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.
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.