Snakebite Severity, Western Rattlesnake Venom, and Prey Digestion

In 2000,  I remember reading an article in Natural History Magazine that stunned me, the author was discussing the fact that rattlesnake bites were showing more neurotoxic symptoms, and that the reason for this was a gene or genes, that produced neurotoxin(s), was spreading through multiple species of US rattlesnake populations through hybridization. The implication was that some Mojave Rattlensnakes with neurotoxic venom were somehow moving cross-country and mating with members of other species of rattlesnakes. As crazy and improbable as this hypothesis is, it was picked up by media and may have fueled  further speculation by physicians in the southwest who reported increases in the number of severe snakebite cases in recent years. William Hayes and Steve MacKessy (2010) have recently addressed this issue and note a greater number of articles that suggest snakes are evolving more toxic venom, perhaps in response to human activity in the environment. Hayes and MacKessy propose other explanations that are more probable and include factors dependent on the snake and factors associated with the bite victim’s response to envenomation. They suggest that while bites could become more severe from an increased proportion of bites from larger or more provoked snakes (ie, more venom injected) that venom evolves slowly, much too slowly, to account for a rapid increase in  the severity of symptoms. The severity of a bite can be influenced by many factors including the demographics of the snakebite victims. So, changes in age, body size, behavior toward the snake (provocation), anatomical site of the bite, clothing (or lack of it), as well as overall general health of the snakebite victim are important. Allergies have increased in the population in recent years, and this may be an overall indicator of the populations' changing sensitivity to foreign antigens. Hospital care of bites has also changed, making comparisons of snakebite severity over time difficult. Media coverage of atypical bites and accompanying speculation has been misleading and is likely to raise public anxiety about snakes.

In a second article,  but one related to the discussion above, MacKessy (2010) examined the venom from the eight subspecies of the the Western Rattlesnake (Crotalus oreganus-viridis species complex ). The Western Rattlesnakes are widely distributed across the western half of North America and while some venom characteristics have been noted for most subspecies there has been no systematic study of venoms from all subspecies. MacKessy extracted venom from all eight Western Rattlesnake snake subspecies collected in the approximate geographic center of their range. The venom was analyzed using  mass spectrometry, enzyme and toxicity assays and electrophoretic. The results demonstrated that small myotoxins, disintegrins and PLA2 were abundant in most venoms and; PIII and PI metalloproteinases were common to all venoms except  the Midget Faded Rattlesnake, the Northern Pacific Rattlesnake, and the Southern Pacific Rattlesnake (C. o. concolor, C. o. caliginis and C.o. helleri). Metalloproteinase activity was highest in the Arizona Black Rattlesnake (C. o. cerberus) and lowest in the Midget Faded Rattlesnake (C. o. concolor) venoms (with a 100-fold difference). Conversely, the Midget Faded Rattlesnake (C. o. concolor) venom was the most toxic and Arizona Black Rattlesnake (C. o. cerberus) venom was least toxic (a 15-fold difference). Overall, MacKessy found  venoms with high metalloproteinase activity were less toxic (type I venoms), while venoms which were highly toxic showed low protease activity (type II venoms). Within the Western Rattlesnake species complex, both extremes of venom composition occur  and it appears that high metalloproteinase activity and high toxicity are incompatible qualities of these venoms. So why are there such dramatic differences in the biochemical characteristics of these related rattlesnakes? likely relates to characteristics of prey consumed, and venoms with low metalloproteinase activity may constrain snake prey selection or foraging activity patterns. Metalloproteinases are common and abundant in viper venoms and contribute significantly to the occurrence of hemorrhage and necrosis following envenomation.  An important role of
 venom has been assumed to be prey digestion, and the metalloproteinases have been implicated as the major family of proteins involved in digestion. However, a more recent study indicated that at 30°C digestive efficiency was not enhanced by venom and that the role of venom in digestion is, indeed, minimal. At this higher temperature, it is not surprising that venom has little effect, another study suggests temperature dependence of venom-enhanced digestion has a highly pronounced effect at cooler temperatures and minimal effect at higher temperatures. Snake gastric processes are highly efficient at “normal operating temperatures,” but venom metalloproteinases likely provide the additional degrading enhancement which facilitates efficient digestion of the prey at the suboptimal temperatures encountered in the field.

Snake venom evolves in response to hunting success or failure. Venom that quickly incapacitates a specific prey species, or venom that starts digestion in cooler than normal environments will be favored over venom that is less efficient. As prey evolves resistance to venom the snake will evolve venom that is more toxic and more efficient, or be less successful and not pass on its genetic material. It seems highly improbable humans are having any impact on venom evolution - other than possibly pushing its source to extinction.

Grenard S. 2000. Is rattlesnake venom evolving? Natural History 109:44–46.

Stephen P. Mackessy 2010. Evolutionary trends in venom composition in the Western Rattlesnakes (Crotalus viridis sensu lato): Toxicity vs. tenderizers. Toxicon,  55:1463-1474

William K. Hayes and Stephen P. Mackessy. 2010. Sensationalistic Journalism and Tales of Snakebite: Are Rattlesnakes Rapidly Evolving More Toxic Venom? Widerness and Environmental Medicine 21:35–45.