Beware of Botulism!

Beware of Botulism!

Botulism is difficult to treat, says Wright, although there are antitoxins that can be effective. Supportive care is also essential in treating botulism.

Photo: Anne M. Eberhardt/The Horse

Botulism is a silent, deadly killer with several forms. While it is not a common disease across the United States, it can crop up anywhere. Botulism occurs when toxins produced by the bacterium Clostridium botulinum get into the body of an animal or human. There they cause weakness because they block the connection between nerves and muscles. Paralysis often occurs.

Unfortunately, horses are perhaps the most sensitive of domesticated animals to botulism. They and other mammals can contract the disease in three basic ways:

  • By ingesting toxins that are present in forage, also known as forage botulism. This is the form that normally strikes adult horses.
  • By having growth of the agent in the gastrointestinal system, also known as toxicoinfection botulism. This form normally afflicts young horses.
  • By contamination of wounds, which is also known as wound botulism.

The ingestion of pre-formed toxins in forage is the usual route of infection in adult horses. This type of botulism occurs when the horse consumes bits of decayed or decaying forage that sets up an environment for Clostridium botulinum proliferation and toxin release. Bits and pieces of dead animals that wind up in hay bales is another way in which the disease is introduced. Another route can be the consumption of silage or haylage that has been contaminated by the organism.

In cases where toxicoinfection is the route for introduction of the disease, young horses ingest the spores, which then germinate and produce toxins in the gastrointestinal tract. When this occurs, the foals often are afflicted with what is termed shaker foal syndrome, so called because the foal's muscles will tremor with neurological weakness as he attempts to stand and walk around the stall.

The least common route of infection is through open wounds. When that does happen, toxins are produced at the site and absorbed into the horse's system.


Normally, outbreaks of botulism in large groups of horses are rare. However, it has occurred. In April 1994, Australia's Thoroughbred industry was rocked by an outbreak of botulism at the Easter yearling sales.

Some 460 yearlings were consigned to the sale, with 41 of them displaying signs of the disease. Demonstrating the deadly nature of botulism, 33 of the 41 afflicted yearlings died or were euthanized. It is likely that the organism was introduced to the yearlings via contaminated feed.

Australia had another outbreak of the disease in 1999 in the Hawkesbury district. Fourteen Standardbreds contracted botulism, and two of them died.

Haylage--Feed Cautiously

Feeding haylage or silage can be a route for introduction of pre-formed toxins, says Bob Wright, DVM, veterinary scientist with the Ontario, Canada, Ministry of Agriculture and Food. Some horse owners have switched to haylage in an effort to avoid dusty hay, he says, but it can be a case where the solution is far more damaging and dangerous than the problem. Wright, who has prepared an informational paper on the subject in collaboration with Dan Kenney, VMD, Dipl. ACVIM, of the Ontario Veterinary College at the University of Guelph, explains:

"Haylage or grass silage is the process where young, respiring (in the growth stage) plants are cut, partially wilted, and placed in a silo or container such as a plastic bag where exposure to air is eliminated. The hay is baled at about 45% moisture and immediately wrapped with plastic or placed in a bag, which reduces the presence of oxygen. The plants use the remaining oxygen in the bag. Fermentation lowers the pH, and the forage goes into a suspended state (as far as fermentation is concerned) when the pH equals 5." (pH is a measurement of alkalinity and acidity. The higher the pH number, the higher the alkalinity; the lower the pH number, the higher the acidity. The neutral point is a pH of 7. When dealing with haylage, a high acid content is desired because it prevents the development of toxins, thus a pH level below 4.5 is desired.)

Wright continues by saying, "Large round bales--four by five feet in size--present a greater risk of botulism because these big bales often have insufficient water-soluble carbohydrates for adequate lactic acid fermentation to achieve a stable pH of 1."

The botulism bacterium is anaerobic, which means it thrives in the absence of oxygen, but goes into a suspended state in the presence of oxygen. Haylage wrapped in plastic is devoid of oxygen. When the pH levels go above 4.5, the botulism bacteria thrive, multiply, and produce toxins, Wright explains.

At one Canadian farm two years ago, according to a report from the University of Guelph, a group of 37 horses developed the classic signs of botulism within hours of ingesting wrapped round baled hay silage. None of the more than 70 additional horses on the premises that were not fed the silage became ill. Seven of the 37 ill horses were euthanized.

What You See

The clinical signs of botulism are consistent, Wright says. Affected horses usually have muscle tremors, might be so weak that they cannot stand up, lose control of their tongues so they might hang from their mouths, can't eat and drool because they can't swallow, walk stiffly with a short stride, are sometimes weak and stumble, and they might lose tail tone. These clinical signs, he says, can occur within several hours or up to seven to 10 days after the contaminated forage is ingested.

Eventually, he says, many afflicted horses die because their respiratory muscles become paralyzed, or because they get other health problems from being recumbent (down).

More Than One Type

Seven distinct types of botulism bacterium have been isolated and identified, says Wright. He outlined five of the most common serotypes and the typical species involved:

  • Types A, B, and C have been seen in cattle.
  • Type A botulinum toxin has been incriminated in several outbreaks in horses in the northwestern United States--Washington, Idaho, Montana, and Oregon.
  • Type B predominates in horses and is referred to as forage botulism because of its association with contaminated forage. It is estimated by Wright and other researchers that more than 80% of equine cases of botulism involve Type B.
  • Type C is referred to as carrion botulism because of its association with the ingestion of feed containing a decomposing carcass, such as a rodent, cat, dog, or bird, or from eating the bones of dead animals. One notable outbreak of type C botulism occurred in California in 1989 from hay cubes that had been produced and processed in Utah.
  • Types C and D are found in poultry and poultry manures, mink, and wild ducks.
  • Type E occurs in fish or in association with the consumption of fish products.


Botulism is difficult to treat, says Wright, although there are antitoxins that can be effective. The problem is that each antitoxin is specific for a particular type of botulism. For instance, if a horse gets Type B and Type A antitoxin is administered, it likely will have no beneficial effect.

There is one antitoxin available that has therapeutic value against all five types--A through E--but it is very expensive. The cost of treating one horse with this particular antitoxin, says Bob Whitlick, DVM, PhD, of the University of Pennsylvania's New Bolton Center (where he and other researchers developed it), is $2,500. This particular antitoxin is available only through the New Bolton Center.

Normally, Whitlock says, therapeutic action involves treating the horse with the antitoxin that works against the type of botulism that is endemic in the area. For example, if a horse in Kentucky becomes ill with botulism, the routine approach would be to treat it with Type B antitoxin because that is the type of botulism bacterium that is generally present there. However, if a case occurs in Montana, it would be more of a guessing game as to which type of botulism bacterium was involved.

"The antitoxin is most beneficial if used when animals are first seen to be sick," says Wright. "With supportive care, horses can recover, but if they are exposed to a large amount of toxin, most will die despite treatment."

The problem with developing antitoxins for other types of botulism, other than Type B, says Derek Brook, DVM, FRCVS, of Templeton, Calif., is that there is a very small market for them. His company has USDA approval for its Type B antitoxin and has developed an antitoxin for Type A, but there is little demand for Type A antitoxin.

Fortunately, botulism does not strike horses frequently. Whitlock estimates that there are between 50-100 documented equine cases in the United States on an annual basis.


The good news is that there is a very effective vaccine on the market that protects against Type B, which Whitlock praises. It is the most efficacious vaccine in the equine world, he declares. "There are no side effects, and I don't know of a single vaccinated horse that has contracted botulism."

The normal approach, Whitlock says, is to vaccinate young horses three times at one-month intervals, then give a booster shot each year. Vaccination has become routine on major equine farms in Kentucky, but horse owners in many other parts of the country do not feel a need for the protection because of the disease's rarity in their respective areas.

Whitlock feels that use of the vaccine in places like Kentucky has put a damper on the spread of the disease. However, he warns, botulism recently has been rearing its head in places where it was unknown. For example, he says, several cases were diagnosed in New Hampshire, a first-time occurrence in that state.

Preventive measures should include feeding non-contaminated hay and feeding it in an area that is clear of all debris, especially from dead and decaying animals. The same would be true for the water supply.

Take-Home Message

Botulism is not seen frequently in horses, but some areas are more prone to this deadly disease than others. If your horses are in an endemic area, vaccinate and become aware of preventive measures you can take. If you are not in an endemic area, protect your horses by carefully selecting hay and other feed products that are clean and free of harmful toxins and animal parts.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

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