Venom flow in rattlesnakes: mechanics and metering

Abstract

SUMMARY The functional morphology of venom injection in Crotalus atrox was explored using high-speed digital videography combined with direct recording of venom flow using perivascular flow probes. Although venom flow was variable, in most strikes the onset of venom flow was coincidental with fang penetration, and retrograde flow (venom suction) was observed prior to fang withdrawal. The duration of venom flow was consistently less than the duration of fang penetration. The occurrence of retrograde flow, ‘dry bites’ (which accounted for 35 % of the strikes) and unilateral strikes all support a hypothesis for venom pooling in the distal portion of the venom-delivery system. No significant difference in temporal or volumetric aspects of venom flow were found between defensive strikes directed at small and large rodents. With the species and size of target held constant, the duration of venom flow, maximum venom flow rate and total venom volume were all significantly lower in predatory than in defensive strikes.

Large Snake Size Suggests Increased Snakebite Severity in Patients Bitten by Rattlesnakes in Southern California

“…However, in contrast to some other studies, snakebites without envenomation ("dry" bites) were very rare. Russell found that 18% to 22% of bites were dry, 7 and Young and Zahn found that 35% of bites were dry, 20 while Tanen et al, like us, documented a low incidence of dry bites (3%). 18 The differences among studies may be due to different ways of judging snakebite severity.…”

“…7 There are data showing that rattlesnakes are able to meter their venom such that smaller prey may receive less venom than larger prey, 19 and one could speculate that children, although they are not prey, may receive less venom than adult patients. On the other hand, there are also data showing that venom flow in defensive strikes does not differ with different target sizes, 20 although arguably the mechanics of venom flow are not necessarily predictive of total venom delivery.…”

“…18 The differences among studies may be due to different ways of judging snakebite severity. For example, the Young and Zahn study measured actual venom flow through the venom duct, 20 whereas another used data collected from multiple other studies to make an estimate. 7 In the present study and that of Tanen et al, 20 the observed clinical effects of envenomation were recorded.…”

“…For example, the Young and Zahn study measured actual venom flow through the venom duct, 20 whereas another used data collected from multiple other studies to make an estimate. 7 In the present study and that of Tanen et al, 20 the observed clinical effects of envenomation were recorded. The way that these effects are judged may differ from study to study, such that one investigator may view a fang puncture with mild limited erythema and pain to be a dry bite, whereas another investigator may score the same findings as mild envenomation (as we did.)…”

Wilderness Medical Society Practice Guidelines for the Treatment of Pitviper Envenomations in the United States and Canada

“…75,76 (Not recommended) THE DRY BITE Venomous snakes may also fail to deliver venom in an event commonly referred to as a dry bite that may occur in 25% or more of crotaline bites. 25,77 Duration of fang contact also affects the amount of venom injected in both predatory and defensive situations, with venom quantities from defensive bites (eg, to humans) being more Even a knowledgeable caregiver may not be able to predict the amount of envenomation from a snakebite or potential sequelae, and therefore observation for the sake of risk stratification is not recommended. [Recommendation grade: 2C] Every effort should be made to evacuate snakebite patients; however, if the patient is in a remote location with a difficult evacuation, further considerations must be weighed, including patient and rescuer safety, likelihood of successful evacuation, and availability of resources required to carry out the evacuation.…”