It's a scenario that's terrifying to imagine: Your horse's muscles are contracting in spasms, he's arching his back, and his body is rigid. The movements of his eyes and lips are unlike anything you've ever seen, rendering him unfamiliar. Brewing and multiplying deep in a puncture wound you might not even know he has is the dreaded Clostridium tetani, which is already wreaking havoc in his nervous system. He has tetanus. The good news is you're only imaging this, and generally tetanus, a disease long-dreaded in humans and other mammals, is preventable through vigilant vaccination.

Tetanus, often called lockjaw, continues to occur in horses unprotected by vaccination. This disease is caused by toxins produced by C. tetani, a spore-forming bacterium present in the digestive tract of many animals and in soils containing manure. Nat T. Messer IV, DVM, Dipl. ABVP, professor in equine medicine and surgery at the University of Missouri College of Veterinary Medicine, says these bacteria are always present in the environment.

"To decrease vigilance by not vaccinating is unwise," says Messer. "The only reason horse owners don't hear about cases is because so many horses are vaccinated. The vaccine is very effective, but like any vaccine is not 100% effective. I've seen cases of tetanus in horses that were appropriately vaccinated, and without vaccination there would be a much higher incidence of tetanus than what we see