Chapter 206

An estimated 3 million bites and 150,000 deaths occur each year from venomous snakes in the world.1 The American Association of Poison Control Centers reports an average of 6000 bites each year, approximately 2000 of them by venomous snakes. Because of underreporting, the true number of snakebites is possibly as high as 45,000 per year in the U.S., with 7000 to 8000 by venomous snakes.2 The major venomous snakes of the world can be divided into three groups: Viperidae (vipers), Elapidae, and Hydrophiinae (sea snakes, see Chapter 207, Trauma and Envenomations from Marine Fauna).

Approximately 20 of the 120 snake species indigenous to North America are venomous. Most bites occur in the warm summer months, when snakes and victims are most active. In the past, it was estimated that mortality from venomous snakebite approached 25%. Because of the availability of antivenom and advances in emergency and critical care, mortality rates today are <0.5%; approximately 5 to 10 deaths occur per year.3

Except for bites by imported species, North American venomous snakebites involve the pit vipers (Crotalinae subfamily of Viperidae) or coral snakes (Elapidae family). The crotaline snakes are represented by the rattlesnakes (Crotalus species), pygmy rattlesnakes, and massasauga (Sistrurus species), as well as the copperheads and water moccasins (Agkistrodon species). Poisonous snakebites from imported exotic species are infrequent but may occur in zoo personnel as well as in amateur herpetologists. A regional poison control center can provide information on snake identification, expected toxicity, and location of antivenom.

The crotaline snakes are called pit vipers because of bilateral depressions or pits located midway between and below the level of the eye and the nostril (Figure 206-1). The pit is a heat receptor that guides strikes at warm-blooded prey or predators. Crotaline snakes are also distinguished by the presence of two fangs that fold against the roof of the mouth, in contrast to the coral snakes, which have shorter, fixed, erect fangs. Within the pit viper group, the rattle distinguishes the rattlesnake from other crotaline snakes. The mistaken belief that rattlesnakes always rattle before striking has persisted for centuries. In truth, many strikes occur without a warning rattle.

Pit viper.

### Pathophysiology

Crotaline venom is a complex enzyme mixture that causes local tissue injury, systemic vascular damage, hemolysis, fibrinolysis, and neuromuscular dysfunction, resulting in a combination of local and systemic effects. Crotaline venom quickly alters blood vessel permeability; this leads to loss of plasma and blood into the surrounding tissue, which causes hypovolemia. Crotaline venom activates and consumes fibrinogen and platelets, causing a coagulopathy. In some species, specific venom fractions block neuromuscular transmission, which leads to cranial nerve weakness (e.g., ptosis), respiratory failure, and altered sensorium.

### Clinical Features

Up to 25% of crotaline snakebites are dry bites: venom effects ...

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