What Is Botulism in Horses?
Botulism in horses is caused by a toxin produced by the bacterium Clostridium botulinum. It is a rare but often fatal illness. The toxin causes muscle weakness that can progress to paralysis, and many horses also show muscle tremors.
In adult horses, botulism is usually a poisoning rather than an infection: the horse eats feed that already contains the toxin. Horses are among the most sensitive species to botulinum toxin (Merck notes they might be up to 10,000 times more susceptible than mice), so a small amount of contaminated forage can be enough.
If your horse suddenly has trouble eating or swallowing, a weak tongue, trembling muscles, or is lying down more than usual, call your veterinarian right away. Treatment works best when it starts early, before more of the toxin reaches the nerves.
What Causes Botulism in Horses?
C. botulinum is a spore-forming bacterium that lives in soil. It grows and produces toxin where there is little oxygen and plenty of moisture and decaying plant or animal material. That is why poorly made silage or haylage, spoiled hay, and feed contaminated by a dead animal are the classic sources.
The toxin blocks the signal from nerve to muscle. The nerve tells the muscle to contract, but the chemical messenger (acetylcholine) never gets released, so the muscle stays limp. This is why botulism causes floppy, flaccid weakness while the horse usually stays bright and alert.
There are several toxin types, labeled A through G. Only types A, B, and C have been reported in horses in the US:
- Type B causes more than 85% of US cases. It lives in soil and is most common east of the Mississippi, especially in the mid-Atlantic states and Kentucky.
- Type A is also soil-borne and is seen mainly in the western US.
- Type C is linked to animal carcasses, for example a small animal accidentally baled into hay, or a carcass in standing water. It turns up sporadically.
The type matters for prevention, because the vaccine covers only type B.
How Horses Get Botulism
There are three recognized forms:
- Forage poisoning (adult horses). The horse eats feed or forage that already contains the toxin. This is the most common route in adults.
- Toxicoinfectious botulism, or "shaker foal syndrome" (young foals). Spores swallowed by a young foal, usually within its first couple of months, grow in its immature digestive system and produce toxin there.
- Wound botulism. Less commonly, the bacteria grow in a wound or other damaged tissue and release toxin from there.
Risk goes up for unvaccinated horses in areas where type B is common, for horses fed fermented or poorly stored forage such as large round or square bales and haylage, and anywhere old feed builds up around feeders or a carcass could contaminate feed or water.
Botulism is not contagious. AAEP reports no demonstrable shedding of the organism once signs appear, and affected horses don't need to be isolated. Several horses can still fall ill together when they share the same contaminated feed, as in a 2026 Cornell report of four draft horses in New York.
Clinical Signs of Equine Botulism
Weakness is progressive, so signs that start mild can become serious. According to the AAEP botulism guidelines, watch for:
- Trouble swallowing (dysphagia). The horse may be slow to eat, leave grain unfinished, or chew without swallowing.
- Feed, water, and saliva coming from the nose or mouth. This looks like drooling and comes from weakness of the tongue and throat, not vomiting.
- Weak tongue tone. The tongue is floppy and slow to pull back into the mouth.
- Weak eyelid and tail tone. The eyelids may droop and the tail offers little resistance when lifted.
- Dilated pupils (mydriasis) that respond slowly to light.
- Muscle trembling, often over the upper forelegs and hindquarters, that gets worse with movement or with standing for a long time.
- Progressive weakness in the legs and body, with stumbling or incoordination.
- More time lying down, and difficulty getting up or standing still.
- Reduced gut movement and colic-like discomfort. Early cases can look like colic.
- Breathing difficulty. As the breathing muscles weaken, the horse breathes poorly, and paralysis of breathing is a usual cause of death.
Through all of this, the horse's mind is usually normal. It stays alert and aware even as its body fails, which helps vets tell botulism apart from brain diseases.
When to Call Your Vet
Call your veterinarian right away if your horse:
- has trouble eating, drinking, or swallowing, or feed is falling from its mouth;
- has a weak or floppy tongue;
- is trembling, weak, or spending much more time lying down;
- seems to have colic and is also slow to eat or swallow.
Speed matters because the antitoxin can only neutralize toxin that has not yet attached to the nerves. AAEP states that every hour of delay in antitoxin treatment reduces survival.
While you wait, keep handling to a minimum and don't offer food or anything else by mouth to a horse that can't swallow. Treat any horse with sudden neurologic signs as a possible rabies case until a vet has made a diagnosis, and keep your hands out of its mouth.
How Vets Diagnose Botulism
Diagnosis is mainly clinical. Your vet will examine the horse and may use two simple bedside tests: a tongue tone (stress) test, which checks how quickly the horse pulls its tongue back in, and a grain test, which checks whether the horse can eat and swallow normally. Don't try these at home. A horse that can't swallow can choke or inhale feed, and rabies hasn't been ruled out yet.
Your vet will also rule out diseases that can look similar. For veterinary professionals, the AAEP differential list includes equine protozoal myeloencephalitis (EPM), equine herpesvirus myeloencephalopathy (EHM), equine motor neuron disease (EMND), arboviral encephalitides, rabies, metabolic derangements, myopathies, and other intoxications.
Laboratory tests exist, but no commercially available test returns results fast enough to guide treatment of the individual horse. The traditional mouse bioassay also often comes back negative in horses that do have botulism. In practice, vets decide to treat based on the clinical picture.
Treatment
Treatment is a veterinary emergency with two parts.
Antitoxin. Your vet gives botulism antitoxin as early as possible and picks the product based on the likely toxin type in your region. Antitoxin neutralizes toxin still circulating in the blood. It cannot undo toxin that has already bound to the nerve endings, so it stops the disease from getting worse rather than reversing it.
Supportive care. Recovery takes time, and until then affected horses often need hospital-level care, which can include:
- nutrition and fluids given by stomach tube while the horse can't swallow;
- deep bedding and regular turning for horses that are lying down;
- eye protection;
- close monitoring of breathing.
Some antibiotics can make the weakness worse, so your veterinarian decides whether any medication is needed and which one.
Prognosis and Recovery
The outlook depends heavily on whether the horse can stay on its feet. In a study of 92 adult horses treated for botulism at one US referral hospital between 1989 and 2013, 48% survived overall. Among horses that remained able to stand, survival was 95%. Horses that can no longer get up have a poor prognosis. These figures come from a single hospital with intensive care available, so results elsewhere can differ.
Recovery is slow. AAEP notes that surviving horses usually regain the ability to swallow within about two weeks, and it can take up to a month before they can get up unassisted.
How to Lower the Risk
Vaccination. A vaccine against type B botulism exists. AAEP classifies it as a risk-based vaccine rather than a core vaccine, so whether your horse needs it is a decision to make with your veterinarian based on where you live and how you feed. It does not protect against types A or C. If you have a pregnant mare in an area where type B is common, ask your vet whether vaccinating her makes sense during the horse gestation period.
Feed and environment. Following the AAEP guidelines, reduce the known risk factors:
- avoid feeding poorly made or spoiled silage, haylage, or other fermented forage;
- be cautious with large round or square bales, and don't feed hay that is spoiled or contains animal remains;
- clear out old feed that builds up around feeders and hay racks;
- keep water sources free of dead animals and decaying material.
Sources
- AAEP: Botulism, Infectious Disease Guidelines
- Merck Veterinary Manual: Botulism in Animals
- Merck Veterinary Manual: Botulism in Horses
- AAEP: Botulism Vaccination Guidelines
- Journal of Veterinary Internal Medicine: Outcome of adult horses with botulism treated at a veterinary hospital, 92 cases (1989-2013)
- Cornell University College of Veterinary Medicine: Botulism in four draft horses in NY
This article is for general information and is not a substitute for advice from your veterinarian.
