Though you may not have thought much about it, botulism is a very real threat to the wellbeing of a horse. However, it is preventable with good husbandry and routine vaccination.
What is Botulism in Horses?
A disease caused by a neurotoxin released from Clostridium botulinum bacteria, botulism neurotoxin is one of the most lethal natural substances. Botulism attacks the nervous system of infected mammals, binding to nerves and impairing their function. In extreme cases, the cardiac and respiratory systems can be affected, rending the animal unable to breathe or maintain a regular cardiac rhythm.
Though there are seven types of botulism—A, B, Ca, Cb, D, E, F and G—the one of most concern to horse health is type B, which causes more than 80 percent of equine botulism cases.
Botulism is a naturally occurring toxin found in the environment that is harmful to mammals.
“Horses are exquisitely sensitive to botulism,” says Rachel Blakey, VMD, of Maryland Equine Center in Reisterstown, Md. “The amount of botulism that can kill a horse may not impair a mouse; horses are simply that susceptible.”
Botulism bacteria are very hardy and live without oxygen and sunlight. They can be found in the soil, in the sediment of lakes and streams, and in the intestinal tracts of mammals.
How Botulism is Transmitted in Horses
A horse can get botulism in three ways, though ingestion of the preformed bacteria is the most common.
1. By eating hay or feed that has botulism toxin already in it (ingestion of preformed bacteria). This is the means most often associated with feed-based poisonings, like the contaminated alfalfa cubes that killed 45 horses in late 2022.
Animals that are accidentally baled into hay can release the toxin from their intestinal tract when they begin to decay, contaminating the hay that surrounds them. Large round bales are particularly susceptible to botulism contamination because their mass allows for more room for contamination than small square bales.
2. By ingesting feed, hay or other organic matter that contain botulism bacteria, which then multiply in the intestinal tract and release neurotoxins (transmission in this manner is most common in foals).
Botulism bacteria favors dark, moist environments, like piles of wet feed or tree roots that aren’t exposed to sunlight. In these environmental conditions, botulism is not accessible. However, when a tree falls over and a horse chews on the roots or when the pile of feed is discovered and ingested, the bacteria can become harmful as it multiplies in the intestinal tract and releases the neurotoxin.
Botulism in foals is also called “shaker foal syndrome” or “barker foal syndrome,” as the foal’s vocal cords are paralyzed, making them sound as if they’re barking when they vocalize.
3. By the introduction of bacteria through wounds. Wounds that encapsulate bacteria while healing provide an anaerobic environment where the bacteria flourish.
It’s important to note that botulism is not a disease that spreads between horses. If one horse on the property gets botulism, he is not “contagious”; in other words, he cannot give botulism to other horses via nasal secretions, shared tack, or in any of the other ways equine viral diseases spread.
Clinical Signs of Botulism
The extent to which a horse is affected by the botulism toxin depends on how much they have ingested, says Blakey. No matter if the illness comes on rapidly or more gradually, the initial symptom is always the same: muscle weakness.
The first muscles in horses that lose tone (meaning they are not able to hold themselves as needed to function properly) include the eyelids, tongue, tail and anus. This means that a horse with botulism will have problems eating, chewing and swallowing, and his muscles will fatigue quickly; he may drool or have upper eyelids that droop.
A horse with botulism will also experience muscle tremors. Progressive paralysis occurs in more severe cases, often affecting the horse’s ability to stand; it can also affect a horse’s respiratory and cardiac systems, causing death.
The biggest predictor of whether a horse will survive a bout of botulism is if he can stand.
“If a horse can stay up, they often survive,” says Blakey. “Sixty percent of horses that go down, but can rise, survive.” A horse that is recumbent and unable to rise without assistance has a poorer prognosis.
So how does a vet determine if a horse has botulism and not another issue with similar clinical signs, such as equine protozoal myeloencephalitis (EPM)?
“The very first thing I ask when I see any signs like this is, ‘Is your horse vaccinated against botulism?’” says Blakey. Knowing if a horse has been vaccinated against botulism allows her to immediately rule out botulism as the cause of illness, saving time (and money!) while drilling down to a diagnosis.
Time is of the essence in botulism treatment, but treatment can be delayed as horse owners and veterinarians work to determine exactly what is happening with the horse’s health. Some horses may lie down more as their muscles fatigue, which can be misdiagnosed as colic, says Blakey.
One telltale sign that the horse has botulism and not another disease is that the muscle weakness the horse is experiencing is symmetrical, says Blakey. Horses with botulism have flaccid, floppy muscles, while horses with tetanus have hard, rigid muscles.
Additionally, one of the biggest things to rule out is trauma, says Blakey.
Treatment and Prevention
Early recognition and prompt treatment is imperative in giving a horse the best chance at a full recovery. If botulism toxicity is suspected, the horse can be treated with plasma containing an antitoxin, which binds to free toxin molecules in the blood, preventing them from attaching to nerve cells. This treatment is not cheap (often running between $1,500 and $3,000), and the antitoxin cannot reverse the effects of the toxin that has already bound to nerve cells.
Eventually the toxin degrades, however. If the horse survives, all muscular deficits he experienced while the disease was active will fade; botulism has no lasting effects on a horse. However, muscling and physical aptitude may take weeks or months to return to normal as new nerve endings grow.
The best preventative measure for ensuring a horse doesn’t contract botulism? Vaccination.
“The botulism vaccine is very effective,” says Blakey. “I insist all my broodmares be vaccinated, so I haven’t seen a shaker [foal] in years,” says Blakey. “And I encourage everyone to vaccinate for botulism. The cost of the vaccine is a pittance compared to the cost of antitoxin treatment for affected horses. I also push hard for those who feed round bales to vaccinate their horses again.”
There is no downside to botulism vaccination and there is not a population of horses that could not benefit from the protection. While there is a chance that the horse may become sore around the injection site (as with any vaccination), the minor soreness is a minimal price to pay for the safeguard from the toxin the vaccination provides.
Are Vaccinations Effective for Everything?There are multiple types of botulism that can affect horses, including types A, B and C, though type A is uncommon. Equine botulism vaccines are effective against type B, but don’t provide protection against the other types. Though a botulism vaccine is not considered a “core vaccine” by the American Association of Equine Practitioners (meaning these vaccines should be given to every horse, no matter where they reside), there are specific populations of horses that should be vaccinated against the disease: ◆ Broodmares ◆ Horses that live in areas of the United States where botulism is endemic: The disease is most prevalent in California, Kentucky, Maryland, Ohio, Pennsylvania and Tennessee, though it can occur in any state. ◆ Horses that eat from round bales Botulism vaccination begins with three doses given at four-week intervals, with boosters given annually. After the initial series, pregnant mares should be vaccinated yearly (two to four weeks before foaling). Foals in high-risk situations can start the series of botulism vaccines as early as two weeks old. Foals born to unvaccinated mares or those that have not ingested enough quality colostrum from the mare (failure of passive transfer) can have botulism vaccination initiated early in life.  Botulism protection provided by the mare’s colostrum generally wanes by about 5 months of age, and vaccination should be considered at this time. |
Key Takeaway
“Botulism is a normal part of our environment,” says Blakey. “It’s not some weird threat, and it doesn’t happen because a horse owner isn’t ‘clean.’”
However, like many other equine-related health concerns, good husbandry can minimize the incidence of disease. This includes not feeding moldy hay or grain or hay that has been baled with a high moisture content, plus keeping pastures and barns free from dead rodents and animals.
If horses live in an area where botulism occurs or if they’re fed round bales, vaccination is the most-recommended route of prevention.
