Medically Significant Bites by “Colubrid” Snakes - "Venomous" Bites from Non-Venomous Snakes - page 157

4.2.3 Genus Thamnophis spp.: Background and General Features of
Documented Bites
North American natricids such as the garter and ribbon snakes ( Thamnophis spp.;
approximately 27 species) are the best-known members of the tribe, Thamnophini
(about 31 taxa), and among the most popular snakes maintained (often by children)
in captivity. They are medium-sized (most species average between 40 and 80 cm in
total length), and prey on a wide variety of small vertebrates and invertebrates ( Table
4.2 ). Rossman et al. (1996) thoroughly reviewed the biology and evolution of the
genus. Although Thamnophis spp. are abundant and among the most studied North
American snakes, their phylogenetic relationships have received relatively limited
scrutiny (de Quieroz et al., 2002). The phylogeography of Thamnophis spp. may have
resulted from at least one invasion of northern North America from Mexico (Alfaro
and Arnold, 2001). Several cases of bites from a couple of these species that caused
mild local effects (e.g., puncture wounds/lacerations, edema, and, uncommonly,
ecchymoses) have been well documented ( Table 4.1 ). As these snakes have long been
popularly viewed as totally “harmless,” some authors have assigned acquired hyper-
sensitivity as a likely factor contributing to these effects (Ernst and Barbour, 1989),
while others have rejected this possibility on the basis of absent “urticaria” after bites
from T. s. similis (Plate 4.53; blue-striped garter snake; Nichols, 1986). As discussed
later in Section 4.6, Type 1 hypersensitivity manifests quite variably and urticaria,
while often present, does not develop inevitably in all cases. Misinterpretation of clini-
cal signs/symptoms features prominently in these reports (see Section 4.5), and com-
monly obfuscates or complicates analysis of real versus perceived risks posed by many
of these species. Several documented cases suggest that two Thamnophis taxa [ T. e. ter-
restris (western terrestrial garter snake), Plate 4.52A, and T. e. vagrans (wandering gar-
ter snake), Plate 4.52D] are capable of inflicting mild local effects, and that two others
[ T. s. similis and some populations and/or large adults of nominate T. sirtalis (common
garter snake, Plate 4.54A, D-J)] may also have this potential ( Table 4.1 ). As noted pre-
viously, these effects may cause greater surprise and concern because of the precon-
ceived perception of complete “harmlessness” of these snakes. Also, medical personnel
confronted by the unfamiliar scenario of a snakebite may be similarly predisposed
to “expect the worst.” This may be more likely in the case of a species of unknown
medical importance or when a culprit specimen is misidentified and/or confused with a
known medically important crotaline or elapid species (see previous section regarding
Philodryas spp. and Section 4.5).
4.2.3.1 Overview of the Duvernoy's Gland and Associated Dentition of
Thamnophis spp.
Some of the members of the genus have slightly enlarged, ungrooved posterior maxi-
llary teeth. Ruthven (1908) noted that members of the genus Thamnophis exhibited
maxillary teeth that were “rather abruptly longer near the posterior end of the maxil-
lary bone than anteriorly.” However, in citing Cope (1892), Ruthven (1908) added
that “the excess in the length of the posterior teeth is so small in many specimens …