Botulism in Horses

Equine botulism is a serious, deadly disease that can cause paralysis in horses, among other symptoms. It's important to understand the signs of botulism in order to get your horse medical attention as soon as possible.

What is Botulism in Horses?

Botulism is a disease caused when spores from the bacteria Clostridium botulinum are either ingested or infect a wound, releasing a neurotoxin called botulinum toxin, which is very potent.

Botulinum toxin specifically targets the body’s nerves, causing paralysis. While it can affect all mammals, horses are one of the most susceptible species.

Equine botulism is often linked to geographical locations (Kentucky and the mid-Atlantic states) and can affect both adult horses and foals.

If you suspect your horse is suffering from botulism, call your veterinarian right away.

Types of Botulism in Horses

While there are seven strains of C. botulinum that produce toxins, only three of these strains have been seen to affect horses. Types B and C have been linked to equine illness:

• Botulism Type B is the cause of Shaker foal syndrome and occurs when the Clostridium spores germinate and produce toxin in the foal’s intestinal tract. This is primarily a concern in foals between 2 weeks and 8 months of age, when the GI microbiome isn't developed enough to prevent the botulism from multiplying and causing disease.

• Botulism Type C is caused by forage contaminated by an infected animal carcass.

Symptoms of Botulism in Horses

Clinical signs of botulism usually occur within 24 hours of exposure to the toxin and may include the following symptoms:

• Weakness

• Exercise intolerance

• Muscle tremors

• Drooling

• Reduced tongue strength

• Lying down and unable to get up

• Progressive paralysis

• Labored breathing

• Difficulty chewing or unable to swallow

• Decreased tail, tongue, and eyelid tone

• Colic

• Death

If you see any of the above symptoms or have concerns your horse may have eaten contaminated feed, call your veterinarian immediately.

Causes of Botulism in Horses

Horses can acquire botulism via the following three routes:

• Ingestion (forage poisoning)

• Infection of a wound (wound botulism)

• Growth within the intestinal tract through ingestion of bacteria or spores

Ingestion of spoiled feed or contamination from an animal carcass is the most common route horses can get botulism. Bacteria can release the toxin once inside the horse’s digestive tract (this is more commonly seen in foals and is due to Type B toxin). Then, the neurotoxin is absorbed from the intestines and spreads throughout the body via the circulatory system.

Botulinum toxin blocks the communication between nerves and muscles in the horse's body. This results in the physical symptoms of botulism as described above.

Clostridium botulinum spores are found widely dispersed in the soil and water throughout the world.

Botulism Type B is the most common cause of botulism in horses and typically occurs due to the horse consuming moldy hay, improperly fermented haylage/silage, or contaminated grain.

Botulism Type C has been linked to decomposing animal carcasses, commonly found in hay.  

Although equine botulism is highly linked to specific geographic areas, it is still considered rare but highly fatal unless treated quickly with antitoxin.

How Veterinarians Diagnose Botulism in Horses

Diagnosing botulism can be very difficult because there aren’t any widely available reliable tests. In most cases, veterinarians rely on the horse’s medical history, clinical signs, and ruling out other diseases to make a diagnosis.

While a blood test for botulism is available, these tests are often inconclusive due to circulating levels of the botulinum toxin being too low to detect in the horse’s body.

The most accurate diagnosis for botulism is to identify the bacteria in the horse's feed or environment where they were infected. This is more likely to be successful during large outbreaks rather than if only one or two horses are infected.

The best way to diagnose botulism in horses is through identification of the specific neurotoxin through PCR (polymerase chain reaction), which checks for botulism DNA. Testing sites may include:

• The affected horse’s feces

• Stomach/intestinal contents

• Any open wound

Treatment of Botulism in Horses

Without quick, aggressive treatment and intensive supportive care, botulism is often fatal in horses. Some treatments may provide a horse with the chance of survival, depending on how early the horse was diagnosed and how soon aggressive treatment began.

Botulism Antitoxin

An intravenous antitoxin is available, with the goal of neutralizing the circulating toxin.

Unfortunately, antitoxin therapy will not reverse existing clinical signs and may not stop the disease from progressing. The antitoxin can also be difficult to obtain and is often expensive.

Supportive Care

Supportive care at a veterinary hospital is often required and may include the following:

• Frequent turning of horses who are lying down (to avoid pressure sores)

• Feeding through a nasogastric tube

• Daily eye lubrication

• Sedation to reduce muscle activity

• Stall rest

• Antimicrobial therapy (for secondary complications such as aspiration pneumonia)

Recovery and Management of Botulism in Horses

Recovery from botulism is dependent on the following factors:

• Dose of toxin (how much toxin is in the horse's body)

• Severity of clinical signs

• Time of diagnosis

• Quick and aggressive treatment

Horses who are only mildly affected with botulism may recover, but the associated muscle wasting may take weeks to months to resolve.

Horses who become unable to stand due to the botulinum toxin often have a grave prognosis.

Botulism can lead to other conditions such as:

• Colic

• Aspiration pneumonia

• Choke

A study from New Bolton Center, the University of Pennsylvania School of Veterinary Medicine campus, reviewed the treatment and survival rate of 92 horses who became infected with botulism. Of the 92 infected horses, 44 survived. Researchers concluded that an important predictor of survival was maintaining the ability to stand.

How To Prevent Botulism in Horses

Botulism can be prevented through good care practices and vaccination.

It’s extremely important to use proper storage, like this durable bucket from Tuff Stuff Products, and inspect all grain and hay before feeding.

Consider these other preventive measures:

• Discard any hay or grain that’s moldy, has an abnormal smell, or is contaminated with an animal carcass.

• Avoid feeding horses silage or fermented forages.

• Do not feed round bales that have gotten wet or have torn plastic wrapping.  

• Keep surrounding areas free of any decaying vegetation and animal carcasses.

• Control rodents, birds, and other animals that could be carriers of the botulinum toxin. 

• Ensure prompt and proper care of wounds to avoid infections.

Botulism Vaccine for Horses

The BotVax B vaccine is available for Clostridum botulinum Type B and is recommended annually in certain areas of the United States, including Kentucky and the mid-Atlantic states.

Follow the manufacturer’s instructions for dosage and frequency. Always work with your vet to determine if the botulism vaccine is right for your unique horse.