Type A botulism in horses in the United States: a review of the past ten years (1998-2008).

The objective of the current retrospective study was to describe naturally occurring type A botulism in horses in the United States. In the past 10 years, the Botulism Laboratory at the University of Pennsylvania's School of Veterinary Medicine has identified 3 isolated cases and 8 outbreaks of type A botulism in horses via samples positive for Clostridium botulinum type A toxin or spores using the mouse bioassay test. Additional information was obtained by review of submission forms and by telephone or email interviews. Almost all type A cases and outbreaks occurred in the western United States, with Oregon and Idaho overrepresented. Type A toxin was identified in only 1 outbreak; all other identified cases and outbreaks were positive for spores but not preformed toxin. Reported clinical signs included progressive muscle weakness, recumbency, decreased tail and/or tongue tone, dysphagia, respiratory distress, and death. Isolated cases involved foals < or =1 month of age; outbreaks involved horses > or =11 months. One hundred and nineteen horses were potentially exposed to the toxin source; 54 out of 119 showed signs of botulism, and 49 out of 54 affected horses were confirmed dead. The number of horses affected per outbreak ranged from 2 to 24. The source of infection was confirmed to be hay or silage in 6 out of 8 outbreaks and was unknown in 2 out of 8 outbreaks. The present report is the first description of outbreaks of type A botulism in horses and has important implications for prevention and treatment. Based on these findings, type A botulism should be considered in suspect cases of equine botulism in the western United States.