Snakebite!

Most people have a strong opinion about snakes-and it is usually bad.

The rattlesnake's triangular-shaped head is one way to distinguish between a poisonous and non-poisonous snake.

We have been told that they are non-aggressive and that they do their part in the ecological scheme of things. Many rodents are kept out of feed rooms and kept from creating burrows in horse pastures by the presence of snakes on our farms.

Reptile experts are always reminding us that snakes would simply like to be left alone and that they are rarely a threat but, to the vast majority of humans, a good snake is a dead snake.

This attitude may be cultural since the snake has been associated with evil since the beginning of time.

Culture casualty

The snake is the tempter and the original "bad guy" in biblical stories and snakes are almost always represented as cold, sneaky and untrustworthy.

Some of that attitude may come from the fact that most of us simply do not understand much about these unique creatures.

But when horses and snakes mix it is a good idea to have a bit of knowledge about these reptiles and to know how to treat snakebites.

It is important that equine veterinarians learn a bit about the nature of snakes and the conditions that can result in snakebites in horses, as well as review the treatment of those injuries.

Knowledge is power

There's some good news and some bad news regarding snakebites. The bad news is that each year more than 8,000 people are bitten by poisonous snakes in the United States. The good news is that fewer than 10 deaths are reported yearly. That means that more people die from bee and wasp stings than from snakebites.

The exact statistics for the number of animal snakebites, and specifically for bites in horses, is unknown because many bites are unreported or unconfirmed. But because snakebites generally cause severe pain, long lasting edema, tissue damage and lameness, even non-fatal bites are to be taken seriously. Since the summer brings good weather for trail riding and other horse-related activities in the woods and pastures, it is more likely that horses and snakes will interact. Most bites occur in the southern and southwestern states between April and October.

Massive forest fires last year destroyed large tracts of forest and natural habitats. We tend to look at these natural disasters and sympathize (rightly so) with the people who lose their homes and neighborhoods. Often forgotten is the fact that countless animals also lose their homes and that they flee the fires into surrounding safer territory-often into more heavily populated areas that wild animals would never venture into if given a choice.

Displaced reptiles

Horses are extremely sensitive to snake venom. They are followed, in order, by sheep, cows, goats, dogs, pigs, and cats. Humans are moderately sensitive and fall somewhere between dogs and pigs.

Equine sensitivity

Horses and cattle, though very sensitive, seldom die as a result of snakebites. A lethal dose of venom is based on body weight and fortunately most horses and cows are simply too large for snakes to kill.

Other factors that affect the severity of the bite should enter into a veterinarian's decision-making process when dealing with a snakebite. The type of snake is important. Some snakes are deadlier than others. The location of the bite is also important. Snake venom enters the circulatory system and is carried throughout the body causing damage. Bites closer to major arteries and areas of large blood supply are potentially far more serious than bites to the limbs and body. Fatalities in horses and cattle have been reported when the snakebite was on the face, head and neck.

Important factors

Size and species of the victim and its age and general health and condition are also important determinants in the severity of snakebites. Generally larger, older horses in good nutritional condition without other medical problems seem to recover from snakebites easier and more quickly. Young foals and weak or sick horses are at a much greater risk of possible death or prolonged difficult recovery and these victims should be treated aggressively from the outset.

Because the type of snake is important in determining a treatment plan for a snakebitten horse, it becomes important to know your adversary. Not all snakes are poisonous. Every year countless harmless and potentially beneficial snakes are destroyed by overzealous animal owners. Non-poisonous snakes can still bite and their bites may need to be treated, but the snake may not need to be destroyed.

Know your adversary

Venomous or poisonous snakes fall into two categories: the elipine snakes which include the cobra, mamba and coral snakes; and the viperine snakes which include the adders, pit vipers, rattlesnakes, cottonmouths, copperheads and moccasins. Elapine snakes have short fangs and tend to chew their victims. In this chewing motion they release a neurotoxic venom into the damaged tissue and vascular structures that eventually paralyzes the respiratory system and thus kills its prey.

Recovery from bites by elapine snakes, if one is large enough, and not sensitive enough to be within the lethal dose of the snake's venom, rarely leaves any lasting effects. The best way to avoid being bitten by these snakes is to avoid travel to exotic locations. They are not generally found in the United States.

Recovery

Viperine snakes are located throughout the Americas, however, and they have hinged fangs that strike, penetrate and withdraw. The motion is not unlike that of an intramuscular injection. The venom of these snakes is mainly hemotoxic consisting of potent enzymes and peptides. A vasculitis quickly occurs and massive edema, local bleeding and eventual tissue necrosis can occur. These problems can lead to tissue damage and skin necrosis even if the victim recovers and some local neurotoxins can lead to persistent lameness in some cases.

Poisonous snakes can be differentiated from non-poisonous snakes in some generally easy-to-remember ways that do not require close contact.

Differentiation

Poisonous snakes have an elliptical pupil while the pupil of non-poisonous snakes is round. Poisonous snakes have a triangular shaped-head that is generally larger than their body. Non-poisonous snakes taper to a head that is normally the same size as their body.

There are other distinguishing features such as the presence, in poisonous snakes, of single scales under their tails and a pit or hole under the nose and above the mouth but these are not practically identified in live snakes in the field. Noting the shape of the head and possibly the shape of the pupils can be done however and this information from a client will be helpful to you as you decide on treatment.

The most common snakes encountered in the U.S. are the copperhead and various types of rattlers.

Common to U.S.

Rattlesnakes are relatively docile and give birth to live poisonous young. There can be many variations in color and markings between adults and their young and even variations noted in different locations within a country.

It's important to be aware of the colors and patterns in your practice area. The eastern diamondback rattler, the most dangerous American snake, can grow up to 8 feet long and weigh as much as 15 pounds. The fangs of this snake can penetrate thick hides. The western diamondback is a similar cousin but is smaller. It is responsible for the majority of recorded deaths in the United States.

The prairie rattler and the sidewinder (named for its unusual method of locomotion) are found in the western states, and the timber or banded rattlesnake is a northwestern snake often camouflaged in forested areas.

Rattlers usually attack when they are startled as when a horse steps over a log in the trail to find a snake dozing in the shade on the other side.

On the attack

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 special segments when its tail is vibrated. A wet rattler makes no noise and this can be important when your clients are riding among wet leaves or grasses.

Perhaps the most interesting fact about snakes is that the decision to inject venom during a bite is a voluntary action on the part of the snake and is totally under the reptile's control. Current theories state that the snake makes a decision as to whether the bite is protective (startled and trying to get away or to simply warn off a large predator that is too big to kill) or whether the bite is aggressive and designed to kill its victim.

Many bites in horses are thought to be "dry" or non-venomous 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 may be another reason why equine deaths from snakebites are not common.

Clients should be aware of a few steps to take in the event of a snakebite.

What to advise

Most snakes will give larger animals a few seconds to get out of the way so encourage clients to be calm and to slowly back their horses away from snakes. Try to confirm the bite and identify the snake if possible. Snakebites can be difficult to locate on the body because of hair, bleeding or swelling. By the time that most veterinarians see affected horses, the swelling will have become severe enough that the original bite punctures cannot be found. Horses that are bitten on the nose or muzzle can swell so much that their nostrils close and breathing can be difficult to impossible.

Encourage trail riders to carry two 6-inch pieces of garden hose. Advise these clients to pass the lubricated hose into the nostrils of a horse that is bitten in this area to keep airways open. These horses then can be transported and treated.

Increased heart rate causes higher blood pressure and the dissemination of snake venom to larger areas of the horse's body so every effort should be made to calm the horse, especially if the horse was exercising heavily prior to the bite.

Instruct clients to keep the horse in one spot and get a trailer to it if possible or to slowly walk the horse to the nearest road if the terrain does not allow trailer pick-up. Apply a wide tourniquet proximal to the bite to slow the toxin's spread. Wash the bite area with Betadine and water. The mouth of a snake contains many potentially harmful bacteria that will contaminate the wound. Do not cut the bite area. Recent research shows that this old practice can allow venom access to even more tissue and may actually contribute to further damage.

Research has also shown that application of cold or hot compresses will definitely worsen the damage. Clients may use the rubber suction cup in snakebite kits to try to remove some of the venom, but these cups are not very effective. Snakebites should never be suctioned by mouth!

On presentation to a veterinarian, the diagnosis of a snakebite has usually been made and the treatment approach should focus on three main areas.

Three focal areas

• First, prevent or delay absorption of venom. Many times there is a substantial time delay between the bite and its discovery or in the call for veterinary assistance. This is common in horses bitten while at pasture or in a stall for the evening. The next morning the owner typically finds a horse in pain with a severely swollen leg. Cleaning and voluminous flushing of the wound, if it can be located, may be all that can be done at this stage. It is thought that infiltrating the bite area with saline in an attempt to dilute the toxin will actually aid in its dispersal throughout the damaged vessels and lymphatics so this should be avoided.

• The second main treatment should be to neutralize any absorbed venom with appropriate antivenin. Many states have hot line numbers linked to a poison control center that will provide antivenin availability information in your area. Rural hospitals are your best bet for locating the antivenin necessary. Be sure to have epinephrine available because of the anaphylactic response possibility, since many antivenins are equine origin products.

• Fight the effects of the toxin and maintain cardiovascular function. Maintain severely affected horses with intravenous fluids. This volume replacement should be tempered with a desire to limit the spread of venom throughout the body. Corticosteroids are advised to counter shock and to help lessen tissue destruction. Broad-spectrum antibiotics should be given and, since many snakes' mouths contain Pseudomonas bacteria, gentamycin and penicillin are the drugs of choice. DMSO can be added to the intravenous fluids. Its action in stabilizing tissue can aid in some cases.

Skin and tissue damage may be the hardest aspects of snakebites to treat. Treatment designed to reduce swelling and to prevent toxin spread in the initial few hours many be the most important action that will help with later tissue repair. Compression bandages that limit edema are very helpful. Treatment with cold laser or monochromatic light in the later healing stages may benefit lymphatic drainage and repair and yield encouraging results.

While discussing snakebites it's important to remember plenty of other potentially serious types of bites are commonly seen in horses.

Other predators

Gila monsters, found in certain areas of the United States, can cause tissue necrosis and painful bites. Scorpions are found throughout the south and southwest. Their bite can cause massive edema with serous leakage through the skin of the affected horse's leg. Laminitis or tendon breakdown in the contralateral limb is a potential complication in all such types of bites if damage is extensive enough.

Leg wraps, heel or frog pads and appropriate anti-inflammatory therapy is necessary in these cases. Brown recluse and black widow spiders account for many serious equine bites each year. The treatment approach is generally the same regardless of the causative agent of the bite.

Dr. Marcella, a 1983 graduate of Cornell University's veterinary college, was a professor of comparative medicine at the University of Virginia. His interests include muscle problems in sport horses, rehabilitation and other performance issues.