Snake bite: Pit vipers, part 1 (Proceedings)
Pit vipers are the largest group of venomous snakes in the United States and are involved in an estimated 150,000 bites annually of dogs and cats.1 Approximately 99% of all venomous snake bites in the United States are inflicted by pit vipers. In North America members of the family Crotalidae belong to three genera: the rattlesnakes (Crotalus and Sistrurus sp) and the copperheads and cottonmouth water moccasins (Agkistrodon sp).
Pit vipers can be identified by their characteristic retractable front fangs, bilateral heat sensing "pits" between the nostrils and eyes, elliptical pupils, a single row of subcaudal scales distal to the anal plate, and triangular shaped heads. The rattlesnakes have special keratin rattles on the ends of their tails. The copperheads and water moccasins, are found throughout the eastern and central United States. Copperheads are responsible for the majority of venomous snake bites in North America. Water moccasins can be pugnacious and have a greater tendency to deliver venom when they bite. Rattlesnakes are found throughout the continental United States and account for the majority of deaths in animal victims.
Pit vipers control the amount of venom they inject during a bite. The amount of venom injected depends on the snake's perception of the situation. Initial defensive strikes are often nonenvenomating. Offensive bites meter a given amount of venom into the victim, and agonal bites deliver the entire venom load and are therefore the most dangerous. A severed snake head can bite reflexively for up to an hour after decapitation.The severity of any pit viper bite is related to the volume and toxicity of the venom injected as well as to the location of the bite, which may influence the rate of venom uptake. As a generalization, the toxicity of pit viper venoms ranges in descending order from the rattlesnakes to the water moccasins and then to the copperheads. The toxicity of rattlesnake venom varies widely. Nine species and twelve subspecies of rattlesnakes have populations with venoms containing proteins that are immunologically similar to the potent neurotoxin mojave toxin (Table 1). It is possible for pit viper venom to be strictly neurotoxic with virtually no local signs of envenomation.
Table 1. Species of Rattlesnakes That Have Populations Containing Neurotoxin
Pit viper venoms are a complex combination of enzymatic and nonenzymatic proteins. The primary purpose of the venom is not to kill but rather to immobilize the prey and predigest its tissues. The venom is 90% water and has a minimum of 10 enzymes and 3 to 12 nonenzymatic proteins and peptides in any individual snake. The nonenzymatic components, called the "killing fraction," have a median lethal dose (LD50) over 50 times smaller than that of the crude venom.
Differences in venom within a species induced by the age of the snake are highlighted by a study of northern Pacific rattlesnakes in which the adult venoms were shown to have approximately fivefold higher fibrinogenolytic protease activity2 . The complexity of the issue of variation of venom components is highlighted by the differences found in fibrinolysis and complement inactivation of venoms from different Blacktail rattlesnakes. In a study of 72 individual Blacktail rattlesnake venoms, the following conclusion was made: there were no venom differences as a function of geographic distribution; however, individual venom variability was significant enough to be identified as an important clinical reality.3
The enzyme phospholipase A is distributed throughout pit viper venoms. This enzyme catalyzes the hydrolysis of fatty ester linkages in diacyl phosphatides, which form lysophosphatides and release unsaturated and saturated fatty acids. There are many antigenically different isoenzymes. Some controversy exists about the extent of any neurotoxic effects that these isoenzymes may possess. Many cellular substances may be released by this enzyme, including histamine, kinins, slow-reacting substance, serotonin, and acetylcholine. The extent of the release of these physiologically active compounds most likely depends on the ability of phospholipase A to degrade membranes. The enzyme phospholipase B may also be present and is responsible for hydrolyzing lysophosphatides. Direct cardiotoxic effects of venom proteins have been exhibited in some pit viper venoms, particularly the diamondback rattlesnakes.