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Venom researcher Peter Mirtschin has published one of the most comprehensive relative toxicity lists for venomous snakes.

Relative Toxicity in Snake Venoms
Peter Mirtschin, Venom Supplies Pty Ltd
PO Box 547   Tanunda South Australia 5352
Phone 61 85 63 0001,  Fax 61 85 63 0020

Snake venoms are like soups. Most of them contain many different components and some of these are toxic to man. For instance, in the common Tiger Snake venom, Notechis scutatus, there have been at least 6 neurotoxins described,2.

There are several haemotoxins 34,5.6, 2 interfering with activation of the clotting factors and some weak haemorrhagins. Some of the neurotoxins also cause muscle damage. There are also toxins which can cause a drop in blood pressure in this venom.

Most venoms have many components, some of which are known to be toxic to man. There are also many components in most venoms that are still un-characterised. It is the combined effect of all the components in snake venoms that give it its overall toxicity.

In the past, and to a lesser extent today, toxicity was measured by injecting test animals with venoms. Many different types of animals have been used. In the 1960's and 1970's, mice became the universally accepted test animal and comparative toxicity was expressed as an LD50 (lethal dose 50%). This is usually measured by testing the crude venom in mice at varying doses and when a particular concentration causes 50% of the test animals to die, it is referred to as the LD50.  Today, these types of animal experiments are not performed as much because of animal welfare concerns, however they have been invaluable in understanding the toxicity of both crude venoms and purified single toxins.

There are other specific tests that look at the effect of single toxins. These can be clotting assays to measure the effect of coagulants, nerve-muscle preparations to measure neurotoxic activity, phosphilipase and phosphodiesterase assays which measure these respective agents in the venoms. Even muscle or myoblast preparations to measure the myotoxic activity of venoms have been used. These tests are useful to investigate the relative toxicity of purified toxins but there is sometimes a poor correlation between these results and the in vivo effects of the venom or toxin. There is still no alternative that surpasses the in vivo LD50 for both purified toxins and crude venoms.

In 1979, the Commonwealth Serum Laboratories in Australia conducted an extensive comparative study of the toxicity of most of the Australian medically significant snake venoms and some venoms from non-Australian snakes. Here they used mice which were subcutaneously injected with the crude venoms.

In the early 1980's, Richard Davis and I published the cobra scale, which compared any venom with that of the well known Indian cobra. Cobra venom is assigned the value of 1 and the toxicity of other venoms are compared to this venom. The following expanded table is the result of this comparison.

Inland Taipan                                            

Oxyuranus microlepidotus
 50.0

Common Brown Snake                              

Pseudonaja textilis
12.5

Taipan                                                         

Oxyuranus scutellatus
 7.8

Reevesby Is. Tiger Snake                           

Notechis ater niger
5.1

Common Tiger Snake                                  

Notechis scutatus
4.2

Western Tiger Snake                                    

Notechis ater occidentalis
4.0

Beaked sea snake                                     

Enhydrina schistosa
2.9

Chappell Is. Tiger Snake                            

Notechis ater serventyi
1.8

Common death adder                               

Acanthophis antarcticus
1.5

Western Brown Snake                               

Pseudonaja nuchalis
1.5

Copperhead                                                

Austrelaps superbus
1.0

Dugite                                                       

Pseudonaja affinis
0.9

Stephens banded snake                           

Hoplocephalus stephensi
0.4

Rough scaled snake                                 

Tropidechis carinatus
 0.5

Spotted black snake           

Pseudechis guttatus
0.3

King Brown Snake                                     

Pseudechis australis
0.3

Collets snake                                            

Pseudechis colletti
 0.2

Red bellied black snake                           

Pseudechis porphyriacus
0.2

Small-eyed snake                         

Cryptophis nigrescens
 0.2

Whip snake                                               

Demansia olivacea
 <0.1

Non-Australian

Indian cobra                          

Naja naja naja
 1.0

Papuan black snake                         

Pseudechis pauanus
 0.4

King cobra                                              

Ophiophagus hannah  
0.3

E. diamond-back rattlesnake

Crotalus adamanteus  
<<0.1

Brazillian viper (Barba amarilla)

Bothrops atrox
<<0.1

This type of comparison still relies on LD50 results being available, but provides a simple comparison of venoms for the lay person. It is important when using this type of comparison, to only use LD50 figures that quoted in the same form using the same test animal with the same injection site.

Minton and Minton (1969)7 quote the s.c. LD50 for the western diamond back rattlesnake Crotalus atrox as 150-385mg/20gm mouse or 7.5-17.9mg/kg. This would make it about 0.07-0.03 times as toxic as Naja naja*. Similarly the sidewinder Crotalus cerates is 0.005 and the Mojave rattlesnake Crotalus scutulatus is 0.08 times as toxic as Naja naja*.

*These comparisons are only approximate since both data sets were carried out by different laboratories with possible differences in the parameters used to assess the LD50's.

References
1. Sutherland,S.K. (1983). Australian Animal Toxins. Oxford University Press. Melbourne. p54.
2. Francis, B., John, T.R., Seebart, C. and Kaiser, I.I. (1996). New toxins from the venom of the common Tiger Snake (Notechis scuatatus scutatus). Toxicon 29(1) 85- 96.
3. Francis, B., Williams, E. S., Seebart, C., Kaiser, I. I. (1993). Proteins isolated from the venom of the common Tiger Snake ( Notechis scutatus scutatus ) promote hypotension and hemorrhage. Toxicon 31(4). 447-458.
4. Tans, G., Govers-Rienslag, J.W.P., van Rijn, J.M.L. and Rosing, J. (1985). Purification and properties of a prothrombin activator from the venom of Notechis scutatus scutatus.The Journal of Biological Chem. 260(16). 9366-9372.
5. Jobin, F. and Esnouf, M.P. (1966). Coagulant activity of Tiger Snake (Notechis scutatus scutatus) venom. Nature 211(5051). 873-875.
6. Picciuto, R., Marshall, L.R.(1994). Unique anticoagulant activity of Tiger Snake venoms (Notechis species). In press
7. Minton, S and Minton, M.R. (1969). Venomous Reptiles. Scribners New York.
8. Nahas, L., Denson, K.W.E. and Macfarlane, R.G. (1964). A study of the coagulant action of eight snake venoms. Thromb. Diath. Treat. 12 355-367.
9. Thesleff, S. (1979). Reptile toxins and neurotransmitter release. Eds. I.W. Chubb and L.B. Geffen. Neurotoxins. Fundamental & Clinical Advances. Centre for Neurosciences. Flinders University. 19-25
10. Broad, A. J., Sutherland, S. K., Coulter, A.R. (1979). The lethality in mice of dangerous Australian and other snake venoms. Toxicon (17). 664-667.
11. Mirtschin,P.J. and Davis,R. (1982). Dangerous Snakes of Australia. Rigby. Adelaide.
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