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Recognizing Dangerous Snakes in the United States and Canada: A Novel 3-Step Identification Method

Published:September 30, 2011DOI:https://doi.org/10.1016/j.wem.2011.07.001
      The rapid and accurate recognition of dangerously venomous snakes following bites is crucial to making appropriate decisions regarding first aid, evacuation, and treatment. Past recommendations for identification of dangerous North American pit vipers have often required subjective determinations of head shape or relied on traits shared with some nondangerous species (elliptical pupils and undivided subcaudal scales). Heat-sensitive facial pits are diagnostic but require close examination of the dangerous head, and cephalic traits are useless when working with a decapitated carcass. Exclusive of cephalic traits, pit vipers north of Mexico can be recognized by the combination of keeled middorsal scales and undivided subcaudal scales. The order of colored rings is usually suggested to identify coral snakes in the United States, yet extension of the colored rings across the ventral scales must be added as an essential identifying factor to ensure elimination of all harmless look-alikes. A novel 3-step flow chart is presented that allows dangerous snakes in the United States and Canada to be recognized quickly and dependably without relying on cephalic traits. This process cannot be used in other countries, however, due to greater variability of these characteristics in snakes from other parts of the world. Finally, close examination of potentially venomous snakes is extraordinarily dangerous and steps to safeguard those making such observations are discussed.

      Key words

      Introduction

      The ability of emergency medical personnel, as well as outdoor enthusiasts themselves, to quickly and reliably recognize snakes capable of producing medically significant injuries is essential to selecting the best course of action following a bite. Does the injury produced by a particular snake require that planned activities be abandoned and emergency evacuation be initiated, or does it simply require routine superficial wound care? When the dead animal (or a piece of it) is presented to paramedics or emergency department staff, are they prepared to safely and reliably assess the danger represented? Such assessments not only facilitate rapid intervention for envenomated patients but, when a nondangerous species is involved, can prevent unnecessary emergency evacuation and antivenom therapy,
      • Norris R.L.
      • Bush S.P.
      Bites by venomous reptiles in the Americas.
      both of which can be hazardous and expensive.
      Even when a carcass has been decapitated, keeled middorsal scales and undivided subcaudal (under the tail) scales—or adjacent red and yellow rings that extend across the ventral scales—will provide rapid dependable recognition of North American pit vipers and coral snakes, respectively.

      The Need for an Improved Method

      Pit vipers are widely distributed in the United States and Canada. Rattlesnakes (genera Crotalus and Sistrurus of the family Viperidae, subfamily Crotalinae) populate much of the southern margin of Canada as well as almost all of the contiguous United States. Copperheads and cottonmouths (genus Agkistrodon) are found throughout much of the eastern and south-central United States. The family Elapidae is represented by 2 coral snake genera (Micrurus and Micruroides) in the southern United States. Maine, Alaska, and most of Canada (north of about 52° north latitude) are devoid of dangerous native snakes.
      • Campbell J.A.
      • Lamar W.W.
      The Venomous Reptiles of the Western Hemisphere.
      Triangular heads, elliptical pupils, and undivided subcaudal scales have historically been suggested as traits for use by lay persons to recognize pit vipers in the United States.
      • Norris R.L.
      • Bush S.P.
      Bites by venomous reptiles in the Americas.
      • Russell F.E.
      • Carlson R.W.
      • Wainschel J.
      • Osborne A.H.
      Snake venom poisoning in the United States Experiences with 550 cases.
      • Parrish H.M.
      Incidence of treated snakebites in the United States.
      • Norris R.L.
      Snake venom poisoning in the United States: assessment and management.
      • Spencer C.
      • Counselman F.L.
      Reptile envenomations: patient evaluation and treatment.
      • Auerbach P.S.
      Medicine for the Outdoors.
      However, each of these traits is either subjective, shared with certain harmless species, or both. The definitive characteristic of pit vipers is the heat-sensitive facial pit; however, detection of these small structures requires close examination of the face, which is dangerous, even after the head has been severed (see Discussion). In any case, cephalic characteristics are not helpful when working with a decapitated carcass—a common method for killing snakes.
      The order of colored rings (“red on yellow, kill a fellow; red on black, venom lack”) is commonly suggested to identify coral snakes in the United States,
      • Norris R.L.
      • Bush S.P.
      Bites by venomous reptiles in the Americas.
      • Parrish H.M.
      Incidence of treated snakebites in the United States.
      • Norris R.L.
      Snake venom poisoning in the United States: assessment and management.
      • Spencer C.
      • Counselman F.L.
      Reptile envenomations: patient evaluation and treatment.
      • Auerbach P.S.
      Medicine for the Outdoors.
      yet that trait alone does not eliminate certain harmless look-alikes (eg, Chionactis palarostris of the family Colubridae).
      • Stebbins R.C.
      A Field Guide to Western Reptiles and Amphibians.

      Concept Development

      The middorsal scales of native viperids are invariably keeled
      • Campbell J.A.
      • Lamar W.W.
      The Venomous Reptiles of the Western Hemisphere.
      (Figure 1A), a trait shared with many nondangerous taxa.
      • Stebbins R.C.
      A Field Guide to Western Reptiles and Amphibians.
      • Conant R.
      • Collins J.T.
      A Field Guide to Reptiles and Amphibians – Eastern and Central North America.
      All native viperids also bear a single row of subcaudal scales, contrasting with the double row of subcaudals (Figure 2) found in all but 3 species of harmless native snakes.
      • Campbell J.A.
      • Lamar W.W.
      The Venomous Reptiles of the Western Hemisphere.
      • Stebbins R.C.
      A Field Guide to Western Reptiles and Amphibians.
      • Klauber L.M.
      The long-nosed snakes of the genus Rhinocheilus.
      Significantly, the 3 harmless species bearing undivided subcaudal scales (Rhinocheilus lecontei, family Colubridae, and the dwarf boas Charina bottae and Charina trivirgata, family Boidae) lack keeled dorsal scales, having smooth dorsal scales (Figure 1B) instead.
      • Stebbins R.C.
      A Field Guide to Western Reptiles and Amphibians.
      • Klauber L.M.
      The long-nosed snakes of the genus Rhinocheilus.
      Therefore, the combination of keeled middorsal scales and undivided subcaudal scales is diagnostic for native pit vipers north of Mexico.
      Figure thumbnail gr1
      Figure 1Keeled dorsal scales (a). Smooth dorsal scales (b). Examine the scales near the dorsal midline and ignore scale shape and color; look only for the tiny longitudinal ridge that forms the keel.
      Figure thumbnail gr2
      Figure 2Undivided (above) compared to divided (below) subcaudal (under the tail) scales. The relatively large anal plate (a) separates the ventral (belly) scales at left from the subcaudal scales at right. The determination of divided or undivided must be based on the majority of the subcaudal scales on the proximal two-thirds of the tail, as a few aberrant scales are common proximally, and subcaudal scales near the distal tip are usually divided in all taxa.
      Although the aforementioned rule is also valid for rattlesnakes, they can be quickly and reliably recognized by the presence of a rattle. Neonates bear a single “birth button.” In one species of the genus Sistrurus, the rattle is noticeably reduced in size.
      • Campbell J.A.
      • Lamar W.W.
      The Venomous Reptiles of the Western Hemisphere.
      • Conant R.
      • Collins J.T.
      A Field Guide to Reptiles and Amphibians – Eastern and Central North America.
      • Klauber L.M.
      Rattlesnakes, Their Habits, Life Histories, and Influence on Mankind.
      As rattlesnakes age, the older distal rattle segments routinely become brittle and eventually break away. The proximal segment, however, contains living tissue and is invariably retained. In the exceedingly rare event of a genetic deformity or traumatic amputation of the distal tail and rattle, the characteristics described herein will properly identify a rattlesnake as a pit viper. No rattlesnake bears a gracefully tapered and pointed tail.
      • Campbell J.A.
      • Lamar W.W.
      The Venomous Reptiles of the Western Hemisphere.
      • Klauber L.M.
      Rattlesnakes, Their Habits, Life Histories, and Influence on Mankind.
      It is worth noting that even the so-called “rattleless” rattlesnake (Crotalus catalinensis) of Isla Santa Catalina in Mexico's Sea of Cortez is not truly rattleless; it produces rattle segments just like other rattlesnakes; it is unique only in that the older segments are lost each time a new one is added.
      • Campbell J.A.
      • Lamar W.W.
      The Venomous Reptiles of the Western Hemisphere.
      • Klauber L.M.
      Rattlesnakes, Their Habits, Life Histories, and Influence on Mankind.
      Native coral snakes are brightly colored serpents, ringed in red, black, and yellow (or white, especially in the Arizona form).
      • Campbell J.A.
      • Lamar W.W.
      The Venomous Reptiles of the Western Hemisphere.
      • Stebbins R.C.
      A Field Guide to Western Reptiles and Amphibians.
      • Conant R.
      • Collins J.T.
      A Field Guide to Reptiles and Amphibians – Eastern and Central North America.
      North American coral snakes can be recognized by the combination of every other ring being yellow (or white) with rings completely encircling the body, including across the ventral scales (Figure 3). Most harmless native tricolored snakes are distinguished by every other ring being black (Figure 4), separating the yellow (or white) and red rings.
      • Stebbins R.C.
      A Field Guide to Western Reptiles and Amphibians.
      • Conant R.
      • Collins J.T.
      A Field Guide to Reptiles and Amphibians – Eastern and Central North America.
      In harmless species with dorsal colors that resemble coral snakes, the belly is colored differently than the dorsum (Figure 5, Figure 6) due to the failure of the red coloration to cross the ventral scales.
      • Stebbins R.C.
      A Field Guide to Western Reptiles and Amphibians.
      Figure thumbnail gr3
      Figure 3The tricolored rings extend all the way around the coral snakes, including across the belly, as in this Texas Coral Snake (Micrurus tener). Compare to Figure 5, Figure 6.
      Figure thumbnail gr4
      Figure 4Several harmless tricolored snakes are native to the United States. Every other ring is black in most of the coral snake mimics, such as this kingsnake (Lampropeltis triangulum).
      Figure thumbnail gr5
      Figure 5Although this harmless Shovel-nosed Snake (Chionactis palarostris) resembles a coral snake dorsally, the red bands do not extend across the belly. Compare to .
      Figure thumbnail gr6
      Figure 6The harmless Long-nosed Snake (Rhinocheilus lecontei) is the species most likely to cause confusion at question one of the flow chart, as some may consider it to be tri-colored. Regardless of which choice is made at question one, the result will be correct: it is eliminated as a coral snake by its predominantly white belly and as a pit viper by its smooth middorsal scales.
      These morphological characteristics are combined in a flow chart (Figure 7) that allows dangerously venomous snakes to be easily recognized based on 3 objective determinations of noncephalic traits for either coral snakes or pit vipers.
      Figure thumbnail gr7
      Figure 7Flow chart depicting the 3-step process using objective noncephalic characteristics to determine if a native snake in the United States or Canada is dangerously venomous.

      Discussion

      The only species likely to be confused in the flow chart is the Long-nosed Snake (Rhinocheilus lecontei, Figure 6) at question one, but it will key out correctly regardless of the choice at the first step. The flow chart does not identify snakes to species. Specific identification is both significantly more complex and unnecessary for the initial decisions in response to a snakebite. For the past 60 years, FDA-approved antivenoms have consisted of a specific product for Eastern and Texas Coral Snakes (Micrurus) and 2 polyvalent antivenoms approved for use against all native pit vipers (genera Crotalus, Sistrurus, and Agkistrodon). Therefore, a determination of coral snake or pit viper is sufficient even for selection of the correct antivenom. It is worth noting that no antivenom is produced for the generally innocuous Arizona Coral Snake (Micruroides).

      Other “Venomous” Snakes

      Some North American snakes belonging to families other than Viperidae and Elapidae are well known for producing toxic oral secretions and for having elongated, grooved teeth in the rear of the mouth for chewing these toxins into their small prey. Bites by these so-called “rear-fanged” snakes, such as lyre and night snakes (Trimorphodon and Hypsiglena, respectively), may produce relatively minor effects (most commonly pain, swelling, and bruising) in humans but are non-life-threatening.
      • Norris R.L.
      • Bush S.P.
      Bites by venomous reptiles in the Americas.
      Similarly, there are occasional reports of mild toxicity produced by the bites of other “non-venomous” snakes.
      • Laskowski-Jones L.
      A case of envenomation from a non-venomous snake?.
      • Weinstein S.A.
      • Keyler D.E.
      Local envenoming by the Western hognosed snake (Heterodon nasicus): a case report and review of medically significant Heterodon bites.
      Recent findings by molecular biologists suggest that many extant snakes and lizards may have evolved from venomous ancestors and retain genes capable of expressing various toxic enzymes and other peptides,
      • Fry B.G.
      • Vidal N.
      • Norman J.A.
      • et al.
      Early evolution of the venom system in lizards and snakes.
      although the vast majority remain harmless to creatures larger than frogs and lizards. It has also been suggested that rattlesnake venoms are undergoing rapid evolution in response to interactions with humans, but that assertion is widely discounted by evolutionary biologists.
      • Hayes W.K.
      • Mackessy S.P.
      Sensational journalism and tales of snakebite: Are rattlesnakes rapidly evolving more toxic venom?.
      One additional dangerously venomous snake is technically native to the United States yet otherwise omitted here because it has been reported on only a handful of occasions and has produced no recorded bites in the United States. The Yellow-bellied Sea Snake (Pelamus platurus of the family Elapidae) is closely related to coral snakes and cobras, is a common animal in coastal waters along the Pacific side of Central America, and has been sighted occasionally in the ocean off of southern California.
      • Stebbins R.C.
      A Field Guide to Western Reptiles and Amphibians.
      It is entirely pelagic but may wash up on beaches or get tangled in offshore fishing equipment. These snakes are easily identified by their bold yellow and black markings and flattened oar-like tails.

      Important Safety Warnings

      This process requires close examination of potentially deadly animals. Headless carcasses are safe to handle. However, freshly killed snakes and even severed heads may bite reflexively and inject venom when handled.
      • Klauber L.M.
      Rattlesnakes, Their Habits, Life Histories, and Influence on Mankind.
      Therefore, severed heads and carcasses with heads still attached should never be manipulated with the hands, even while wearing gloves.
      In the event that a live snake is presented for identification, the danger of another person being bitten is high. Snakes presented in cardboard boxes, buckets, and other improvised containers are particularly dangerous and should generally be kept out of ambulances and emergency rooms, as snakes are well-known escape artists. Attempts by inexperienced personnel to open such containers, transfer live snakes from one container to another, or examine them under such circumstances, are likely to result in additional people being bitten. Cloth bags are favorite containers of herpetologists but are extremely dangerous when used by nonexperts. These precautions cannot be overemphasized.
      Following a bite, it may be tempting to kill and decapitate a snake to facilitate its close examination; however, intentional interaction with snakes produces a high percentage of the annual snakebites in the United States
      • Norris R.L.
      • Bush S.P.
      Bites by venomous reptiles in the Americas.
      • Spencer C.
      • Counselman F.L.
      Reptile envenomations: patient evaluation and treatment.
      and the risk must be carefully weighed against the potential benefit. Aroused snakes, especially pit vipers, can be very quick and unpredictable and close approach (within the length of the snake), even momentarily, clearly risks another bite.
      The proliferation of camera phones and the ability to easily transmit digital photographs makes it possible to rapidly identify snakes by transmitting a digital photo to an expert. In most cases, detailed close-ups are not necessary; a well-focused full-body image will usually suffice. Nonetheless, photography of a live snake is dangerous and must be accomplished while remaining farther away from it than the length of the snake. And the hazard of opening a container housing a live snake, even for a photograph, likely outweighs the immediate need to identify it.

      Geographic Limitation

      It is crucial that this key not be used outside of the United States and Canada. Latin America is home to pit vipers with divided subcaudal scales (eg, Bothrops asper and Bothrops atrox), as well as greater than 50 additional species of coral snakes, most of which violate the identification rules described herein.
      • Campbell J.A.
      • Lamar W.W.
      The Venomous Reptiles of the Western Hemisphere.
      Afro-Eurasia and Australian serpents are even more diverse. This identification method should not be relied upon following bites by captive snakes, as some hobbyists keep deadly exotic species and may not know or admit the animal's true identity.
      Finally, escaped exotic snakes must be considered, although reports of innocent persons (ie, other than the owners/keepers) being bitten by escaped nonnative venomous snakes are almost nonexistent. Nonetheless, signs and symptoms of potential envenomation should never be ignored, regardless of the appearance or tentative identification of the biting snake.

      Acknowledgments

      I thank Jim McGuire of UC Berkeley's Museum of Vertebrate Zoology, as well as 3 anonymous reviewers, for valuable comments on the manuscript.

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