Treatment of Snakebites in Field Dogs
Any time a field dog works, he encounters many dangers, one of which is a potentially fatal envenomation from a poisonous snake. These can occur on land or in the water. There are approximately 15,000 dogs and cats bitten by poisonous snakes in the United States annually. The highest envenomation fatality rates occurred in Arizona, Georgia, Florida, Alabama, South Carolina and Texas. In dogs, 70-80 percent of bites occur on the face and head, and 20-30 percent occur on the legs, with only rare cases occurring on the body. Rattlesnakes account for 80 percent of dog envenomations, while cottonmouth, water moccasin and copperheads are responsible for the remaining 20 percent.
The poisonous snakes of the United States belong to 3 groups: the pit vipers, the elapids and the colubrids. More than 99 percent of venomous bites in the U.S. are caused by the pit vipers. This article will focus on the treatment of pit viper envenomations of dogs caused by rattlesnakes, copperheads and cottonmouths.
Pit vipers are named for heat sensitive pits located between the eyes and nostril. Other characteristics of pit vipers are their triangular shape heads, elliptical pupils, retractable fangs, and a single row of subcaudal plates. These snakes can strike at 8 feet per second and may strike to a distance up to one-half their body length. Their recurved fangs, 10-12 mm long, are capable of penetrating 1-8 mm deep.
The primary biological purpose of snake venom is the immobilization of the victim, to cause its death quickly and to start predigestion of tissue before the victim is ingested by the snake. Pit viper venom consists of many enzymes and nonenzymatic proteins. It contains 10-25 distinct biologically active components. In general, the venoms of the rattlesnake and other New World crotalids produce alterations in the resistances (and often integrity) of the blood vessels, changes in the blood cells and blood coagulation mechanisms, direct and indirect changes in cardiac dynamics, alteration in the nervous system, and depression of respiration. The venom of snakes varies considerably in both its volume and toxicity.
Numerous factors affect the severity of the snake bite. One of these factors is the amount of venom injected by the snake into the victim. In a typical feeding strike, 15-20 percent of the venom is released, 50 percent is released in a defensive strike, and up to 75 percent or more in multiple strikes. It has been shown that the pit vipers will replenish their venom in 16-54 days. Not all snake bites result in death or severe medical problems due to the fact that 20 percent of the bites are dry bites where no venom is released, 30-35 percent are mild envenomations and 45-50 percent are envenomations severe enough to be a medical emergency. It is important to understand that you cannot wait to see how bad a bite is. You cannot tell how severe the envenomation is simply by looking.
The amount and composition of the venom is determined by the species of snake, the time of year, the regenerated volume of venom, the age of the snake, aggressiveness of the snake, and the motivation and size of the snake. The quantity and the toxicity of the venom are perhaps the two most important factors affecting the severity of envenomation.
Factors affecting the severity of envenomation of the victim include the body mass or size of the victim, the location of the bite, the length of time to an emergency clinic or hospital, the excitability of the patient, physical activity after the bite, type of first aid given, and medications administered.
The field diagnosis of snake bites involves seeing punctures where the fangs penetrated the tissues, severe pain at the site of the bite, edematous swelling, ecchymosis (pinpoint bruises), nausea, weakness, severe hypotension and shock.
The best and safest advice on first aid is to seek medical attention at an emergency clinic or hospital as soon as possible. Call ahead to allow the clinician to prepare the antivenin and to set up other emergency medications.
AVOID medications of any type, but especially pain medications, tranquilizers, cortisone and/or DMSO. Do NOT use ice, tourniquets, alcohol, cut and suck techniques, or electrical shock devices. The Food and Drug Administration of April 9, 1990, banned the use of electrical devices for the treatment of envenomation of animals and humans.
Medical treatment of envenomations are (1)Antivenin given IV; (2)Intravenous fluids; (3)Antibiotics; and (4)Diphenhydramine, if necessary.
Antivenin (Fort Dodge) is a refined and concentrated preparation of equine serum globulins which neutralize the venom of the viperene snakes, including rattlesnakes, copperheads and cottonmouth moccasins.(1)Antivenin takes 20-30 minutes to go into solution after mixing and should not be shaken. It should be administered IV only. It is NOT to be given IM or at the bite site. Studies using radioactively labeled antivenin demonstrated that when administered IV, 85 percent was at the site within two hours; when given IM, only 1.4 percent went to the site; and when given SQ, only 5.6 percent appeared at the site.The dose of antivenin given should be guided by the severity and progression of edema as well as the clinical and laboratory findings. The total dose must be titrated to the clinical need of the patient. The patient should be monitored for an allergic reaction while the antivenin is being given. Signs include petechia on the pinna of the ear, and an itchy face. If this occurs, slow the rate of antivenin administration and administer Benadryl, 10 mg IV or SQ to a small dog, or 25 mg IV or SQ to a large dog. If extreme or severe reactions occur, epinephrine (0.25 to 0.5 ml IV of a 1:1000 solution) may be administered.
Antivenin can have beneficial effects when administered as long as 24 hours after the bite.
(2)Intravenous fluids are used at the time of presentation. A large bore catheter should be placed and rapid IV administration of lactated Ringers or 0.9 percent sodium chloride should be given at 40 ml per pound of body weight as an initial dose to a patient with signs of shock. The primary cause of death from envenomation is cardiovascular collapse due to hypovolemic shock. Fluids are administered as the primary treatment. Steroids are contraindicated in the treatment of envenomations. Most experimental studies indicate that steroids have no place in the acute phase of snake venom poisoning and may be contraindicated. They are of little, if any, value in the snake venom hypotensive crises and they may tend to confuse the diagnostic tests.
(3)Antibiotics are used following any snakebite to prevent abscess formation at the site of the bite. There have been 58 different bacteria isolated from the fangs of snakes, with Proteus vulgaris, Escherichia coli and Corynebacterium being the most common isolates. Administration of a broad spectrum antibiotic that provides gram negative coverage is advised.
(4)Diphenhydramine (Benadryl) can be used in evenomation patients. It pre-treats against allergic reactions from the use of antivenin and seems to calm the patients. A large dog (50 lbs) receives 25 mg, and a small dog receives 10 mg, which is given IV or SQ.
When envenomations are presented, there is little time to waste in the emergency room, but it is important to draw blood samples to establish baseline values on each case. Tests which are suggested include a complete blood count (CBC) with platelets, a chemistry profile, prothrombin time (PT), partial thromboplastin time (PTT), fibrinogen, bleeding time, fibrin split products and urinalysis. As many of these tests as possible are repeated at six hour intervals to monitor the patient's progress. The venom of pit vipers causes a severe coagulopathy syndrome characterized by a persistent drop in platelets, prolonged clotting times, decreased fibrinogen and increased fibrin split products. Laboratory findings should be used as the guide to determine if more antivenin is needed and thereby become our most important prognostic tool.