Hi all,
I don't get the time to visit SAReptiles very often these days, but as I was working on researching a paper I am writing I came across some publications that reminded me of a conversation I had with a couple of people about why they believed tourniquets were the only option for managing Dendroaspis envenoming.
They had been particularly emphatic that Dr Roger Blaylock had said that mamba venoms are absorbed via the bloodstream, and not via the lymphatic system, and that this evidence proved the need the tourniquets after mamba bites.
Well today as I was reading Dr Blaylock's 1994 editorial titled Pressure immobilisation for snakebite in southern Africa remains speculative., in the South African Medical Journal [SAMJ. 1994. 84(12): 826-827.], in which he argued against the concept of using pressure immobilisation bandages for first aid treatment of most southern African snakebites, I came across the section where he states "Christensen showed that southern African elapid polypeptide toxins are absorbed rapidly into the bloodstream as shown in the subcutaneous LD50 dose's being only 10-20% higher than the intravenous LD50 dose in mice." and it occurred to me that here at last was proof in writing that what my two friends had said about Blaylock's assertion may actually be true, so I took a quick look at the reference list, and armed with the Christensen citation he had given, went hunting through the SAMJ online archive to see exactly what evidence Christensen had presented to prove this apparent dichotomy which had established that southern African elapid venom polypeptides travel via a different route to the venom polypeptides of elapid snakes elsewhere in the world.
Christensen's paper, The treatment of snakebite., was published in the SAMJ in October 1969 [SAMJ. 1969. 43: 1253-1258], and what was suprising however is that this paper contained absolutely no primary scientific evidence whatsoever demonstrating that southern African elapid polypeptide toxins are absorbed rapidly into the bloodstream! Blaylock appears to have been rather dis-ingenious!
What Christensen's paper actually says is "The lethal toxins in elapid venoms are polypeptides with little or no local action, but, being rapidly absorbed into the blood stream, they may exert their neurotoxic action soon after a bite. Consequently, the subcutaneous median lethal dose (LD50) of elapid venoms for mice is only about 10-20% larger than the intravenous LD50. As a reference to this statement, Christensen very ironically cited the well-known 1941 study by Drs Barnes and Trueta titled Absorption of bacteria, toxins and snake venoms from the tissues., which appeared in the esteemed journal, The Lancet [The Lancet. 1941. 1: 623-626.], so it was to that paper I turned next.
Those who have not read the Barnes & Trueta paper might wonder why I say that Christensen's citation of this work was ironic. Well to understand, remind yourselves that Blaylock in 1994 was geographically precise in stating that "Christensen showed that southern African elapid polypeptide toxins are absorbed rapidly into the bloodstream as shown in the subcutaneous LD50 dose's being only 10-20% higher than the intravenous LD50 dose in mice.", and yet Christensen said nothing of the sort, stating specifically that "The lethal toxins in elapid venoms are polypeptides with little or no local action, but, being rapidly absorbed into the blood stream... with no geographic qualification whatsoever. Remember also that Blaylock's 1994 statement was made in the context of rejecting an Australian method of snakebite first aid (PIB) by seeking to imply that southern African elapid venoms are absorbed via a different route to those of elapids elsewhere. And I might add that after Dr Struan Sutherland wrote to the SAMJ rejecting Blaylock's editorial in 1995 [SAMJ. 1995. 85: 1039-1040.] Blaylock actually went a step further in trying to make this point [SAMJ. 1995. 85: 1040-1041]. Again citing as his evidence the 1969 Christensen paper, he stated that "...dendroapsis venom is absorbed directly into the bloodstream and not via lymphatics." Nowhere in Christensen's paper is there any statement to the effect that Dendroaspis venom specifically is absorbed into the bloodstream. What was Blaylock playing at? Was he really so determined to have the last word against his rival Sutherland, that he made it up? Christensen's paper certainly doesn't say what he claims it does. Now getting back to irony ... the elapid venoms used in the Barnes and Trueta (1941) experiments were none other than Australian black tiger snake venom (Notechis ater) and Indian spectacled cobra (Naja naja) venom!! No southern African elapid venoms were tested and so no proof whatsoever exists in this paper trail to support any of Blaylock's statements in the 1994 and 1995 SAMJ publications, and the only evidence at the end of the trail of publications actually refers to non-African venoms!! Blaylock really should have checked Christensen's paper more carefully, and it's cited papers as well!!
As to the experiments conducted by Barnes & Trueta, they demonstrated that Notechis ater venom injected into the legs of rabbits whose lymphatic transport vessels had been occluded or severed survived longer than control rabbits with intact lymphatic transport, but found that there was no difference in survival between control and test rabbits injected with Naja naja venom. Likewise while they found that immobilisation of rabbits injected with Notechis ater venom led to prolonged survival, the same was not true of rabbits immobilised and then injected with Naja naja venom. One of the fundamental problems with the Barnes & Trueta work however is their simplistic concept of venom molecular weight. They considered that Notechis ater venom had a MW of ~20 kDa and that Naja naja venom was only 2.5-4.0 kDa in size, when the reality is that both venoms are actually complex mixtures of many different toxins ranging in size from a few kDa to several hundred kDa. Hence their conclusions, which were that Naja naja venom (which they not had significant local effects) appeared to be absorbed into the bloodstream because lymphatic occlusion and body immobilisation failed to prevent death, whereas Notechis ater venom (which cause inconsequential local effects) must travel via the lymphatics because lymphatic occlusion and body immobilisation prolonged survival, really cannot be accepted in the light of what we know about these venoms today. The view that Naja naja venom appeared to be absorbed into the bloodstream simply holds little merit as the sole explanation for the observed outcomes, given the complexity of that venom as we now understand it, and only a series of more carefully designed and executed experiments would be likely to resolve this question further.
Certainly there was no basis for Blaylock to use this data to extrapolate his 1994 and 1995 statements about southern African elapid snakes (and specifically mambas) possessing toxins that are absorbed directly into the bloodstream, and it is well past time that this myth perpetrated by Blaylock during an acrimonious exchange between two men known for their egotistical determination to have the last word, be recognised for being nothing more than a case of Chinese whispers of the literary kind ...
To my two friends, who will no doubt read this sooner or later ... I'm sorry to kick sand on firmly held beliefs, but I believe we must question what people tell us, and we have to avoid falling into the trap of simply accepting what experts tell us, simply because of who they are. Blaylock and Sutherland's tussle in the SAMJ back in 1994 and 1995 is a great illustration of this, and neither are blameless for letting their egos and their belief in their own doctrines to ride roughshod over the truth and over impartial, unbiased scientific query. Sutherland took great delight in claiming that Blaylock's citation of a handful of clinical case reports showing PIB to be of limited value were "...quite inadequate for this purpose." - pure hypocrisy from a man who delighted in pointing to isolated clinical anecdotes from Australia as absolute proof that is invention of PIB was the only effective first aid for snakebite, making him no better than Blaylock for twisting facts to suit his agenda.
So what of first aid for snakebite? ... That's a whole other discussion, perhaps to be debated in depth at a later date. Certainly the truth is that the jury is still out right now, and that a clear answer will only come about though rigorous, unbiased investigation using cleverly designed experiments and adequate numbers to prove theories beyond doubt.
Cheers
David Williams