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Veterinarian Technician December 2011 (Vol 32, No 12)

Equine Essentials: Preparing a Practice for Rattlesnake Envenomation in Horses

by DeeAnn Wilfong, BS, CVT, VTS-EVN

    This article focuses on envenomation due to Crotalus spp (rattlesnakes) within the Crotalidae family (pit vipers)—one of three families of venomous snakes in North America. Rattlesnake envenomation in horses is often unwitnessed, resulting in delayed treatment based only on clinical signs. Preparing a practice for rattlesnake envenomation and educating veterinary personnel on local venomous snakes can be advantageous for dealing with this type of emergency. The mortality rate associated with rattlesnake envenomation in horses ranges from 9% to 25%,1 with the higher rates being associated with delayed treatment.2

    Rattlesnake Identification

    Rattlesnakes have a triangular head; vertically elliptical pupils; hollow, retractable fangs; a “pit” between each eye and nostril; and a “rattle” on the end of the tail. Rattlesnake species have various colors and patterns, most of which serve as camouflage. The rattle is the most distinguishable feature—the only one that can be assessed from a safe distance (FIGURE 1) . The rattle is a series of interlocked keratin segments, which are added to each time a rattlesnake sheds. The rattle creates a buzzing sound to warn enemies and can be heard from a distance of 3 to 160 ft (0.9 to 48.8 m).3

    Venom

    Rattlesnake venom is a complex mixture of high-molecular-weight proteins, including peptides and enzymes.4 The effects of venom include cytotoxicosis, neurotoxicosis, coagulation anomalies, endothelial damage, myonecrosis, and thrombocytopenia.5 The venom of crotalid species can have one or more of these effects, which vary widely among species.

    Factors Affecting Envenomation

    Rattlesnake envenomation is highly controlled in that a rattlesnake can meter the release of venom from each fang.6 The variable effects of envenomation are due to many factors, including the snake’s motivation (e.g., degree of aggressiveness), age, species, and condition (e.g., injured or ill versus healthy); the number and quality of bites; the duration of the bite; and the victim’s reaction.7 Venom is a rattlesnake’s means of securing prey, so a snake’s natural tendency is to withhold venom rather than inject the total volume in one bite.6 However, dire circumstances (e.g., being stepped on by a horse) may intensify a snake’s reaction, resulting in injection of more venom than in a typical bite.3 Degree of aggressiveness and potency of venom vary by species; for example, some species’ venoms have been shown to be 60 times more potent than others.3 The volume of venom injected usually increases with the number of bites; however, rattlesnakes can deliver “dry” bites in which no venom is injected. If bite penetration is imperfect (e.g., only one fang penetrates; the bite is not well seated), less venom may be delivered. If the victim jumps back or withdraws a limb just as a snake strikes, a less efficient bite may be delivered; if the victim is unaware that a snakebite is imminent, an efficient bite with a larger volume of venom is more likely. Rattlesnake venom glands may be depleted by illness or recent feeding. Fangs that are broken or ready to shed may impair the delivery of venom; recently erupted fangs may improve the delivery of venom.

    A popular myth is that juvenile rattlesnakes cannot meter the amount of venom they deliver during envenomation, making their bites more potent than adult bites. Because juvenile rattlesnakes are heavily preyed on, they can be more ready to bite; however, they can control their injection of venom just like adult snakes. The major difference between adult venom and juvenile venom is the proportion of digestive proteins to toxin. In adult venom, the higher level of digestive proteins aids the snake’s digestion of large prey; the lower level of toxin allows adult prey to live longer after envenomation, allowing digestive proteins to spread throughout the prey’s body. Because juvenile venom has a lower level of digestive proteins, the toxin level is 2.3 times higher by volume than that of adult venom. However, compared with adult snakes, juvenile snakes have smaller fangs and venom glands and, in turn, a smaller venom supply (adult snakes can release 17 times more venom); therefore, bites from juvenile snakes are not often fatal.3,6

    Receiving an Emergency Call and Administering First Aid

    Clients calling because of a snakebite emergency are often fearful not only for the safety of the horse but also because of their encounter with a snake. It is important for clients to remain calm, seek immediate veterinary attention, and keep the horse calm to decrease distribution of venom through the bloodstream.8

    After delivering a bite, snakes often leave the area but occasionally stay close to where the bite occurred. Advise clients not to interact with a snake even if they have killed it: rattlesnakes can deliver postmortem bites when disturbed.

    Rattlesnake envenomation requires immediate medical care; therefore, depending on the horse’s location, the client may have to provide basic first aid before the horse is transported or the veterinarian arrives. The wound should be gently cleaned with antibacterial soap and water as soon as possible after the bite. Severe swelling can occur when horses are bitten on the muzzle. Sections of a garden hose or syringe cases can be secured in the nares to ensure an open airway. A compress can be applied to the wound to minimize profuse bleeding.

    Applying a tourniquet to a limb with a snakebite wound and cutting and/or sucking the wound to remove venom are suggested in some of the literature, much of which pertains to human first aid when medical assistance may be hours away and the victim’s condition is critical.9 The use of a tourniquet compromises venous return, which decreases the spread of venom throughout a victim’s body but also compromises the health of the limb. Placing a tourniquet on a horse’s limb is dangerous because it increases the need for limb amputation, which is not practical in horses as it is in humans and small animals.7 In addition, placing a tourniquet on a horse’s head or nose is impractical. Cutting a snakebite wound to remove venom is contraindicated for the following reasons:

    • It puts the human first-aid provider at risk for exposure to venom9

    • It is ineffective because snakebites in horses are often not witnessed, so venom has already circulated in the body by the time a horse shows signs of a bite

    • It further compromises the wound area, unnecessarily putting vital tissue at risk7

    Therefore, clients with horses should be advised against applying a tourniquet or cutting or sucking snakebite wounds to try to remove venom, all of which can increase destruction of tissue.9

    Clinical Signs of Envenomation

    Local snakebite wound reactions are seen immediately and are mild to severe, depending on the severity of the bite and the time elapsed since the bite occurred. Two of the most recognizable clinical signs, edema and swelling of the bite area, can be particularly pronounced in bites on the muzzle and can compromise the airway (FIGURE 2) . Other common local reactions include bleeding from the puncture wounds, erythema, tissue discoloration and necrosis, and pain at the bite wound. If a bite occurs on a limb, significant lameness and swelling are common.

    Systemic reactions can be immediate or delayed as well as life-threatening. These reactions can include tachycardia or bradycardia, various arrhythmias, shock, colic, a decrease in the capillary refill time, diarrhea, pulmonary edema, laminitis, and respiratory distress.

    Diagnostic Tools

    Physical examination, a complete patient history, gross examination of the bite wound, and the presence of clinical signs are important for diagnosing a snakebite. Time should not be spent trying to identify the rattlesnake species because this can delay treatment and risk another snakebite and usually does not change the treatment plan.10 A complete blood cell count, a serum chemistry profile, and blood clotting times should be obtained. While hypofibrinogenemia and anemia can be present and prothrombin and partial thromboplastin times can be prolonged, only thrombocytopenia can be used as a prognostic indicator.1 Urinalysis may demonstrate hematuria, proteinuria, and myoglobinuria and may help dictate fluid therapy.5 Electrocardiography can be used to diagnose an auscultated dysrhythmia and/or to establish a baseline for monitoring the cardiac effects of venom. Antivenin therapy can be cost prohibitive, but the response to it is diagnostic.

    Treatment

    Treatment of envenomation is supportive, with the goals of preventing or treating asphyxiation, controlling shock, neutralizing the venom, controlling inflammation, preventing secondary bacterial infection, and minimizing tissue damage.

    Airway Maintenance

    In a recent retrospective study of snakebite envenomation in horses,1 tracheostomy was required for airway maintenance in 57% of cases, making it the most commonly performed surgical procedure for snakebite envenomation. Tracheostomy is commonly performed by veterinarians while the horse is standing. To perform tracheostomy, a 6 × 10–cm area over the trachea at the junction of the proximal and middle thirds of the neck is clipped and sterilely prepared. Local anesthetic is injected over the incision site and into the underlying tissue, allowing a 10-cm vertical incision on the midline. The incision is made through the skin and subcutaneous tissue, and the underlying paired sternothyrohyoideus muscles are bluntly dissected to either side. With care to avoid the jugular veins, the trachea is incised horizontally between adjacent cartilage rings, extending the incision approximately 1 cm to either side of the midline. A tracheostomy tube is inserted, pointing ventral, and secured with gauze or tape around the neck11 (FIGURE 3) . A small percentage of horses with limited, rostral swelling may benefit from the placement of tubing within the nares to help maintain airway patency (FIGURE 4) .

    Drug Therapy

    To treat snakebite envenomation in horses, tetanus toxoid therapy should be administered per the manufacturer’s label (typically 1 mL), especially if the patient’s vaccination history is unknown or incomplete. Broad-spectrum antibiotics are routinely administered to prevent or treat secondary bacterial infections, which are common, due to gram-positive and gram-negative organisms in the mouths of pit vipers.9 Because swelling can be severe, NSAIDs are indicated but should be used judiciously during the acute phase of treatment because of their anticoagulant properties. After the initial treatment period, NSAIDs can be used to reduce pain and swelling in well-hydrated patients. Antihistamines (e.g., oral hydroxyzine, IV diphenhydramine) can also be used.1 Corticosteroid use, which is controversial, is indicated only if absolutely necessary to control the effects of shock.12 Because corticosteroids are thought to render antivenin less effective, they should not be used if antivenin will be administered.8 Because pain can vary from moderate to severe, analgesia should be considered.

    Fluid Therapy

    Fluids should be administered as needed to treat shock and dehydration. Many horses are initially reluctant or unable to eat or drink.

    Antivenin

    Simplified antivenins are antibodies that bind to circulating unbound venom. While the literature is divided on the timing of antivenin use, the sooner that antivenin is administered, the more beneficial it is for the patient. Because of the large physical mass (e.g., 1001 lb [455 kg]) of the average equine patient, antivenin use is not necessarily indicated and can be cost prohibitive. Foals, smaller horses (e.g., miniatures, ponies), adult horses with severe bite wounds, and valuable equids may benefit from the use of antivenin. In a recent retrospective study of 58 horses, no patients that received antivenin (16%) died or required euthanasia.1 A suggested dose of antivenin is three to seven vials per 143 lb (65 kg) of body weight; however, because of limited availability of antivenin, a typical starting dose is one vial diluted in 1 L of 5% dextrose and given at a rate of 5 mL/min.8 Adverse reactions to antivenin should be prepared for by closely monitoring the patient during administration and by having a crash kit, including epinephrine, on hand.

    Rattlesnake Vaccine

    A rattlesnake vaccine is available for horses, but its use has not been well documented.

    Conclusion

    Although rattlesnake envenomation is serious, immediate first aid followed by supportive care can prevent death in most healthy adult horses.

    1. Fielding CL, Pusterla N, Magdesian KG, et al. Rattlesnake envenomation in horses: 58 cases (1992–2009). J Am Vet Med Assoc 2011;238(5):631-635.

    2. Dickinson CE, Traub-Dargatz JL, Dargatz DA, et al. Rattlesnakevenom poisoning in horses: 32 cases (1973-1993). J Am Vet Med Assoc 1996;208(11):1866-1871.

    3. Klauber LM. Rattlesnakes: Their Habits, Life Histories and Influence on Mankind. Berkeley, CA: University of California Press; 1982.

    4. Rose RJ, Hodgson DR. Manual of Equine Practice. 2nd ed. Philadelphia, PA: Saunders; 2000:458-459.

    5. Hudelson S, Hudelson P. Pathophysiology of snake envenomization and evaluation of treatments—Part II. Compend Contin Educ Vet 1995;17(8):1035-1038.

    6. Bonnell M. Sex, lies and rattlesnakes [presentation]. REI Stewardship Program, Denver, CO. https://www.auroragov.org/AuroraGov/index.htm. Accessed November 2011.

    7. Beasley V. Toxicants that affect peripheral circulation and/or that may cause reduced lactation. In: Veterinary Toxicology. Ithaca, NY: International Veterinary Information Service; 1999. http://www.ivis.org/advances/Beasley/Cpt9/IVIS.pdf. Accessed September 2011.

    8. Driggers T. Venomous snakebites in horses. Compend Contin Educ Vet 1995;17(2):235-241.

    9. Hopper K. Rattlesnake envenomation: pathogenesis and treatment. Proc NAVC 2005.

    10. Stewart A. Snakebite. Equus 2010;396:35-45.

    11. Orsini JA, Divers T. Equine Emergencies: Treatment and Procedures. 3rd ed. St. Louis, MO: Saunders; 2007:233-234, 448, 687-688.

    12. Rees C. Bites and stings of venomous insects, spiders, and reptiles: snake bite. In: Reed SM, Bayly WM, Sellon DC, eds. Equine Internal Medicine. 2nd ed. St. Louis, MO: Saunders; 2004:711-712.

    References »

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