Type A Botulism in Horses Described in Case Review

An exhaustive review of 10 years of data involving three isolated cases and eight outbreaks of type A botulism in horses is anticipated to play an important role in establishing prevention and treatment strategies.

The botulinum toxin, produced by the bacterium Clostridium botulinum, is the most potent and lethal toxin known to mankind. To make matters worse, there are eight different kinds of the toxin, three of which are known to affect horses in the United States.

"In the United States, horses are most commonly affected by Type B and only rarely by types C and A," explained lead researcher Amy L. Johnson, DVM, Dipl. ACVIM-LAIM, from New Bolton Center's section of medicine.

Because equine outbreaks of C. botulinum type A have not been previously described despite the fact that the type A toxin is a predominant cause of botulism in humans, Johnson and colleagues reviewed records of test results submitted to the University of Pennsylvania's School of Veterinary Medicine Botulism Laboratory between 1998 and 2008. Key findings were:

  • Almost all type A cases and outbreaks occurred in the western United States;
  • Oregon and Idaho were overrepresented;
  • The type A toxin itself was only identified in one case (the remaining cases were diagnosed based on the presence of spores);
  • Affected horses had progressive muscle weakness, recumbency, decreased tail and/or tongue tone, dysphagia, respiratory distress, and death;
  • Isolated cases involved foals less than or equal to 1 month old, whereas outbreaks occurred in horses greater than or equal to 11 months;
  • Of the 119 horses thought to be exposed to the toxin, 54 showed signs of botulism and 49 of these 54 horses died, and
  • Silage was the source of infection in 6/8 outbreaks.

"This is the first report to describe naturally occurring type A botulism in horses. Type A appears to be more common in horses than previously assumed, produces similar signs of disease as type B, and may have a higher case fatality rate," Johnson said. "Equine practitioners should consider type A botulism in suspected cases west of the Mississippi river when choosing an antitoxin product to use."

Future studies to more fully characterize the different types of equine botulism are indicated and a faster, validated diagnostic test that is toxin-specific is needed.

The study, "Type A botulism in horses in the United States: a review of the past ten years (1998-2008)," was published in the March 2010 edition of the Journal of Veterinary Diagnostic Investigation. The abstract is available on PubMed.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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