SASTM Newsflash - Deadly snakes

One of the most-asked questions about snakes is what is the most poisonous. Most snakes are of course not poisonous but venomous. Some plants, like certain mushrooms, are poisonous if eaten but snake venom, on the other hand is not really poisonous but rather venomous. You have to be envenomated for the venom to take effect. So generally speaking, provided that you do not have major lacerations in the throat or open wounds in the stomach, you can safely swallow snake venom without any ill effect. Not that I am recommending this as some people may well be allergic to snake venom and may go into anaphylactic shock if exposed to venom. And with dire consequences.


So then the question changes – which snake is the most venomous. This is quite a debate as the most venomous snake in Africa is often not considered very dangerous – the Boomslang (Dispholidus typus). Drop for drop the Boomslang has the most potent venom of any snake in Africa and the amount of venom that it requires to kill a human is so small that one can barely see it with the naked eye. But this snake is extremely reluctant to bite and seldom does so. There are seldom more than one or two Boomslang bites a year in South Africa and the victims are often snake handlers. Or people who accidentally stood on a Boomslang. In trees they are reluctant to bite and stories of them hanging from branches and biting people as they pass beneath are myths. Even if you had to climb a tree with one or more Boomslang in it, you have virtually no chance of being bitten. Unless you actually grab one. Even then, the snake is unlikely to bite and if severely provoked, it will inflate its neck and once it does this it will strike out with intent. The Boomslang is back-fanged with short fixed fangs far back in the mouth and it is often incorrectly believed that it can only bite onto a small digit like a finger. Like most snakes it can open its mouth very wide – up to 170˚- and easily latch onto an arm or a leg. While most venomous snakes have full control over their venom glands, back-fanged snakes have quite primitive glands and to envenomate their prey they need to chew onto their prey, putting pressure on the venom glands to ensure that venom carries along a duct and runs down a groove in the front of each fang. It often happens that a Boomslang will bite a chameleon, release it, bite it again and repeat the process a few times. This is to make sure that envenomation takes place. It is not uncommon for people to be bitten by a Boomslang (or any other snake for that matter) without being envenomated. Such bites are called dry bites.

Boomslang venom is haemotoxic, affecting the blood clotting mechanism, and very slow to take effect. Victims seldom experience serious symptoms in the first few hours and untreated cases may result in human fatalities after 12 hours or even after a few days. There is a monovalent antivenom made especially for Boomslang envenomation and this is kept at the South African Vaccine Producers and supplied when required.


To measure the strength of snake venom, we used to rely on a test called an LD50. Increasing quantities of a specific snake venom would be injected into 100 mice or rats until 50 of them would succumb. The result would then be reflected in milligram per kilogram animal. Because of rules around animal ethics, these tests are used less often nowadays. One of the problems with such tests is that humans are not mice and the effect of the venom may well be very different on humans. We know, for instance, that many snakes have prey-specific venoms that are highly effective at killing some prey but have little or no effect on humans.


So perhaps the question should be what the deadliest snake is. Some of the sea snakes have venom that is far more toxic than that of land snakes but as people very seldom get bitten by sea snakes (you cannot easily accidentally stand on one) they don’t really feature in snakebite accidents. If you do an internet search you might find that the Australian Taipan is rated as one of the deadliest snakes in the world and this view may well be because of the LD 50 test. But as most Australian Taipans live in the outback where few people live, they seldom account for bites and the first human mortality from a Taipan bite happened very recently. So can you really consider the Australian Taipan to be one of the deadliest snakes in the world if one person has been killed by it in recent years?

In Africa, where there are around 20,000 snakebite deaths a year, the main culprit is the Saw-scaled Viper (also known as the Carpet Viper) of the genus Echis. It has a potent haemotoxic venom that affects the blood clotting mechanism but part of the problem is a lack of antivenom and medical facilities in northern Africa.


In Southern Africa the Mozambique Spitting Cobra (Naja mossambica) accounts for the majority of serious snakebites, followed by the Puff Adder (Bitis arietans) and the Stiletto snake (Atractaspis bibronii). These three snakes account for more than 80% of our serious snakebites but the mortality rate is extremely low as the venoms are cytotoxic, causing severe pain, local swelling and tissue damage and the majority of victims are treated successfully although some may lose limbs or suffer severe tissue damage. Our Stiletto snakes have not caused any human deaths but those further north into Africa have.


Of our cobras the Cape Cobra (Naja nivea) has the most potent venom and along with the Black Mamba (Dendroaspis polylepis), accounts for the most human fatalities. Information on snakebite deaths are hard to come by but totals around 12-24 or more deaths a year in Southern Africa. These snakes have predominantly neurotoxic venom that quickly affects breathing.


As for the most dangerous snake in Africa, or in the world for that matter, I would go with the Black Mamba. It is by far the largest venomous snake in Africa, historically reaching 4.5 m although specimens over 3.8 m are unheard of nowadays. Because of its size it has a lot of venom, it bites readily (often more than once), the venom absorbs rapidly and may have a severe effect on breathing within 20 minutes in serious bites. Although often labelled an aggressive snake the Black Mamba is very shy and nervous and quick to escape when it has the choice but if cornered or hurt it will not hesitate to strike. Another problem is that because of its length it may bite quite high up in the chest region and such a bite would be far more severe than a bite on an extremity.


There is no two-step snake – the mythical snake that bites and you die after two steps. A severe untreated bite from a Black Mamba may kill a human in anything from 4 – 16 hours and in severe cases within an hour but that is unusual. Having said that, people die within minutes from a bee sting or eating a peanut but that is the exception and not the rule.


Communicated by: Johan Marais - African Snakebite Institute