by Kenneth L. Marcella, DVM

Snakes commonly found in the United States can cause serious injury to a horse. Horses are at the top of the list of sensitivity to snakebites and are followed in order by sheep, cows, goats, dogs, pigs, and cats. However, because a lethal dose of venom is based on body weight, most horses and cows are simply too large for snakes to kill. Other factors that affect the severity of the bite are:

• Type of venom. Some snakes are deadlier than others.
• Location of bite. Bites to the head, face, and other areas of major blood supply are far more serious than bites to limbs and the body. Fatalities in horses and cattle have been reported when the snakebite is on the muzzle, head, or neck.
• Size and species of the victim and its age and general health and condition. Dogs are most commonly bitten because of their aggressive and inquisitive nature. Most dogs, like horses, are bitten on the muzzle face as they sniff to investigate. Given the relatively small size of the dog compared to that of the horse, many snakebites in dogs are fatal.

Poisonous snakes fall into two categories: the elapine snakes, which include the cobra, mamba and coral snake, and viperine, which include the pit vipers, such as rattlesnakes, copperheads, cottonmouths, and moccasins.

Elapine snakes have short fangs and tend to chew their victims. Their venom is mainly neurotoxic in that it affects the nervous system and kills its target by paralyzing the respiratory system.

Viperine snakes, located throughout the Americas, however, have long, hinged fangs that strike, penetrate, and withdraw. The venom of these snakes is mainly hemotoxic and causes massive damage to blood vessels and tissue loss even if the victim recovers. The main ingredients of snake venom consist of potent enzymes, peptides, and neurotoxins.

Poisonous snakes can be differentiated from nonpoisonous snakes in some generally easy-to-remember ways. Poisonous snakes have an eliptical pupil, while the pupil of nonpoisonous snakes is round.

Nonpoisonous snakes have small teeth rather than fangs, and they will have a rounded head that is about the same size as their bodies. Poisonous snakes have a triangular head that is somewhat larger than their bodies and will have single scales under their tail and a pit or hole above the mouth and under the nose. Some time spent learning to recognize poisonous snakes will save some worry and may save a harmless snake’s life.

The most common snakes encountered in the U.S. are the copperhead and various types of rattlers. Rattlesnakes have both day and night vision and give birth to live, poisonous young. Most subspecies are relatively docile.

The Eastern diamondback rattler is the most dangerous American snake. It can grow up to eight feet long and weigh as much as 15 pounds. The fangs of this snake can be three-quarters of an inch long and capable of penetrating thick hides. The Western diamondback is a similar cousin but smaller in size. It is responsible for the majority of recorded deaths in the U.S. The prairie rattler and sidewinder are found in the Western states, and the timber or banded rattlesnake is a Northeastern snake often camouflaged in forested areas.

Rattlers are usually startled when they attack, such as when a horse steps over a log in the trail to find a snake dozing in the shade on the other side. Rattlers coil before striking with a strike distance of one-third to one-half of their overall length. The sound a rattler makes is caused by the clicking together of rattle segments when the tail is vibrated. The sound has been described as similar to the crackling sound of frying fat.

Perhaps the most interesting fact about snakes is that the decision to inject venom into a bite is a voluntary action and totally under the reptile’s control. Current theories are that the snake makes a decision whether the bite is protective, such as when a larger predator startles it, or whether it is aggressive and meant to kill its victim.

Many bites in horses are thought to be nonvenomous because the snake has to put a lot of biological work into making its poison and does not seek to waste it. Because the snake can sense the size of the horse, it bites only to get away. This is perhaps another reason why equine deaths from snakebites are not common.

If a horse is bitten, there are a few steps to take. If riding, prevent the horse from looking down and slowly back away. Most snakes will give larger animals a few seconds to get out of the way. The snake is trying to avoid confrontation, and you want to avoid further strikes. Try to confirm the bite and attempt to identify the snake—it may be important later in trying to determine the correct antiserum, but do not waste time looking for the snake. The primary concern is the horse.

Snakebites can be difficult to locate on the body because of hair, bleeding, or swelling, but a close examination should reveal fang marks. Viperine snake venom causes immediate swelling. Horses bitten on the nose or muzzle can swell so much that their nostrils almost close and breathing can be difficult or impossible.

Seasoned trail riders and ranch hands carry two six-inch pieces of old garden hose that can be lubricated and inserted into the nostrils of a snakebitten horse; more than one animal has been saved with this simple procedure. The hose allows the horse to breathe until the swelling subsides with treatment.

Intense pain, nausea, muscular weakness, and shock follow a typical snakebite. If the horse has become excited or was exercising heavily prior to the bite, it is important to quiet him down. Increased heart rate causes higher blood flow and the dispersal of the poison to larger areas of the body.

A wide constricting band (handkerchief or shredded clothing) should be placed about two inches above the bite if it is on the leg. Obviously, tourniquets on the face are not indicated. The band should only be tight enough to compress the veins and lymphatic vessels and not the arteries. Your goal is to keep the venom in the bite area. This band should be as tight as the band a nurse applies when drawing blood.

Wash the bite with soap and water. If possible, trailer the horse to its stall. If you have to travel, walk the horse slowly to the nearest trailer. Do not cut the bite area. Recent research shows that this old practice actually may contribute to further damage. Also, never suck venom from a bite by mouth; you can use the rubber suction cup in a snakebite kit if one is available, but this rarely does much good and usually only serves to give you something to do until help arrives. Do not apply cold or hot compresses. Recent research positively show this to worsen the damage.

Antivenin has proved useful in horses even when given 24 hours after a head bite. There are some potential problems with antivenin, however, because it is produced in horses and therefore anaphylactic shock can occur. A veterinarian may use epinephrine to help lessen the threat of reactive shock to the serum. Corticosteroids and fluids may be necessary to counteract the effects of shock, and good management will be required to treat the tissue damage that may result.

If the owner has seen a horse bitten by a snake then the diagnosis is easy to make. Most times, however, the horse is found at pasture or in the stall with a severely swollen leg and exhibiting signs of shock. Bites by scorpions, spiders and the occasional Gila lizard must also be considered in these cases. Treatment for all of these injuries is generally similar with the use of antivenin being the biggest difference with snakebites. The appropriate antivenin must be used for individual snakes. Many states have Hot Line numbers in your area. Rural hospitals are your best bet for locating the antivenin you may need. Because of the anaphylactic response possibility be sure to have epinephrine available.

Shock is the most common problem following snakebites. The aims of treatment for snakebit horses focus on three areas—prevent or delay absorption of venom; neutralize any absorbed venom with the use of the appropriate antivenim; and fight the effects of the venom and maintain cardiorespiratory function. To this end, fluid therapy to maintain blood volume is tempered with the desire to limit the spread of venom through the body. Corticosteroids should be utilized to counter the shock and to minimize tissue destruction. Broad-spectrum antibiotics should be given and since many snakes’ mouths contain Pseudomonas bacteria, Gentacin and Penicillin are the drugs of choice. Intravenous fluids containing dextrose and DMSO (a potent antioxidant) can be given if needed.

Skin and tissue loss is managed as it would be in any other traumatic case, but the actions taken in the first hour following a snakebite will ultimately do more for the outcome than almost anything else. A compression wrap will limit the venom to the immediate area. Reducing activity will slow down the pumping of the horse’s heart and limit the spread of the venom. Tetanus protection should be given as well. Wrapping a severely swollen leg will also limit the edema that develops after a bite and may actually reduce the amount of fibrous tissue formed in the leg.