From the University of Kentucky’s College of Agriculture
The long hot days of summer bring with them an increased snakebite risk to all animals, including horses. The major venomous snakes-the pit vipers-in the United States include several species of rattlesnakes, copperheads, and water moccasins. Coral snakes, another poisonous snake found in the U.S., do not pose a risk to horses because of their small mouth size.
Pit vipers are so named because of the heat-detecting holes, or pits, on each side of their head that help the snake locate prey. Pit vipers can be differentiated from other snakes at a distance by their triangle-shaped heads and narrowing of the neck area just behind the head.
The major types of poisonous snakes in Kentucky are copperheads, cottonmouths, timber rattlesnakes, and pygmy rattlesnakes; there are also some reports of Eastern Diamondback rattlesnakes. Risk of severe, fatal envenomation (poisoning from a venomous bite or sting) is highest with Diamondback rattlers, less with water moccasins, and lowest with copperheads.
Most snake bites to horses occur when the horse encounters a snake in the pasture or on the trail. Severe bites can occur if a horse steps on a snake and the snake releases all of its venom in one bite as it dies. Snake venom components vary tremendously by snake species, but most venoms contain substances that cause digestion and breakdown of tissues and blood vessels, impair blood clotting, and damage the heart. Some snakes’ venom also contain neurotoxins. Ultimately, many factors influence how severe a particular bite will be (i.e., snake species, size, age, recent feeding, number of bites, etc.). Some bites are “dry bites,” where little if any venom is injected.
Victim factors can also influence how severe a bite’s effects will be; these factors include size and age of the horse, concurrent medical conditions, drugs the animal might be receiving, and the location of the bite. It is important to understand that while one snakebite incident in one horse might be quite mild and not require treatment, a different snakebite involving a different snake, a different horse, or different circumstances might be much more severe and even fatal.
Clinical signs of snakebite in horses can vary widely but generally include pain, swelling at the bite site, one or more puncture wounds, and sometimes sloughing of tissues near the bite site. Some bite wounds might not be readily apparent. Copperhead bites or dry bites with little venom injected often cause only mild signs. Severe bites from more dangerous snake species or larger doses of venom can cause marked pain and swelling, coagulopathy (blood clotting defect) and hemorrhage, cardiac arrhythmias, shock, collapse, and in some cases acute death. With neurotoxic venoms, paralysis can occur. Horses bitten on the nose can develop swelling of the nasal passages resulting in respiratory distress. Signs of envenomation can occur within minutes of the bite incident or can be delayed for many hours depending on the bite site, dose of venom injected, snake species, and other factors.
Seek veterinary care immediately if a horse is bitten by a venomous snake. No first-aid treatments performed by owners in the field have been shown to be particularly helpful for venomous snakebite. Many anecdotal or folk remedies can cause more harm than good. Additionally, suction devices designed specifically for venom removal have not been shown to be beneficial (in pig models of snakebite). The best first-aid is to keep the horse calm and quiet and arrange for immediate veterinary care.
Veterinary treatment will vary depending on the severity of the bite, but might include treatment for shock, fluid therapy, pain medications, wound treatment and antibiotics, tetanus prophylaxis, and antivenin. Antivenin can be especially helpful in cases of severe envenomation and can decrease the amount of tissue damage and hasten recovery times. Antivenin is dosed according to the estimated amount of venom injected, not the size of the patient, so even one vial of antivenin can help counteract venom toxins in a horse. Cardiac arrhythmias occur in many horses and might require treatment. Horses with severe nasal passage swelling might need treatment to maintain a patent airway and nutritional support if swelling impairs the horse’s ability to eat and drink.
Even after horses have recovered from a snakebite’s more immediate effects, subsequent complications such as chronic heart failure, kidney damage, and hemolytic anemia (a disease in which the body’s immune system attacks and kills its own red blood cells) can sometimes occur. Cardiac failure can occur weeks to months after the bite incident and appears to be more common in horses than other species. Horses that have recovered from snakebites should be evaluated every few months for cardiac health, and owners should be watchful for signs that might suggest cardiac problems (e.g., shortness of breath, slow recovery after exercise, an increased effort to breathe, and general weakness).
A vaccine is now available for use in horses to help prevent complications of snakebite, but its efficacy is not yet well documented. Contact your veterinarian or a veterinary toxicologist to obtain more information on snakebite and available vaccinations.