Snakebites in Animals

Between 150,000 and 300,000 animals are bitten every year by pit vipers in the US.1 Fatal snakebites are more common in dogs than in other domestic animals. Because of the relatively small size of some dogs in proportion to the amount of venom injected, the bite of even a small snake may be fatal. In dogs and cats, mortality is generally higher in cases of bites to the thorax or abdomen than bites to the head or extremities.

Because of their larger sizes, horses and cattle seldom die as a direct result of snakebite; however, deaths may follow bites on the muzzle, head, or neck when dyspnea results from excessive swelling. Serious secondary damage sometimes occurs; livestock bitten near the coronary band may slough a hoof.

North American elapids have short fangs that deliver a neurotoxic venom that paralyzes the respiratory center. Except for death adders (Acanthophis spp), the venom delivery system of which resembles that of crotalids, Australian elapids tend to have relatively short, grooved to tubular fangs. The venoms of Australian elapids may have neurotoxic, myotoxic, procoagulant, anticoagulant, or hemolytic properties; pronounced cardiovascular abnormalities have been associated with envenomation by tiger and brown snakes.

Crotalids have long, hinged, tubular fangs with which they strike, inject venom (a voluntary action), and withdraw. Many bites by vipers reportedly do not result in injection of substantial quantities of venom and are therefore termed “dry bites.” Crotalid venom is typically hemotoxic, necrotizing, and anticoagulant, although a neurotoxic component is present in the venom of some species, such as the Mojave rattlesnake (Crotalus scutulatus scutulatus).

Nonvenomous snakebites cause only local injury, usually multiple superficial teeth marks (in contrast to the fang marks suggestive of a venomous snakebite), with accompanying signs of pain.

Typical crotalid bites are characterized by severe local tissue damage that spreads from the bite site. The tissue becomes markedly discolored within a few minutes, and dark, hemorrhagic fluid may ooze from the fang wounds, if not prevented by swelling. The epidermis may slough when the overlying hair is clipped or parted; hair may hide the typical fang marks. Sometimes, only one fang mark or multiple punctures are present. Neurologic signs, including muscle fasciculations, are possible if neurotoxic crotalid venom is involved.

In elapid snakebites, pain and swelling are minimal, and systemic neurologic signs predominate. Clinical signs of coral snake envenomation include tetraparesis, ptyalism, tachypnea, shallow or abdominal breathing, depressed gag reflex, ataxia, muscle fasciculation, decreased spinal reflexes, and quiet mentation. Australian elapid bites may cause collapse, vomiting, ptyalism, tremors, tachypnea, urinary or fecal incontinence, tetraparesis, hemolysis, coagulopathy, rhabdomyolysis, swelling at the bite site, renal failure, or delayed immune-mediated hemolytic anemia (in the case of a bite from a red-bellied black snake).

• Clinical signs (ie, presence of bite wound)

• When possible, identification of the snake

• In crotalid envenomations, presence of echinocytes or coagulopathy

In many instances, the bite has been witnessed, and diagnosis is straightforward. Snakes can be distinguished by some physical features; consultation with a zoo or aquarium can help in identification of snake species. However, owners may not recall details of the snake’s appearance. Also, fractures, abscesses, spider envenomations, or allergic reactions to insect bites or stings can all be confused with snakebites. Owners should not try to capture the snake, but when possible, the dead snake should be brought in, along with the bitten animal; mutilation of the snake’s head should be avoided, because this may hinder proper identification. Some bites do not result in envenomation or have been made by nonvenomous snakes. In Australia, venom detection test kits have been developed to detect the various snake venoms and determine the appropriate antivenom to use; however, these appear to be infrequently used for veterinary patients.

• Monitoring spread of tissue damage (with crotalid envenomations), and supportive treatment

• Sometimes, antivenom

Snakebite with envenomation is a true emergency. Rapid examination and appropriate treatment are paramount. Owners should not spend time on first aid other than to keep the animal quiet and limit its activity. The following commonly touted measures are ineffective and can be potentially harmful: use of ice, cold packs, or sprays; incision and suction; tourniquets; electric shock; hot packs; and delay in presentation for medical treatment (waiting until problems develop).

Intensive treatment should begin as soon as possible because irreversible effects of venom begin immediately after envenomation. Bite sites should be shaved and wounds cleansed thoroughly with germicidal soap. For animals bitten by crotalids, the leading edge of tissue swelling should be marked on the skin with a skin marker at frequent intervals to monitor the spread of tissue injury. All snakebite patients should be monitored closely for a minimum of 24 (crotalid) to 48 (elapid) hours for the development of clinical signs.

Treatment for crotalid envenomation should be directed toward preventing or controlling shock, neutralizing venom, preventing or controlling coagulopathy, minimizing necrosis, and preventing secondary infection. Any dog or cat evaluated for treatment within 24 hours after a snakebite showing signs of crotalid envenomation requires intensive treatment, starting with IV administration of crystalloid fluids to combat hypotension. Rapid-acting corticosteroids may be of benefit in the first 24 hours to help control shock, protect against tissue damage, and minimize the likelihood of allergic reactions to antivenom (also known as antivenin); however, prolonged use of corticosteroids is not recommended. Monitoring for the development of echinocytosis or coagulopathy is recommended because these are often early signs of severe envenomation.

Antivenom is the only direct and specific means of neutralizing snake venom. Antivenoms available against North American pit vipers include equine-derived polyvalent antivenom, ovine-origin polyvalent F(ab) fragment antivenom, and equine-origin polyvalent F(ab)² fragment antivenom. The F(ab) antivenoms use the F(ab) components of the immunoglobulin molecule, resulting in an antivenom that has lower risk of allergic reaction, faster reconstitution, and potency similar to that of the polyvalent immunoglobulin.

Antivenom is most effective if administered in the first 6 hours after the bite, although improvement in clinical condition may occur after antivenom administered ≥24 hours after the bite. In the unlikely event of an anaphylactic reaction to the antivenom in a dog or cat, antivenom administration should be discontinued and epinephrine (0.5–1 mL of 1:1,000 solution, SC, once), with or without antihistamines such as diphenhydramine, should be administered. In severe envenomations, multiple vials of antivenom may be required, although this is frequently cost-prohibitive in veterinary patients.

Antivenom generally helps considerably in managing the pain of a crotalid bite. Opioid analgesics may be used as needed for residual pain; NSAIDs are not recommended.1 If coagulopathy (thrombocytopenia, disseminated intravascular coagulation, etc) occurs, appropriate treatment, including blood replacement products and heparin sodium (5–10 U/kg, SC, every 60 minutes [mini dose]) or 50–100 U/kg, SC, every 8 hours [low dose]),2,3 should be administered. Hemoglobin glutamer-200 (bovine) or hetastarch may be helpful to manage hypovolemia; however, colloids should be used with caution because of their potential to leak out of damaged vessels and pull fluids into tissue beds.

Several potential pathogens, including Pseudomonas aeruginosa, Clostridium spp, Corynebacterium spp, and staphylococci have been isolated from the mouth of rattlesnakes. However, the incidence of wound infection after snakebites is low, and many veterinarians use antimicrobials only when notable tissue necrosis is present. Broad-spectrum antimicrobials such as amoxicillin potentiated with clavulanate or cephalosporins are preferred.

Tetanus antitoxin also should be considered, especially in horses, and other supportive treatment should be administered as needed (eg, blood or plasma transfusions in the case of hemolytic or anticoagulant venoms). In most cases, surgical excision of tissue is impractical or unwarranted. Antihistamines have been reported to be contraindicated; however, diphenhydramine hydrochloride (10–50 mg, SC or IV, once) has been shown to be helpful to manage fractious patients and may possibly assist in minimizing risk of allergic reactions to antivenom.

Animals bitten by elapids may be treated with supportive care as needed (IV fluid therapy, ventilatory support, anticonvulsants, etc) and antivenom, if available. Ventilatory support is frequently required for 6−8 hours in animals with Australian elapid envenomation; additional vials of antivenom can reduce ventilator time. Antivenom against coral snake venoms is no longer manufactured in the US, although some practitioners have received special permission to import coral snake antivenom from Mexico.

In Australia, several antivenoms are available for use in veterinary patients. A polyvalent antivenom is available for use when the identity of the snake cannot be ascertained, and many veterinarians prefer to use the polyvalent antivenom for all envenomations. Additionally, animals bitten by Australian elapids should be monitored for development of coagulopathy, hemolysis, renal injury, cardiovascular abnormalities, or rhabdomyolysis; appropriate treatment should be instituted as needed. As with crotalid bites, broad-spectrum antimicrobial therapy may be indicated if there is risk of infection of the bite wound.

The prognosis of snakebite depends on the type and species of snake, location of the bite, size of the victim, extent of envenomation, and time interval between the bite and the institution of treatment. Animals that survive elapid bites generally make full recoveries; however, crotalid bites can result in long-term sequelae due to tissue necrosis (amputation, loss of function, etc), depending on severity of the bite and promptness and aggressiveness of treatment.

• Also see pet health content regarding snakebite.