Thursday, March 14, 2013

Medical risks from colubroid snakes


Rhabdophis subminiatus, a Hazard Level 1 species. JCM
Facts that don fit  existing knowledge are frequently clues to a greater understanding of nature. And, the fact that snakes without front fangs  could deliver venom has produced a greatly improved knowledge of snake evolution. as well as evolution in a general sense.

In the late 1960's a visit to a Miami animal dealer revealed the diversity of snake species in the pet trade. Besides the usual large boas and pythons, the dealer had green mambas, forest cobras, bushmasters, and cages filled with other species that I could not identify. But there was one cage that had a pile of green and brown snakes, that I did recognize, boomslangs.

The front-fanged snakes were known to carry venom and the anatomy to deliver it. But, at the time I was aware that some colubrid snakes, snakes thought to be harmless,  could also do serious harm to human health. The best know instance at the time  was the death of K. P. Schmidt in 1957 . Schmidt was Curator Emeritus at the Field Museum, when he was bitten identifying a boomslang for the Lincoln Park Zoo. Envenomation from colubrid snakes was relatively rare and reactions to bites from colubrid snakes was frequently attributed "allergic reactions" to saliva.

Antivenom for front-fanged snakes has been around for the last century, but venom  for rear-fanged snake bites as been slow in coming. In a forthcoming article in Toxicon, Weinstein and colleagues look at the evidence for health risks from the bites of non-front-fanged colubroid snakes (NFFC).  Until now a relatively few species of NFFC have been documented to be a medical risk to humans, but the authors point out a growing number of NFFC are implicated in medically significant bites. More NFFC species are entering the commercial snake trade and pose an uncertain risk. Weinstein and colleagues examined published case reports describing NFFC bites and used a meta-analysis yo generate a hazard index for select taxa. They also included cases on which they had consulted or personally treated.

About 120 species met the selection criteria, and a small subset were designated Hazard Level 1 (most hazardous), this group contained five species with lethal potential.Three species, Dispholidus typusRhabdophis tiginisRhabdophis subminiatus, have available antivenom. The other two - birds snakes -in the genus Thelotornis lack commercially available antivenoms. Bites from these snakes are treated with plasma/erythrocyte replacement therapy and supportive care. Heparin, antifibrinolytics and plasmapheresis/exchange transfusion have been used in the management of some Hazard Level 1 envenomings, but evidence-based analysis positively contraindicates the use of any of these interventions. Hazard Level 2/3 species involved in cases that contained mixed quality data and implicated three species: Boiga irregularisPhilodryas olfersii, and Malpolon monspessulanus. The bites of these snakes have rare systemic effects. Management of these bites may include use of acetylcholinesterase inhibitors and wound care on a case-by-case basis. Hazard level 3 is a large group of taxa capable of producing significant local effects (no systemic effects), symptoms are often associated with a protracted bite, species in this group include: Heterodon nasicusBorikenophis portoricensis, and Platyceps rhodorachis. Bite management is restricted to wound care. Hazard level 4 species comprised the majority of species surveyed and these showed only minor effects of no clinical importance.

As snakes become more popular as pets bites from these animals are likely to increase in frequency. But perhaps more important than knowing the risk to humans is the knowledge gained about the evolution of venom  and the increased potential for new molecules for research.

Citation
Weinstein, S.A., et al., Non-front-fanged colubroid snakes: A current evidence-based analysis of medical significance, Toxicon (2013), http://dx.doi.org/10.1016/j.toxicon.2013.02.003

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