Hi Nicole,
The standard measure of venom potency for many years was the mouse LD50 (
LD50 = lethal dose that will kill 50% of uniform test animals in a given time period).
Essentially the protocol involves that groups of (
typically) 4 or 6 mice of the same breed or strain that are of equal age and weight, and then injecting each mouse with a predetermined dose of venom or toxin. Normally this is done is series ... one group may receive 0.1mg, the next 0.15, the next 0.2 and so on ... the dose that kills 50% of a test group in the time period specified (from 4 to 24 or even 48 hours) is the LD50. Normal procedure requires that the result be reproducible in successive groups of mice a this dose.
The test is uniform and thus gives equality so that the only variable when comparing venoms from different species is the type of venom itself and the dose of that venom needed to achieve an LD50. It is however commonly argued that the test is only relevant in the species being tested and that physiological differences render it inconsequential in relation to other forms of life.
In recent times it has also been largely discontinued due to the ethical considerations surrounding the mass killing of (
often) several hundred or even thousands of rodents to perform a full series of experiments. A number of alternative assays have been used that test the specific activities of venom: neurotoxicity, haemotoxicity, myotoxicity, procoagulant or anticoagulant function etc ... but the LD50 remains the test most quoted by people trying to define relative toxicity.
Now where does this all fit in with the inland taipan (
Oxyuranus microlepidotus)???
In the late 1970's the late Professor Sutherland's research group at CSL performed LD50 tests to gain a measure of how certain species of Australian snakes ranked in comparative terms against some overseas snakes.
Using a protein known as bovine serum albumin as a diluent to help stabilise the venoms they found that the LD50 of the inland taipan was a mere 0.010mg/kg body weight in groups of 18-21 gram mice - the average venom yield of the species at that time was reportedly 44.1mg dry weight.
By comparison coastal taipans (
Oxyuranus scutellatus) with an average venom yield of 120mg dry weight had an LD50 of 0.064mg/kg (6.4 times less toxic).
In mouse-killing terms an inland taipan could therefore
theoretically kill 217,821 18-21 gram mice with 44 mg of venom. The maximum recorded yield of 110mg at that time would have been able to
theoretically kill 544,554 18-21 gram mice.
By way of comparison the 120mg average yield of a coastal taipan would
theoretically kill 94,488 18-21 gram mice. The maximum reported yield of 400mg would have been sufficent to take out 314,961 18-21 gram mice.
If we take the
theoretical extrapolation of this data further, 244,975 mice weighing 18 grams each have a collective body mass of 4409.55 kilograms. This is approximately equivalent to 63 humans with equal body weight of 70 kilograms. The
very generous extrapolation therefore is that an inland taipan may be capable of killing more than 60 average adult humans with the venom expressed in a single milking.
In the real world however it simply is not reasonable to attempt to compare the theoretical estimates of the number of mice a species may be capable of killing, with the ability to produce death in humans (
or cats, dogs, horses or elephants for that matter). But what it does give us is a
uniform means of comparing the relative toxicity of different species under equivalent test conditions in which the venom itself is the only variable ... and this does give a
valid means of ranking the
potential toxicity of those species.
The important caveat is that
it does not give us a ranking that is applicable to different "test" species, or even to the same species using different routes of injection/administration.
As for the effects of snake venoms on plant species ... I have never heard of this being examined. There is however considerable work being done to attempt to identify plant compounds that neutralise snake venom toxins.
Anyway I hope that all this babble is comprehensive given the hour at which I am writing this. There are some links to PDF files re this subject at the bottom of the post. You could also visit my colleague
Dr Bryan Fry's Website to see comparisons of toxicity tests via different routes of venom administration.
Fire away with further questions.
Cheers
David
REFERENCES:
Electrophoretic, enzyme, and preliminary toxicity studies of the venom of the small-scaled snake, Parademansia microlepidota (Serpentes:Elapidae), with additional data on its distribution. (1979) BROAD AJ, SUTHERLAND SK, TANNER C, COVACEVICH J.
Mem. Qld. Mus. 19(3):319-29.
The lethality in mice of dangerous Australian and other snake venom. (1979) BROAD AJ, SUTHERLAND SK, COULTER AR.
Toxicon 17:661-664.
Intravenous dose-lethality study of American pit viper venoms in mice using standardized methods. (1992) CONSROE P, GERRISH K, EGEN N, RUSSELL FE.
Journal of Wilderness Medicine. 3:162-167.
