People often want to know which snake has the most potent venom. Expert Johan Marais provides some answers.
Historically, snake venom was tested on animals, more specifically mice and rats. It seems these tests are becoming less popular nowadays, perhaps because of concerns about animal ethics.
Testing snake venom on mice obviously gives good comparative results, but such tests are not necessarily an indication of how the venom would affect humans. Some snakes have prey-specific venom. In other words it is designed to be most effective on specific animals.
One that comes to mind is the black-headed centipede-eater (Aparallactus capensis). I wrote a small note on the potency of the venom of this snake for the newsletter of the Herpetological Society of Africa about a year ago after I got one to bite my finger and allowed it to chew away for about five minutes. The symptoms were minor, with a bit of itching, minimal swelling and a sore finger for a day or two. However, when this snake bites its prey, the centipede dies almost instantly!
The following is my understanding of how the test, known as an LD 50, is conducted on mice and rats:
Tiny quantities of a specific snake’s venom are weighed out and injected into mice or rats, using 100 individuals per experiment. The moment enough venom is injected to kill 50 of the 100 rodents, the LD 50 for that venom has been established and is usually represented as mg/kg animal.
I have been going through a great deal of literature to obtain some accurate figures for southern African snakes. This is no easy task as different measures are used in different experiments. Some of the LD 50s are reflected for intramuscular injections and others for intravenous injections or as intra-peritoneal injections (those injected into the membrane that lines the walls of the abdominal cavity).
One of the deadliest snakes in the world, on the LD 50 scale, is the Australian taipan (Oxyuranus microlepidotus) with a subcutaneous LD 50 of 0.025 mg/kg. It is followed by the Australian eastern brown snake (Pseudohaja textilis) which has an LD 50 of 0.0365mg/kg.
Then there are several sea snakes with extremely toxic venoms including the yellow-bellied sea snake (Pelamis platurus) with an LD 50 of 0.067mg/kg. This is the only sea snake found off our shores, but is very seldom seen as it is a pelagic snake that lives in streams within the ocean far off our coast. The odd individual gets washed up during storms but they are usually battered by waves and close to dead.
The black mamba (Dendroaspis polylepis) has an LD 50 of 0.32mg/kg and the Cape cobra (Naja nivea) one of 0.72mg/kg. This is about the same strength as that of the West African green mamba (Dendroaspis viridis).
The venom of the rinkhals (Hemachatus haemachatus) has an LD 50 of 2.65 mg/kg — more potent than that of the local green mamba (Dendroaspis angusticeps) at 3.05mg/kg.
What also interests me is how much venom snakes possess at any one time, and how much is needed to kill an adult human.
The black mamba has 100-400mg of venom and needs about 10-15mg to kill an adult. The green mamba has only 60-100mg but also needs 10-15mg for a lethal bite. The Cape cobra has 120-250mg of venom and needs 15-20mg for a lethal bite, compared with the rinkhals that has 80-120mg and needs 50-60mg. The Gaboon adder (Bitis gabonica) has the largest venom yield of our snakes at 400-650mg and needs 90-100mg for a lethal bite.
The boomslang (Dispholidus typus) has minute venom glands with 1.8-8mg of venom but needs a mere 0.07mg for a lethal bite – less venom than one can see with the naked eye. But they are back-fanged and very seldom bite.
Don’t forget that most snakes have full control over their venom glands and can bite without injecting any venom whatsoever.