A recent study was published that examined the effect of a snakebite on the horse's heart muscle. Rattlesnake bites in horses are fairly common in rattlesnake territories. Symptoms vary from just a mild swelling to severe swelling and even death depending on location, the amount of venom injected, and the size of the horse. Most horses are bitten on the nose because they are curious. Horses cannot breathe out of their mouth, and the bite causes so much swelling in the nose that they cannot breathe. This swelling is usually the most serious initial problem in snakebite cases that must be dealt with as an emergency. Some horses will require a tracheotomy to breath, in which the vet makes an incision into the trachea in the horse's neck and places a tube for air so the horse does not have to breathe out of the nose. And certainly horses cannot drink if the mouth is also very swollen, so many horses require intravenous fluids to prevent dehydration; if they are bit on the nose, you certainly cannot pass a tube in their stomach to give them water.
Anti-inflammatories and pain medication are also used, and a plasma antivenin has been shown to help. Depending on the report you read, anywhere from 9 to 25 percent of horses bitten by rattlesnakes die. A recent study in Veterinary Internal Medicine indicated that 70 percent of the horses bitten developed cardiac arrythmias and 40 percent developed damage to the heart muscle. This may be another reason some horses die after a bite when they seem to be doing well. A rattlesnake vaccine and antivenin is available for horses. We do not know if either of these options decrease damage to the heart in the event of a bite.