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"Venomous” Bites from Non-Venomous Snakes: A Critical Analysis of Risk and Management of "Colubrid” Snake Bites PDF

348 Pages·2011·26.5 MB·English
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1 An Overview of the Artificial Assemblage, the Colubridae: A Brief Summary of Taxonomic Considerations The taxonomy of this former assemblage is in dynamic transition and subject to frequent and often conflicting recommended rearrangement. This very large artifi- cial grouping often functioned as a sort of “disposal depot” (colloquially known as a “taxonomic garbage can” or “rag bag”) for taxa of unestablished affinities. This resulted in the incorrect assignment of many diverse and phylogenetically unrelated ophidian species. For over 30 years, numerous taxonomists have devoted increasing attention to resolving this complex issue by using methods involving morphologi- cal (based on osteology, dentition, hemipenal morphology, lepidosis/meristics, etc.) or molecular (analysis of nuclear or mitochondrial DNA sequences—sometimes inferred from ribosomal RNA sequences—allozyme electrophoresis, immunodif- fusion, etc.) methods or, less commonly, combined morphological and molecular systematics (Cadle, 1988; Dessauer et al., 1987; Dowling et al., 1983, 1996; Heise et al., 1995; Jenner and Dowling, 1985; Kraus and Brown, 1998; Lawson et al., 2005; Pinou et al., 2004; Vidal et al., 2000, 2007, 2010; Zaher, 1999; Zaher et al., 2009). Discussion of the current status of these ongoing reassignments is far too volumi- nous to detail here. However, some recommended changes are summarized in Table 1.1. Analyses of previous taxonomic assignments have increasingly been subject to more of a “splitting” (e.g., division of a given species, genus, subfamily, or family into separate entities) approach, rather than “lumping” these together. For instance, the crayfish-eating snakes (Regina spp.) of the tribe Thamnophiini are polyphyletic, and the Thamnophiini itself has at least three major clades (Alfaro and Arnold, 2001). Ongoing phylogenetic investigations will probably modify numerous subfamilies, genera, and re-define existing lineages (Hedges et al., 2009; Pyron et al., 2011). It should be noted that several medically important species have been recently reassigned. For example, Vidal et al. (2007) and Zaher et al. (2009) recommended raising the previous subfamily, Natricinae, to a full family, Natricidae (see Table 1.1). This family contains Rhabdophis tigrinus and R. subminiatus, two species that have inflicted fatal or life-threatening bites (Section 4.2). Such reassignments reinforce the call for the use of precise and current taxonomy in clinical toxinology (Wüster et al., 1998). Some species can be reassigned; this can lead to controversy and confusion in the literature. For example, historically, the front-fanged mole vipers, burrowing asps, or stiletto snakes (Atractaspis spp., approxi- mately 18 species; Plate 1.1A), were first considered viperids or elapids, then reassigned to the colubrid subfamily, Aparallactinae (the centipede-eating snakes), then placed in “Venomous” Bites from Non-Venomous Snakes. DOI: 10.1016/B978-0-12-387732-1.00001-4 © 2011 Elsevier Inc. All rights reserved. 2 “Venomous” Bites from Non-Venomous Snakes Table 1.1 Summary of Proposed Taxonomic Reassignments for the Superfamily Colubroidea and Other Medically Relevant Taxaa,b Superfamily Colubroidea Colubridae Colubrinae (including Boiga, Dispholidus, Hierophis, Platyceps, Thelotornis)* Grayiinae Calamarinae Dipsadidae Dipsadinae (including Leptodeira, Sibynomorphus)* Heterodontinae (Heterodon)* Xenodontinae (including Alsophis, Boiruna, Clelia, Hydrodynastes, Phalotris, Philodryas, Tachymenis)* Natricidae (including Natrix, Rhabdophis, Thamnophis)* Pseudoxenodontidae Superfamily Elapoidea Elapidae Elapinae Hydrophiinae Lamprophiidae Atractaspidinae* Lamprophiinae Psammophiinae* (including Malpolon) Pseudoxyrhophiinae Superfamily Homalopsoidea Homalopsidae (including Cerberus, Homalopsis, Enhydris)* Superfamily Viperoidea Viperidae Azemiopinae Causinae Crotalinae Viperinae a Taxa discussed in the text are marked with an asterisk. b Further revision of the Colubroidea is underway and may modify the relationships shown here. For instance, Pyron et al. (2011) supported the earlier definition of the Colubroidea, and thus recognized the subfamily status of a number of clades. This includes the Natricinae, rather than the full family, Natricidae as assigned by Vidal et al. (2007), and Zaher et al. (2009). An Overview of the Artificial Assemblage, the Colubridae 3 (A) (B) (C) Plate 1.1 (A–C) Mole viper, burrowing asp, or stiletto snake (Atractaspis spp.). These unusual fossorial snakes have long been subject to taxonomic revision. They possess notably enlarged, canaliculated fangs that are freely rotatable on the maxilla. This makes manual handling impossible as gripping these snakes behind the head in the conventional manner allows a penetrating jab from the laterally highly mobile fang(s) (Plate 1.1A, A. fallax, Kenya; Plate 1.1B, West African mole viper; slender burrowing asp, A. aterrima, Nigeria; Plate 1.1C, fangs from Reinhardt’s burrowing asp; variable burrowing adder; itiuiu, A. irregularis, Niangara, Congo). Their venoms contain a wide array of components, including multiple isoforms of cytotoxins and novel vasoconstrictor peptides (e.g., sarafotoxins). Envenomations may be severe; life-threatening cases are well- documented. Current taxonomic reassignments have recommended placement of these snakes from their own family Atractaspididae into a subfamily, Atractaspidinae, of the Lamprophiidae, thereby including one other taxonomically problematic front-fanged genus, Homoroselaps. However, some investigators consider Homoroselaps spp. as members of the Elapidae. Many little-known colubroids remain of uncertain taxonomic affinity (see text). Photos copyright to David A. Warrell (Plate 1.1A and B) and Arie Lev (Plate 1.1C; AMNH specimen #12355). their own family, Atractaspididae (Underwood and Kochva, 1993). These distinctive snakes are now assigned by some investigators to the superfamily Elapoidea, as a sub- family (Atractaspidinae) of the Lamprophiidae (www.reptile-database.org/; see Table 1.1). The lamprophiids include a number of species that are commonly kept in captivity. Among the approximately 8-12 genera (depending on the author [s]) grouped within the atractaspidids are taxa with mid-maxillary enlarged, grooved, and noncanaliculate 4 “Venomous” Bites from Non-Venomous Snakes (B) (A) (C) Plate 1.2 (A–C) Maxilla and enlarged posterior maxillary teeth of the Natal black snake (Macrelaps microlepidotus). The natural history of this rare semifossorial species is poorly known. A non-front-fanged colubroid, it has traditionally been grouped with the unusual front- fanged genus, Atractaspis (see text). There are a number of anecdotal cases of bites by this species. Unfortunately, there is no documented clinical review of any of these victims. Effects have allegedly included loss of consciousness and possible cranial nerve involvement, but further information is required in order to critically evaluate the potential hazard associated with bites from this uncommon species. As illustrated in the comparison of two specimens in Plate 1.2A, the most posterior maxillary teeth are markedly enlarged and gently recurved. They contain a shallow groove that extends along almost the entire medial-posterior surface of the tooth (the position of the groove from an antero-lateral view is indicated by the arrow in Plate 1.2B). The groove is visible (arrows) in Plate 1.2C. The uppermost specimen in Plate 1.2A is AMNH #5897; the other specimen in Plate 1.2A and Plate 1.2B and C is AMNH #18227. See Appendix E for locality data; photos copyright to Scott A. Weinstein. dentition (e.g., the Natal black snake, or Natal swartslang, Macrelaps microlepidotus, Plate 1.2A–C), and front-fanged (“proteroglyphous”) canaliculated morphology (e.g., the dwarf garter snakes, Homoroselaps spp.), with Atractaspis spp. exhibiting markedly enlarged distensible canaliculated fangs (Plate 1.1B and C) and notably elongated venom glands. Deufel and Cundall (2003) noted the similarities between unilateral fang use in Atractaspis and unilateral “slashing envenomation by some rear-fanged snakes.” However, the loss of pterygoid teeth and associated maxillary movement resulted in the inability of Atractaspis spp. to perform “pterygoid walk” prey transport.1 These authors remarked 1 “Pterygoid walk” prey transport refers to the alternating pterygoidal movements employed during active deglutition of a seized prey item. This generally advances the maxillae, thereby drawing the grasped prey into the snake’s esophagus, and facilitates swallowing. An Overview of the Artificial Assemblage, the Colubridae 5 that “Atractaspis spp. appear to represent the evolutionary endpoint of a functional conflict between envenomation and transport in which a rear-fanged envenomating system has been optimized at the expense of most, if not all, palatomaxillary transport function” (Deufel and Cundall, 2003). Therefore, although all of the fossorial and/or nocturnal genera included in this group share many traits: slender body form with short tails, and lacking a loreal scale; possessing smooth, shiny scales; relatively small heads and eyes (Shine et al., 2006; see Plate 1.1A), and their monophyly is supported by morphological and molecular data (McDowell, 1986; Underwood and Kochva, 1993; Vidal et al., 2008; Zaher, 1999; Zaher et al., 2009), some genera possess markedly dif- ferent dentitional morphology that notably influences their potential medical impor- tance. Bites from Atractaspis spp. have caused serious envenomings (Kochva, 1998; Kurnick et al., 1999; Wagner et al., 2009; Warrell et al., 1976a), while the medical importance of uncommonly encountered genera such as Homoroselaps and Macrelaps is unclear. Bites from M. microlepidotus have been reported to cause loss of conscious- ness, and the species is considered “potentially lethal” by some authors (Vitt and Caldwell, 2008), although all of these cases are anecdotal, without any formal medi- cal evaluation or verification (Branch, 1982; Chapman, 1968; FitzSimons, 1919, 1962; FitzSimons and Smith, 1958; Visser and Chapman, 1978). FitzSimons (1919) stated that M. microlepidotus bites were insignificant, and Chapman (1968) described a bite with minimal local effects. Similarly, the single well-documented bite from a dwarf- spotted garter snake (Homoroselaps lacteus) consisted of only local pain and edema (Branch, 1982). Thus, the atractaspidids are a distinctive series of snakes assigned to a shared taxonomic status on the basis of strong morphological and molecular systematic evidence. However, this taxonomy has placed together several species previously con- sidered either “colubrids,” elapids, or viperids. Therefore, the revised biological classi- fications of many species previously considered part of the “Colubridae” may result in reassignments that can impact their perceived clinical importance by altering previous taxonomic relationships, or by formulating new perceptions on the basis of relation- ships to newly reassigned taxa. It must be noted that to date, some of these reassignments (such as those involving Atractaspis spp.) are not universally adopted or recommended by consensus and may be subject to further changes or returned to their previous classification(s). Therefore, the clinician must be proactive in seeking a thorough biological history (through consultation with a knowledgeable toxinologist or professional herpetolo- gist and/or via current literature) for any colubroid of unknown clinical importance involved in a medically significant bite. This may provide clues about the potential medical importance of the species involved as derived from observations of bites by related taxa. It emphasizes the need for careful verification of recommended taxo- nomic reassignments prior to their publication and general adoption. Such a standard may frustrate taxonomists eager to achieve recognition of their findings, but this cau- tion may also temper incorrect, premature assertions that can affect categorization of an “envenomed” patient. 2 Differences Between Buccal Gland Secretion and Associated Delivery Systems of “True” Venomous Snakes and “Colubrid” Snakes: Low- Versus High-Pressure Gland Function and Canaliculated Versus Solid Dentition A half-truth, like half a brick, is always more forcible as an argument than a whole one. It carries better. Stephen Leacock 2.1 Basic Considerations Regarding Gland Structure and Function The functional morphology of venom glands in “front-fanged” or “true” venomous snakes (viperids, elapids, and atractaspidids) differs notably from the gland appara- tus and associated dentition of other colubroids. An unknown number of these spe- cies lack their homologous counterpart, the Duvernoy’s gland (Kardong, 1996; Taub, 1966; Weinstein and Kardong, 1994; Weinstein et al., 2010; Zalisko and Kardong, 1992). Taub (1967) reported that about 17% (approximately 30 species) of colubrid snakes studied (120 genera, 180 species) lacked evidence of Duvernoy’s glands and, in some discrete groups, as many as 90% examined lacked these glands. Most Duvernoy’s glands lack any significant storage capacity, exhibit a duct sys- tem distinguishable from that of venom glands of front-fanged snakes, and usually have no direct striated muscle insertion to pressurize the fundus of the gland. The consequence is a low-pressure secretion-injecting system (Kardong, 1996; Kardong and Lavin-Murcio, 1993; Taub, 1967; Weinstein and Kardong, 1994; Weinstein et al., 2010). Figure 2.1 (Panels A–C) illustrates the basic functional morphology of a typical Duvernoy’s gland with its limited muscle attachment and associated dentition. Some members of the tribe, Dispholidini (Section 4.3), are exceptions to this, as they do have some limited striated muscle attachment on the gland fundus and thereby have “Venomous” Bites from Non-Venomous Snakes. DOI: 10.1016/B978-0-12-387732-1.00002-6 © 2011 Elsevier Inc. All rights reserved. As As Cg As As (A) (D) Ld Cld Se Lu Pd Cc Md Oe Vg Avg (B) (E) Mx Md Sd Pk Fs Ep Ep G F F (C) (F) Figure 2.1 Comparison of a Duvernoy’s gland system in an “opisthoglyphous” (“rear-fanged”  non-front-fanged) snake (left) and a venom gland system in a model “proteroglyphous” or “solenoglyphous” snake (right). Panel A. In the non-front-fanged (“opisthoglyphous”) snake, Duvernoy’s gland (shaded) is located in the temporal region. Adjacent striated muscles (e.g., adductor superficialis) run medially past the gland, but usually are not directly attached. Dispholidus typus is an exception to this, as it does have limited muscle attachment to the gland. Panel B. A cross-sectioned view of the Duvernoy’s gland that reveals the arrangement of the internal duct system draining the extensive parenchyma. A single duct departs from a small, central cistern within the gland, and runs to a cuff of oral epithelium surrounding the posterior maxillary tooth (F). Panel C. When the posterior maxillary tooth penetrates the integument of the prey or human victim, the cuff of the oral epithelium remains on the surface, thereby receiving Duvernoy’s secretion, which flows around the tooth that may (as depicted here) or may not be grooved (an “open system” with inherently low pressure). Panel D. The venom gland (shaded) of this model proteroglyphous or solenoglyphous snake includes a main venom gland, main duct accessory venom gland, and secondary duct that empty into the base of the canaliculated (hollow) fang. Striated jaw muscles (e.g., adductor externus superficialis in “proteroglyphous” elapids or compressor glandulae in “solenoglyphous” viperids) act directly upon the venom gland to raise the intraglandular pressure and send a pulse of venom from the gland through the duct to the fang. Panel E. A sagittal view of the venom gland reveals the secretory epithelium, and extensive storage reservoir of venom. Panel F. When the fang penetrates the integument of the prey, the attachment of the venom duct to the fang tightens in order to maintain the relatively high-pressure head, and venom passes down the hollow core of the fang to be delivered deeply into the tissues (a “closed system” with inherently high pressure). Abbreviations: jaw muscles, adductor mandibulae externus superficialis (As), compressor glandulae (Cg), accessory venom gland (Avg), central cistern (Cc), common lobular duct (Cld), epithelium of prey integument (Ep), fang or enlarged maxillary tooth (F), fang sheath (Fs), groove on surface of maxillary tooth (G), lobular duct (Ld), lumen holding secretory product (Lu), main duct (Md), maxilla (Mx), oral epithelium (Oe), pocket of oral epithelium around tooth (Pk), primary venom duct (Pd), secondary venom duct (Sd), secretory epithelium (Se), main venom gland (Vg). After Weinstein and Kardong (1994) used with permission. Low- Versus High-Pressure Gland Function and Canaliculated Versus Solid Dentition 9 some pressurization of the system (Fry et al., 2008; Taub, 1966; Weinstein et al., 2010). The muscle insertion into the venom glands typical of front-fanged snakes exerts a high-pressure head (often in excess of 30 psi; Kardong, 2009) facilitating rapid deliv- ery of a significant volume of pre-stored venom bolus through the associated venom ducts and canaliculate fangs (Figure 2.1, Panels D–F) that may be fixed (essentially, permanently erect, “proteroglyphous,” Plate 2.1A–E) or erectile with varying mobility (or, distensibility) according to size or species (“solenoglyphous,” Plate 2.2A; Kardong, (A) (B) Plate 2.1 (A and B) Fangs of the common Asian cobra (Naja naja). The fixed, erected, canaliculated fangs are representative of the “proteroglyphous” dentition present in the Elapidae. Many elapid species have short fangs, but some such as the coastal taipan (Oxyuranus scutellatus) have long, slightly recurved fangs. Many elapid venoms contain multiple isoforms of postsynaptic or presynaptic neurotoxins. These venoms may be delivered less deeply than those administered by the often larger fangs of viperids, but this does not decrease their in vivo lethality. (A) Profile of fangs of Naja naja, Sri Lanka. (B) Close-up of fang of Naja naja, North India. Note the elongated oval-shaped bevel (arrows) that closely resembles that of a hypodermic needle. The canaliculated (hollow or containing a lumen) morphology facilitates deep injection of venom into the integument of prey or human victim (see text). (C) Fangs of the yellow-lipped sea krait (Laticauda colubrina), Madang, Papua New Guinea. Hydrophiine sea snakes and laticaudiines (sea kraits) exhibit the “proteroglyphous” dentition associated with high-pressure venom glands. Bites from laticaudiines are rare, but may be life threatening when they occur (see text). (D) Yellow-lipped sea krait (Laticauda colubrina), Madang, Papua New Guinea. While hydrophiine sea snakes are ovoviviparous, the laticaudiines are oviparous, and come ashore to lay their eggs in rock crevices. Their venoms contain postsynaptic neurotoxins (e.g., erabutoxins), and phospholipases A myotoxins. 2 (E) Close-up view of fangs of the olive sea snake (Aipysurus laevis), Roebuck Bay, Western Australia. The fixed canaliculated fangs of this hydrophiine sea snake are closely set (arrows), probably to establish a firm grip on struggling prey and possibly increase the likelihood of effective venom delivery to the fishes of multiple niches belonging to at least 17 families and six different morphological types that comprise a major part of the diet of this species (Heatwole and Cogger, 1993). Aipysurus laevis also preys on crustaceans, cephalopods, and fish eggs. Plate 2.1A, C, and D, photos copyright to David A. Warrell; Plate 2.1B, AMNH specimen #64418; and Plate 2.1E, AMNH specimen #86176, photos copyright to Arie Lev. 10 “Venomous” Bites from Non-Venomous Snakes (C) (D) (E) Plate 2.1 C–E (Continued) 1979; Weinstein and Kardong, 1994). Some elapids exhibit incomplete fang can- nula without complete fusion of the venom duct groove (Bogert, 1943). None of the known colubroids of the former colubrid assemblage possess such dentition. The teeth associated with Duvernoy’s glands are never canaliculate (i.e., never with a lumen), but instead are solid, often enlarged, and sometimes deeply grooved (Fry et al., 2008; Weinstein and Kardong, 1994; Weinstein et al., 2010; Young and Kardong, 1996). Duvernoy’s gland morphology may vary considerably among non-front-fanged colubroid species, and enlarged teeth associated with glands may be present mid- [e.g., Pampas snake, boipemi, cobra espada comum (other names as well), Tomodon dorsa­ tus, Plate 2.3A–D], or notably posterior (e.g., Malpolon monspessulanus, Plate 2.4A–C), in the maxilla (Broadley and Wallach, 2002; Fry et al., 2008; McKinstry, 1978; Taub, 1967; Weinstein et al., 2010). Several medically important species (i.e., some members of the tribe, Dispholidini, see Section 4.3) have multiple enlarged, deeply grooved pos- terior maxillary teeth with modifications probably adapted for enhanced conduction of secretions into inflicted bite wounds (Broadley and Wallach, 2002; Meier, 1981; Section 4.3). It is important to note that other medically important species (e.g., R. tigrinus and

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This book is the first significant contribution to thoroughly examine the potential hazards associated with snakes of the former family, Colubridae. This family contained >65% of living snake species (approximately 3,000 taxa) and has recently been split into multiple families. Many of these snakes
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Most books are stored in the elastic cloud where traffic is expensive. For this reason, we have a limit on daily download.