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in which the toxin produced potent neuromuscular blockade at the avian neuromus-
cular junction, but was three orders of magnitude less effective in blocking the mam-
malian neuromuscular junction (Pawlak et al., 2009). Thus, this toxin may account
for the prey-specificity shifts reported for this snake (e.g., juveniles favor lizard prey;
Greene, 1989) as well as the decreasing level of neurotoxin with increasing size and
age (Mackessy et al., 2006; Weinstein et al., 1993).
4.4.1.4 What Is the Etiology of the Medical Effects of
B. irregularis
Bites?
The distinctive circumstances involving medical risks from
B. irregularis
on Guam (and
other islands) present a rare opportunity to scrutinize the source of risks from bites of
a given non-front-fanged colubroid species. There is a relatively large series (
n
450)
of well-documented emergency-room presentations after bites from
B. irregularis
.
Also, as these occurred in an insular environment, factors influencing variability in
snakes, victims, and medical facilities were likely to be relatively restricted. The vast
majority of these cases featured puncture wounds, lacerations, mild edema/ecchy-
moses, and bleeding (
Table 4.1
). Only five cases, all involving infants younger than
6 months old, reportedly contained clinical signs that suggested systemic effects (“pto-
sis,” “respiratory distress” or “respiratory difficulty,” “spasticity”;
Table 4.1
). However,
the size-related/ontogenetic variability in
B. irregularis
Duvernoy's secretion properties
and the predatory behavior of these snakes complicate interpretation of these limited
cases that suggest systemic effects, including neurotoxicity. For example,
B. irregula-
ris
swallows smaller prey directly while large prey is actively constricted (Rochelle and
Kardong, 1993), and “envenomed” rodents retain in their integument almost 50% of
the secretion introduced during predation (Hayes et al., 1993). As noted previously, the
secretion of this species exhibits low lethal potency, contains a postsynaptic neurotoxin
that is avian/saurian-specific, and is in significantly higher concentrations in secretions
from smaller/younger snakes (Mackessy et al., 2006; Pawlak et al., 2009; Weinstein
et al., 1993). Also, metalloprotease and acetylcholinesterase activities increase with size
and age, along with the variable increase in lethal potency (Mackessy et al., 2006). Aside
from
B. irregularis
, only specimens or populations of the crotaline viperids,
Lachesis
muta stenophrys
(Central American bushmaster,
L. stenophrys
; McDiarmid et al., 1999;
see
Plate 4.86
), and the jararaca (
Bothrops jararaca
) exhibit an ontogenetic increase
in venom toxicity, respectively (Gutiérrez et al., 1990; Antunes et al., 2010). All other
ophidian species studied to date that have detectable ontogenetic change in venom prop-
erties typically exhibit ontogenetically decreased lethal potency and increased proteolytic
activity (Chippaux et al., 1991; Mackessy, 1988; Minton and Weinstein, 1986).
Therefore, the potential basis for systemic effects after
B. irregularis
bites in this
small group of pediatric patients is unclear and contrasts with available biomedical
data regarding the biology of this species. The following points are considered: some
larger snakes (
1.4 m) produce a secretion that exhibits a higher murine toxicity and
has very low postsynaptic neurotoxin content; all of the concerning bites inflicted on
humans (principally, four of the five pediatric cases mentioned earlier;
Table 4.1
) were
from large snakes (average length about 1.17 m); juvenile/smaller snakes have a lower
murine toxicity and higher postsynaptic neurotoxin content; larger prey are actively
