A dozen horses that died in a pasture west of Casper, Wyo., last month had botulism, according to a report appearing on kcwy13.com. 

George Marble, DVM, said three horses at the ranch had died, and he saw two more that were unable to walk when he visited the ranch. “Within two days, all 12 the of the rancher’s horses were dead or euthanized,” reported the article. “Almost all the horses suffered paralysis of the legs and tongue which are both classic signs of botulism. The vet says no other pastures were affected by the toxin.”

He explained to the news station that horses downstream from the affected ranch (a small creek ran through the property) were unaffected.

Horses usually become infected with botulism by ingesting the neurotoxin produced by the anaerobic bacterium Clostridium botulinum in contaminated feed or water. Feed contamination can occur when the decomposing carcass of a rodent or bird is baled in hay. This is seen more often in round bales. Feed can also be contaminated through improper storage or poor fermentation. Rarely, horses can get botulism when C. botulinum from the soil gets into an open wound