Definitions and Characteristics

Two main families of venomous snakes indigenous to the United States are Crotalidae (pit vipers) and Elapidae (coral snakes). Most snakebites are caused by the pit vipers, so called because of a depression, or pit, in the maxillary bone. Rattlesnakes (see Plate 7), the cottonmouth (water moccasin) snake (see Plate 8), and the copperhead snake (see Plate 9) are members of the pit viper family. The major snakes of medical importance outside of North America are the cobras, mambas, kraits, coral snakes, Australian elapids, sea snakes, vipers, rattlesnakes, asps, and colubrids (rear-fanged snakes). The fastest pit viper can crawl at a maximum speed of approximately 4.8 km/hr (3 mph). The speed of a pit viper’s strike has been clocked at 2.4 m/sec (8 ft/sec); the snake can reach distances of approximately one-half its body length.

Identifying characteristics of pit vipers include the following (Fig. 37-1):

Two members of the coral snake family, Elapidae, are found in the United States: the western coral snake (Micruroides euryxanthus) (see Plate 10), found in Arizona and New Mexico, and the eastern coral snake (Micrurus fulvius) (see Plate 11), distributed from coastal North Carolina through the Gulf states to western Texas. The elapids differ from pit vipers in having very short fangs, round pupils, and subcaudal scales in a double row. Because many nonpoisonous mimics occur in coral snake territory, the rule of thumb for identifying a venomous species is that red bands bordered by yellow or white indicate a venomous reptile, whereas red bands bordered by black indicate a nonvenomous reptile. This rule applies to all coral snakes native to the United States but does not apply to species found south of Mexico City and in other non-U.S. countries. A few essentially harmless, rear-fanged colubrid snakes in the United States, such as the night snake and lyre snake, possess elliptic pupils but lack facial pits.

Pit Viper Envenomation

The clinical presentation of pit viper envenomation is quite variable and depends on the circumstances of the bite. Important factors include the species, size, and health of the snake; age and health of the victim; circumstances that led to the bite; number of bites and their anatomic locations; and quality of the care rendered to the victim, both in the field and hospital. Most bites occur to the lower extremities followed by an upper extremity and often coincide with the victim’s intentional interaction with the snake (e.g., tormenting or attempting to capture the animal, or handling a captive specimen). Most bites occur around dawn or dusk and during warmer months, when snakes and people are more active outdoors. About 75% to 80% of pit viper bites result in envenomation. Approximately one in every four to five bites is “dry,” meaning no venom has been injected. Approximately 35% of snakebites result in mild envenomation, 25% moderate, and 10% to 15% severe. Observe the patient closely for signs and symptoms.

Coral Snake Envenomation

In the United States, bites by the eastern coral snakes (M. fulvius) tend to be more severe than those by Texas coral snakes (M. fulvius tenere), and both are significantly more dangerous than those of Sonoran coral snakes (M. euryxanthus). Because they have a less efficient venom delivery system, coral snakes effectively envenom only 40% of their victims. Following envenomation, the earliest symptoms may be nausea and vomiting, followed by headache, abdominal pain, diaphoresis, and pallor. Severe envenomation results in predominantly neurotoxic sequelae.

Envenomation by Non–North American Snakes

Venomous Lizard Bites

The Gila monster (see Plate 12) and Mexican beaded lizard (see Plate 13) are found in North America. Both possess venom glands and grooved teeth. Human envenomation most often occurs when the lizard retains its grasp and chews on the patient.