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mild local effects that constitute the vast majority of clinical presentations after
B. irregularis
bites. The possible role of Type 1 hypersensitivity reactions in some
patients bitten by
B. irregularis
and other colubroids is discussed in Section 4.6.
4.4.1.5 Conclusion and Assessment of
B. irregularis
The profound ecological impact of
B. irregularis
has resulted in concurrent and sig-
nificant medical repercussions in Western Pacific islands (especially Guam) invaded by
this species. This snake exhibits remarkable durability as well as fecundity, and has pre-
sented a challenge to efforts focused on controlling its geographic expansion. It also dis-
plays disconcerting behavior suggestive of active predatory attempts on human infants.
Only five documented cases of bites inflicted on infants have resulted in clinical fea-
tures suggestive of systemic envenomation. The vast majority of bites have caused only
mild local effects. The etiology of medically significant effects from these bites remains
unclear. Although to date, the characterized postsynaptic neurotoxins in Duvernoy's
secretion of
B. irregularis
are specific for avian and saurian prey, it is possible that
other neurotoxins may contribute to the clinical syndrome in human infants. This pos-
sibility would be likely associated with protracted bites (allowing increased introduction
of secretion) inflicted by smaller specimens on neonates or infants (due to low body
weight, as well as vulnerability to bites and trauma to the airway via concomitant con-
striction). Smaller specimens would probably have a higher concentration of such neu-
rotoxins in their secretion. On the other hand, an uncharacterized mammalian-specific
neurotoxin could increase in concentration with size or age and thus play a role in the
variable concomitant increase in murine toxicity with size or age. However, the ontoge-
netic/size-related variability in secretion components (especially detectable neurotoxic-
ity) complicates analysis of the potential factors contributing to these cases. A possibly
relevant consideration may be drawn from the highly specific neurotoxicity of distinc-
tive neurotoxins from other ophidian species. For instance, the specificity and onto-
genetic nature of the AchR-subunit composition at the murine motor end plate dictate
the action of waglerin 1 from venom of the crotaline viperid,
Tropidolaemus wagleri
(
Plate 4.87A and B
; Aiken et al., 1992). Waglerin 1 specifically binds to the ε-subunit
of the AChR. Therefore, it is active only in mature mice as neonate mice have AChR
with g-subunits that are later ontogenetically switched to ε with maturity. Thus, neo-
nate mice are unaffected by waglerin 1 whereas the peptide is rapidly fatal to adult mice
(i.p. LD50 of waglerin 1 [adult mice]
0.370 mg/kg). This peptide exhibits potent activ-
ity in the murine nerve-muscle assay. However, the venom has modal lethal potency in
mice, and the purified peptide shows no AchRB activity when tested in assays using
human or avian tissues (McArdle et al., 1999; Weinstein et al., 1991). Human enven-
omations by
T. wagleri
typically feature mild-to-moderate local edema and pain with-
out manifestations of neurotoxicity (S.A Minton, personal verbal communication with
SAW, March 1982). It is conceivable that
B. irregularis
may produce other neurotox-
ins yet to be identified that also exhibit target tissue ontogeny-dependent potency and
specificity. Although irditoxin from
B. irregularis
Duvernoy's secretion is distinctive, it
is highly specific for lizards and birds, and to date, there are no data to support the pres-
ence in this secretion of other neurotoxins that have significant toxicity for mammals.
