Snake Envenomation: Slideshow
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The vast majority of snakebites are from nonvenomous species, with only 15% of snake species considered dangerous to humans. Snakebite reporting is not mandatory, so all reported envenomations are estimates. The most dangerous snakes in the world live in Australia, Southeast Asia, and Africa; however, because of the large number of private and public snake collections, venomous snakes may be transported throughout the world. Image courtesy of Wikimedia Commons.
Venomous snakes in the United States are responsible for 7000-8000 bites and about 5 fatalities every year. The 2 native venomous snake families in the United States are elapids and vipers. The elapidae family consists of coral snakes and non-native sea snakes, cobras, and brown snakes. The viper family, specifically the subfamily crotalines or pit vipers, consists of rattlesnakes, moccasins, and copperheads. Images clockwise from top left are a pygmy rattlesnake, water moccasin, copperhead, and coral snake. Images courtesy of the US Fish and Wildlife Service and Wikimedia Commons.
The presence of a rattle distinguishes rattlesnakes from other pit vipers. All rattlesnakes are venomous. Some nonvenomous snakes mimic rattlesnakes by rapidly vibrating their tails, although they lack the buttons (pictured here) responsible for the characteristic sound of rattlesnakes. Newborn rattlesnakes lack functional rattles but develop a button segment with their first molt. Image courtesy of Wikimedia Commons.
Venomous pit vipers typically have a more triangular-shaped head than nonvenomous snakes. The broad head accommodates large venom glands at the base of the jaw. Another feature that can distinguish venomous pit vipers is the shape of the pupil. Pit vipers generally have vertical pupils in contrast to the round pupils of nonvenomous snakes. Pictured here is a cottonmouth, a pit viper that lacks a rattle. Image courtesy of Wikimedia Commons.
Pit vipers can be found all over the United States, with specific species located in various geographic and climatic regions. Generally speaking, cottonmouths or water moccasins prefer moist habitats, including wetlands and rivers. Rattlesnakes are often found in dry grasslands and rocky hillsides. Copperheads inhabit both aquatic and dry environments. The US Fish and Wildlife Service can provide regional information about specific species of poisonous snakes. Image courtesy of New York State Department of Environmental Conservation.
Vipers have a characteristic pit between the eye and nostril on each side of the head that houses a sensitive infrared detecting organ. Large rattlesnakes can have fangs that grow up to 20 mm in length. With these hollow fangs, vipers can rapidly deliver a large dose of venom in a single bite injection. Different species of vipers have different venom profiles, but there is some combination of neurotoxic, hemorrhagic, and proteolytic effects. Vipers usually cause significant localized wounds with local tissue destruction. Image courtesy of Wikimedia Commons.
Coral snakes are members of the Elapidae family, a group that also includes cobras. The bright yellow, red, and black coloring pictured here is characteristic of coral snakes. Approximately 40-50 different species of venomous coral snakes can be found in North and South America, with the greatest variety in Mexico and South America. Coral snakes are considered to be timid, with bites accounting for fewer than 1% of venomous snakebites in the Untied States. Most bites occur during intentional handling. A western coral snake can be seen here.
In the United States, the harmless milk snake has coloration that mimics the deadly coral snake. The Mexican milk snake (top) has red stripes surrounded by black stripes, whereas the Texas coral snake (bottom) has red stripes surrounded by yellow stripes. The mnemonic "red on yellow, kill a fellow; red on black, venom lack" only differentiates between wild snakes in the United States. This rule of thumb is unreliable in locations south of the United States, where venomous snakes can have both coloration patterns. Photo by Charles Alfaro.
Coral snakes have small, fixed, hollow fangs. Unlike pit vipers, coral snakes must hang onto their prey and inject venom more slowly by a chewing motion to achieve envenomation. As a result, many human coral snake bites are "dry," without delivery of venom. The venom is neurotoxic, leading to neuromuscular dysfunction, and has little proteolytic or enzymatic activity. Patients may be found with a small wound and later respiratory failure may develop as a result of neuromuscular blockade. It is considered to be among the most powerful venoms among North American snakes.
Cobras are the quintessential venomous snake to many people. They can be identified by their characteristic hood that unfolds when they feel threatened. There are no native species in North America, but the large public interest has led to many snakes being held in captivity. They are responsible for the majority of non-native snakebites in the United States. An Indian Cobra is shown. Photo by Robert Norris, MD.
Cobra bites frequently do not result in envenomation and are so-called dry bites. Some species of cobra have a unique defense mechanism where they can spit jets of venom at an enemy, usually at the eyes. Cobra venom is among the most toxic of snakes venoms and has been studied extensively. The venom can be some combination of neurotoxic, cardiotoxic, complement activating, and enzymatic at the postsynaptic level. Image Courtesy of Wikimedia Commons.
Snake venom is mostly water with variable types of enzymatic proteins. Hyaluronidase disrupts mucopolysaccharides to allow subcutaneous spread of venom. Phospholipase A2 causes hemolysis through esterolysis of red cell membranes and promotes muscle necrosis. Thrombogenic enzymes produce a consumptive coagulopathy, leading to hemorrhage. The amount of venom injected per bite is dependent on the elapsed time since the last bite, the degree of threat perceived by the snake, and the size of the prey. Image courtesy of Wikimedia Commons.
The enzymatic concentration of venom and the volume of venom injected vary among species, resulting in the wide variety of symptoms from bites of different species. The most common symptoms among all snakebites are anxiety and fear, which if severe enough may lead to nausea, vomiting, diarrhea, tachycardia, and fainting. Specifically, copperhead bites are generally limited to localized tissue destruction. Coral snakes have limited local effects, but cause significant neurologic compromise. Rattlesnakes can cause both localized symptoms and systemic toxicity. Cobra are known for neurotoxicity, and their spit may cause acute ocular inflammation. Image courtesy of Wikimedia Commons.
Local tissue destruction commonly accompanies bites of many pit viper species. Typical findings include edema, bullae, streaking, erythema, and ecchymosis. Rapid swelling is usually a sign of significant envenomation. The patient shown was bitten on the fingers by a copperhead 3 days prior to being photographed. Note the large bullae and erythema.
Cobra bites can cause severe localized damage complicated by extensive tissue necrosis, often occurring within 48 hours. The pattern of injury usually begins with skin darkening around the bite site, followed by blistering. Later, necrosis of the skin and subcutaneous tissues may occur. Photo by Sherman Minton, MD.
Treatment of snake envenomation is species specific, but antivenom is the only proven therapy for significant snakebites. For North American pit viper bites, ovine Crotalidae polyvalent immune Fab, or CroFab®, is available to help neutralize toxins. For coral snakes, the Micrurus fulvius antivenom is no longer manufactured and all remaining stocks expire in fall of 2009. Efforts are being made to get approval to import Mexican coral snake antivenom. For cobra bites, a number of different antivenoms are available, as shown. However, none of them are approved by the US Food and Drug Administration, and are all considered experimental drugs. Local or even regional medical centers may not stock appropriate antivenom, particularly for exotic snakes. Consultation with regional poison control centers, which have access to the Antivenin Index, may help identify and locate an appropriate product for use. For maximum efficacy, antivenom should be administered as soon as possible, ideally within 4-6 hours. Unfortunately, antivenom does little to retard the neurologic effects of some snake venoms. Image courtesy of the US Food and Drug Administration.
Pressure immobilization may be performed in the prehospital setting for bites from snakes known to not cause local necrosis. Pressure immobilization preserves arterial blood flow, but slows venous return and lymphatic flow and can delay the systemic absorption of elapid venoms. A broad pressure bandage is tightly wrapped from distal to proximal over as much of the extremity as possible, as shown. A splint is placed to immobilize the limb while the victim is transported for definitive care. Once the compress is removed, systemic spread of venom may occur. Used with permission from Commonwealth Serum Laboratories.
Supportive care for snakebites may include aggressive fluid resuscitation, the administration of cholinesterase inhibitors and analgesics, blood product transfusions, and airway support. Patients who receive antivenom are at risk for allergic reactions because many preparations are made from ovine or equine sources. Patients may require antihistamines, steroids, and blood pressure support. Surgical interventions include fasciotomy which is reserved for patients with compartment syndrome who do not improve with antivenin. Image courtesy of Wikimedia Commons.
A large number of purported snakebite treatments have no proven benefit and may worsen tissue necrosis. Mouth suction, incision across fang marks, negative pressure venom extraction devices (as shown), tourniquets, and ice water immersion are contraindicated. Efforts to identify the snake should be made, but efforts to capture or kill the snake should be avoided. Photographs at a safe distance of at least 6 feet are advised. Photo by Sean Bush, MD.
For more information, see the following resources:
- Australian Venom Research Unit
- US Fish and Wildlife Service
- American Association of Poison Control Centers
- eMedicine, Emergency Medicine: Snake Envenomation
- eMedicine, Emergency Medicine: Snake Envenomation, Coral
- eMedicine, Emergency Medicine: Snake Envenomation, Cobra
- eMedicine, Emergency Medicine: Snake Envenomation, Brown
- eMedicine, Emergency Medicine: Snake Envenomation, Moccasins
- eMedicine, Emergency Medicine: Snake Envenomation, Mohave Rattle
- eMedicine, Emergency Medicine: Snake Envenomation, Rattle