Dogs and snakes don't get along very well. Canine encounters with cottonmouths, also known as water moccasins, are actually fairly common in the United States. While snakes prefer flight to fight, your pup probably doesn't understand the situation. Cottonmouths are venomous and more aggressive than many local snake species.
If you come running at the sound of sudden desperate barking only to find your pet holding half a snake in his mouth, then he's probably been bitten at least once. Most dogs win the battle of strength against cottonmouths, but the snake's agility allows him to strike his canine foe with relative ease at least once. Whether the snake is dead or alive, your first priority is removing your dog from the situation immediately. Cottonmouths can bite and envenom their target multiple times, and each successive bite makes the encounter much more dangerous. Call to your dog and tug his leash or chain to get him away from the snake. Take care to avoid the snake if it's still alive, as cottonmouth venom is dangerous for humans as well.
If your dog has been bitten, expect to see significant swelling within minutes of the altercation. You may also notice pairs of fang marks where the snake bit your dog and small amounts of blood near the wound. The absence of noticeable bite marks does not necessarily mean your dog is unscathed, according to Regional Emergency Animal Care Hospital of Asheville. Take your pet to the vet immediately if he was struck by a cottonmouth or if snake bite wounds start to swell up. Signs of advanced symptoms warrant an immediate trip to an animal hospital or clinic.
Cottonmouth venom is deadly to small creatures, but it's not immediately fatal for most dogs. However, more serious symptoms start to develop in the half hour or so following the bites. Dizziness, tremors, bodily weakness and nausea are all symptoms of advancing venom, so don't waste any time seeking professional assistance if your dog starts acting weird after being bitten. Fatal symptoms including kidney failure can develop in less than 24 hours after the bite if the dog's body contains too much venom and it's left untreated.
Call the local vet or animal hospital to inform them of the situation and ask about their supply of anti-venom if you believe your pet's been bitten by a venomous snake. Smaller clinics may not carry the appropriate chemicals to manage a cottonmouth bite, so you may be redirected to a larger institution if it's close enough so your pet can get the best treatment possible. In addition to anti-venom, the vet may also administer the painkiller morphine as well as antibiotics and fluids to keep your pet hydrated. With prompt vet treatment, the prognosis is generally good and recovery should take less than a week. Your vet my advise an alternate course of treatment if the bite injuries are too numerous or severe.