With warm weather comes the increased risk of snakebite. The major venomous snakes in the United States are the pit vipers, including rattlesnakes, water moccasins and copperheads. Pit vipers are named after the heat-detecting holes, or pits, on each side of the head that help the snake locate prey. Pit vipers can be differentiated from other snakes by their triangle-shaped heads, narrowed necks and tail rattles (rattlesnakes only). Coral snakes, another type of poisonous snake in the U.S., do not pose much risk to horses because of their small mouth size.
Venom components vary tremendously by snake species, but most venoms contain substances that cause breakdown of tissues and blood vessels, impair blood clotting and damage the heart. Venoms from some species of snake also contain neurotoxins. Snakebite severity depends on multiple factors such as snake species, size, recent feeding and number of bites. Some bites are “dry bites,” where little venom is injected. Other bites, such as when a snake is stepped on and releases all of its venom agonally, can be very severe. Victim factors such as horse size, age, disease conditions, medications and bite location also influence bite severity.
Clinical signs of snakebite in horses vary widely, but generally include pain and swelling at the bite site, and often sloughing of tissues near the bite. Bite wounds may not be readily apparent. Dry bites with little venom injected or bites from copperhead snakes often cause only mild signs. Bites from dangerous species of snakes and large doses of venom can cause marked pain and swelling, coagulopathy, hemorrhage, cardiac arrhythmias, shock, collapse and even death. With neurotoxic venoms, paralysis can occur. Horses bitten on the nose can develop nasal swelling and respiratory distress. Signs of envenomation can occur within minutes or be delayed for many hours.
The best first aid is to keep the horse calm and arrange for immediate veterinary care. No first-aid treatments performed by owners in the field have proven particularly helpful, and many folk remedies can even be harmful. Suction devices have not been shown to be beneficial in animal models of snakebite.
Treatment varies with the severity of the bite, but may include fluids, pain medications, wound care, antibiotics, tetanus prophylaxis and antivenin. Antivenin can decrease the amount of tissue damage and hasten recovery times, and can be especially helpful in cases of severe envenomation. Antivenin is dosed according to the estimated amount of venom injected, not the patient size, so even one vial of antivenin can have beneficial effects. Cardiac arrhythmias occur in many horses and may require treatment. Horses with severe nasal passage swelling may need treatment to maintain a patent airway; nutritional support may be required if swelling impairs the horse’s ability to eat and drink.
Even after horses have recovered from the immediate effects of snakebite, subsequent complications such as heart failure or kidney damage are possible. Cardiac failure can occur weeks to months after the bite incident, necessitating continued evaluation and monitoring.
A vaccine is now available for use in horses to help prevent complications of snakebite, but efficacy in horses is not yet well documented. Contact your veterinarian for more information about snakebite in your region.
This article was written by Dr. Cynthia Gaskill of the University of Kentucky's Veterinary Diagnostic Laboratory. The Equine Disease Quarterly is funded by underwriters at Lloyd's, London, their brokers and Kentucky agents.