Botulism FAQ

February 05, 2002

What is equine botulism?

Equine botulism is a progressive neuromuscular disease that quickly leads to weakness and flaccid paralysis (lack of muscle tone with reduced ability to move).  This disease can be severe and typically leads to death via respiratory failure when not treated soon after the onset of clinical signs.  Horses can aquire botulism in a variety of ways, including forage poisoning, wound botulism, and toxicoinfectious botulism (shaker foal syndrome).

What causes botulism?

Equine botulism is caused by botulinum toxin, a dangerous protein produced by the Gram-positive anaerobe, Clostridium botulinum. The spore-forming bacterium dwells naturally throughout the environment and is commonly found in soils across the country.  The majority but not all of C. botulinum Type B is found in the east and especially high risk areas.  Under the right conditions, C. botulinum produces one of many different toxins (A-H), with type B being the most common botulium toxin affecting horses (more than 85% of cases).

How do horses acquire botulism?

Forage poisoning occurs in adult horses that ingest preformed botulinum toxin in improperly dried, processed, or stored forages (baled hay, hay cubes, etc.) or feed contaminated with animal carcasses containing C. botulinum.

Wound botulism ensues when C. botulinum establishes infection within a sealed wound, including injection site abscesses, umbilical infections, deep puncture wounds, and castration sites.

Toxicoinfectious botulism (shaker foal syndrome) sporadically affects foals from a few days old to several months of age (70% of cases occur between 2 and 5 weeks of age).  Soil borne C. botulinum spores are ingested by the foal, infect the gastrointestinal tract, and locally produce type B botulinum toxin that the foal absorbs systemically.

How dangerous is the toxin that causes equine botulism?

Botulinum toxin is considered the most poisonous naturally occurring substance in the world.  One milligram (0.000035 ounce) of toxin can kill a 1,200-pound horse.  One teaspoon of toxin is sufficient to kill 5,000 horses.

Clinical signs of equine botulism

The onset of clinical signs is variable.  Usually clinical signs are observed within 24 hours of exposure to botulinum toxin; however, signs may be seen in as little as 12 hours post-exposure.  While the route of toxicosis may differ, clinical signs exhibited by the equine patient are generally the same.

  • Dysphagia (difficulty in swallowing)
  • Eating more slowly
  • Leaking milk from mouth while suckling (in foals)
  • Poor muscle tone
  • Reduced tongue tone
  • Weak eyelid tone (droop)
  • Poor tail tone
  • Reduced ability to retract tongue
  • Weakness
  • Muscle fasciculations/tremors
  • Recumbency/Inability to rise
  • Colic

Equine botulism is difficult to diagnose because its clinical signs mirror other diseases, and existing diagnostic procedures for the disease are lengthy and can be inconclusive. To save the horse, treatment must usually be started before a definitive diagnosis can be made. In lieu of a definitive test, veterinarians can use simple tests to aid in the diagnosis of botulism. In one, the horse’s tongue is gently pulled out, released, and observed to see how quickly the horse retracts it back into its mouth. The other is a “grain test,” in which a small amount of sweet feed is offered in a large flat tub, and the horse is monitored for the how long it takes to eat the feed. Reduced tongue tone and dysphagia are often present before other clinical signs of botulism appear, making these two tests useful to make a presumed diagnosis of botulism.

Are there effective treatments for equine botulism?

If equine botulism is recognized early, there is a chance to save the horse using specific antiserum (antitoxin) treatment in addition to good supportive therapy.  The affected horse typically requires several days of round-the-clock intensive nursing care at an equine hospital, in order to potentially survive.  Prognosis depends on a number of factors but tends to be poor with equine botulism, despite well-intended efforts and costly hospitalization ($1,000+ for the antitoxin alone – not including the potential intensive hospitalization and nursing care).

Can botulism be prevented?

Yes.  Prevention of equine botulism is easily achieved through the administration of three doses of BotVax B vaccine, each one month apart.  BotVax B is the only USDA-approved Clostridium botulinum type B * toxoid licensed for preventing equine botulism in healthy horses.  First approved in 1987 and for sale exclusively to veterinarians, BotVax B has safely and successfully protected hundreds of thousands of horses and foals from equine botulism and shaker foal syndrome.  Preventing botulism is the simple, inexpensive, effective alternative to the futile outcome that this severe neuromuscular disease often brings.

*Does not confer protection against other Clostridium botulinum toxin types

Unvaccinated horses should receive three 2 mL doses of BotVax B in the muscle at one-month intervals, including broodmares (starting at the ninth month of pregnancy).  Previously immunized horses are vaccinated once annually.  BotVax B is safe to administer to foals as young as 2 weeks.

Owners also can take other proactive steps to help prevent botulism, including good husbandry techniques. Check hay and grain for decay and for the presence of dead animals, which may contain botulism toxin. Additionally, if using round bales to feed horses, ensure they are properly process and stored. For example, bales stored outdoors should be stacked on pallets to keep it off of the ground. Avoid wrapping hay in plastic if stored outdoors, rather simply cover it. Finally, owners should carefully manage the presence of rodents and birds and promptly dispose of carcasses and decaying feed.


Category: Animal Safety, Equine, Veterinary, Animal Health