism — infection of a would and ingestion. This disease is not passed directly from horse to horse, or from horses to humans. But if several horses are consuming the same infected feed, there can be an outbreak of botulism.
“Contaminated feed its the most common method of contracting botulism, whether because it was improperly stored and held too much moisture, which allows the bacteria to grow in there, or if it’s been contaminated by a carcass that had botulism,” Madrigal said.
Some of the early signs of botulism in a horse include the horse being unable to swallow or retract its tongue, Madrigal said. They may have difficulty standing, along with other neurological signs such as weakness in other parts of the body.
“They may also have weak eyelids — they may be unable to really open up their eyelids,” Madrigal said.
The horse’s condition will deteriorate the more exposure the horse has to the toxin, Madrigal said.
“As they ingest more toxin, they’ll continue to get weaker, until they become recumbent, or are unable to stand,” Madrigal said. “Usually once that happens, the most common thing to do, or the best thing to do for them, is to unfortunately put them down. These horses are unable to clear the toxin in a quick enough manner to recover.”
Historically, diagnosis of botulism was difficult, as it was performed using a mouse bioassay. This meant that samples from affected patients were injected into mice and each mouse was treated with antitoxin to each type of botulism to help identify the toxin type. Madrigal says newer testing including polymerase chain reaction (PCR) can help find the DNA for each toxin type, although few labs are currently performing this testing.
Madrigal says the only treatment available for horses is administering plasma with an antitoxin, paired with supportive care. The disease cannot be reversed. There is no way to make the toxin release from the neuroreceptors, but with time, they can slowly release on their own.
“Foals tend to have a better outcome, but foals may end up needing to be put on ventilators,” Madrigal said. “But there’s no good way to keep an adult horse on a ventilator.”
Botulism starts affecting a horse’s ability to contract its muscles, and it affects the muscles needed to stand first, Madrigal said. Once the muscles are affected, horses are unable to stand, leading to recumbence, and then the muscles for respiration become affected, leading to asphyxiation. Once a horse has become recumbent, chances of survival are extremely low.
“If you get plasma in the horse, you’re hoping to stop progression of the disease, but you will not be able to reverse the clinical signs,” Madrigal said. “For some of these horses, as long as they’re still standing and eating, we can keep them going. If they’ve already progressed to clinical signs of difficulty swallowing, that can take weeks to resolve. They can fully recover, but it takes time.”
There is a vaccine to cover toxin B, so horse folks can vaccinate against this strain of botulism, Madrigal said. This is helpful in areas where botulism is common, particularly when given to mares in the last three months of pregnancy to give foals immunity to that one strain.
The only other method of prevention is keeping a close eye on your feed selection, particularly manufactured feed, and hay.
“Try to choose feed that has been well-maintained from a forage point is the main thing,” Madrigal said.
Fortunately, botulism is not very common in horses. The current outbreak in alfalfa cubes is a rarity, says Madrigal.
“Here in Texas, we don’t really see botulism, because [many people] feed round bales and square bales, and it’s not commonly seen in that forage source,” Madrigal said.
Your best bet is to pay attention to signs in your horse of neurological issues, particularly with eating and drinking.
“The earlier you catch it, the better results you’ll get, if you’re able to treat it,” Madrigal said.
This article was originally published in the March 2023 issue of Barrel Horse News.