Botulism Kills Five Indiana Horses

Five horses in Reddington, Ind., are dead after contracting botulism, according to an alert from the International Society for Infectious Diseases (ISID). The alert indicates the botulism toxin was ingested via an "unknown food source," likely the hay the horses consumed.

The ISID alert also noted that five additional horses residing at the same facility consumed the same hay as the deceased animals; those horses are currently being treated for botulism. The report indicates the sick horses received a botulism antitoxin and had not consumed as much potentially contaminated hay as the five animals that died. As of March 19, the report said, all were still alive.

It was not reported if any of the affected horses were vaccinated against botulism.

Botulism is a fatal neurologic disease of horses caused by toxins produced by the anaerobic, spore-forming, soil-dwelling bacterium Clostridium botulinum. Horse can contract botulism in three ways:

By consuming forage or feed containing the bacteria, which will then produce the toxins in the intestinal tract (more commonly found in foals, known as shaker foal syndrome, or toxicoinfectious botulism);

By consuming feed or forage containing the preformed toxins of C. botulinum (known as forage poisoning); or

Through wounds contaminated with the bacteria (wound borders will close, providing an anaerobic environment, which is a favorable condition for the bacteria to produce the toxins).

Once a horse is infected with C. botulinum, his health typically deteriorates rapidly. Clinical signs of botulism in horses are weakness; decreased muscle tone of the tail, eyelids, and tongue; trembling; dilated pupils; lying down; difficulty in swallowing; drooling; and green or milky nasal discharge. The horse will eventually become recumbent, as the toxins target only the nerves responsible for motor function. As the chest muscles and diaphragm become weakened, respiratory failure can set in, killing the horse via asphyxiation.

Botulism is often fatal in horses because of the rapidity of disease onset. When treatment is an option, intensive supportive care and the administration of botulism antitoxin are the main options.

According to the American Association of Equine Practitioners (AAEP) vaccination guidelines, botulism is considered a risk-based vaccine. The AAEP offers the following administration recommendations for horses deemed at-risk (it's advisable to consult a veterinarian to determine the need for botulism vaccination):

  • Previously vaccinated pregnant mares should receive an annual a single dose 4 to 6 weeks before foaling;
  • Previously unvaccinated pregnant mares should be vaccinated during gestation with a series of 3 doses administered four weeks apart, so that the last dose will be administered four to six weeks before foaling;
  • Foals of vaccinated mares (in endemic areas) should receive a primary series of three doses administered four weeks apart, starting at 2 to 3 months of age;
  • Foals of unvaccinated mares (born in, or moving to, endemic areas) should receive a primary series of three doses four weeks apart beginning at 1 to 3 months of age (the AAEP notes that foals at high risk may have the vaccination series initiated as early as 2 weeks of age; again, consult a veterinarian to determine if a particular foal is at-risk); and
  • All other horses (where indicated) should receive a primary series of 3 doses of vaccine given four weeks apart, followed by annual revaccination.


About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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