The pit vipers (Crotalidae family) indigenous to the United States include rattlesnake species, cottonmouths, and copperheads. Physical characteristics of pit vipers include a triangular head, heat-sensing pits, elliptical pupils, and a single row of subcaudal ventral scales. Pit viper venom is complex and produces hematologic, cardiovascular, and neuromuscular effects. Clinically, pit viper envenomations are divided into four categories. Bites without envenomation are characterized only by the direct tissue damage caused by the strike. Minimal envenomations consist of fang marks and local swelling only, with no systemic symptoms. Moderate envenomations include progression of tissue changes beyond the immediate location of the bite and/or systemic symptoms with mild changes in coagulation parameters. Severe envenomations include marked local and progressive swelling and significant systemic symptoms with coagulopathy. Coagulopathy is manifested by subcutaneous ecchymosis and/or signs of bleeding).
Eastern Diamondback Rattlesnake. The eastern diamondback is the largest US rattlesnake and has a characteristic diamond-shaped pattern on its dorsal aspect. Note the triangular head, which is characteristic of pit vipers. (Photo contributor: R. Jason Thurman, MD.)
Western Diamondback Rattlesnake. The western diamondback causes the most fatalities from snakebites in Mexico and the second most in the United States behind the eastern diamondback. (Photo contributor: Mike Cardwell, MS.)
Mojave Green Rattlesnake. The Mojave green is found in the southwestern United States and Mexico. There are two subspecies, Types A and B, with Type A thought to have the most toxic venom of all North American snakes. Unlike most rattlesnake venoms, Mojave Type A venom contains a potent neurotoxin. (Photo contributor: Mike Cardwell, MS.)
Red Diamond Rattlesnake. The elliptical pupils and heat-sensing pits in this red diamond rattlesnake are characteristic of pit vipers. (Photo contributor: Sean P. Bush, MD.)
Cottonmouth. The cottonmouth is a semiaquatic venomous pit viper that may crawl or swim with its head raised at an angle of 45 degrees. When disturbed, it may open its mouth wide to reveal a white lining. (Photo contributor: Stephen J. Knoop.)
Copperhead Snake. The copperhead is frequently encountered in wooded mountains, abandoned buildings, and damp, grassy areas. It is able to climb low bushes and trees in search of food. (Photo contributor: R. Jason Thurman, MD.)
Juvenile Copperhead Caudal Lure. Juvenile pit vipers may have bright yellow coloring on their distal tails, known as a caudal lure. The snakes are known ...