Since August, 1998, twelve horses and a mule have died in the Ash Fork, Ariz., area after displaying similar symptoms. Although the exact cause of the illness has not been specifically determined, evidence gathered by several veterinary practitioners involved in the cases, and test results on samples submitted by them, all but ruled out contagious diseases, many heavy metals, and organophosphate pesticides. Based on the clinical appearance of the affected horses, veterinary practitioners suspected botulism as the cause of deaths.

On Feb. 1, 1999, the Arizona Department of Agriculture and U.S. Department of Agriculture established an animal health investigation team to look into the situation. The team consisted of field veterinarians from the Office of the State Veterinary Diagnostic Laboratory, two pathologists from the National Veterinary Services Laboratory, and an epidemiologist from USDA/APHIS/Veterinary Services. The team spent the week of Feb. 1st in the Ash Fork/Chino Valley area gathering information, and some environmental and healthy animal samples. No deaths nor clinical cases were seen during the week. The team concluded its initial on-site investigation on Feb. 4th.

Results from gut samples collected from a horse that died at the end of November were released late last week. The University of Pennsylvania was successful in isolating Clostridium botulinum type C spores. According to the University of Pennsylvania scientists, isolation of the actual toxin from the gut is nearly impossible. The isolation of C. botulinum type C spores along with compatible clinical signs is highly suggestive of a diagnosis of botulism as the cause for the deaths. Although this is the most probable diagnosis, the animal health team plans to continue analyzing the available samples.

Botulism is a flaccid neuromuscualar paralysis that can affect all mammals. Horses, however, are one of the most susceptible species. The paralysis is a result of interference with acetylcholine release at the end plate caused by the potent exotoxin. Ingestion of the preformed toxin is the most common route of infection. The symptoms include muscle tremors, weakness, dysphagia (not characteristic of the Ash Fork horses), loss of tongue, tail and eyelid tone, recumbency, and finally death from respiratory failure. The source of the toxin is unknown however, it appears likely that ravens in the area may have acted as a “physical carrier” of the toxin to feed and waterers from improperly disposed carcasses nearby. These carcasses have since been properly disposed of and it is hoped that there will be no new cases of the illness. Veterinarians, as well as owners, in the area are encouraged to be on the lookout for suspicious cases and to contact authorities if cases appear in order to assist in the collection of diagnostic samples