Deadly Snakes

Preparing a list of the most deadly venomous snakes is fraught with difficulties from start to finish. A quick trip to the Internet or a look into some general reference books easily leads to a listing of the “10 most venomous snakes in the world,” all of which by some coincidence come from Australia and generally list the inland taipan (Oxyuranus microlepidotus) as the world’s most venomous snake. In fact, most references state that almost all the snakes of Australia are venomous, and some make you think you’d be taking your life in your hands just by walking through a garden in Sydney. Reality, however, is very different from easily found listings.

For the purposes of this article, I’ve made the assumption that readers want to know which snakes are most dangerously venomous to humans, those that are most likely to kill you if you should have an unfortunate accident. This is not nearly the same assumption that leads to the most published list, where snakes are ranked by the toxicity of their venom to mice in standardized laboratory tests.

Although mouse tests (commonly called LD50 studies) are important so researchers can have standardized baselines for venom studies, they really have very little to do with what happens when a snake bites a human. LD50 is the smallest amount of venom (stated in milligrams venom per kilogram mouse weight) that when injected into a standardized group of mice will kill half the subjects. Many standard lists of the most venomous snakes are based on a study published in 1979 by A. J. Broad and colleagues working out of Australia, which partially explains the strong Australian bias of the list. The well-known venomous snakes of the Americas, Africa and Asia are mostly absent from the list and simply were not included in the study.

Does it do any practical good to list venomous snakes by how easily they can kill mice? Of course not — keepers and readers want to know which snakes will kill humans and which ones have repeatedly done so. Today, snakebite deaths are rare in most of North America, Europe, Australia and other areas where outdoor workers wear shoes and homes are not easily invaded by snakes. Australia itself has had only 30 deaths from snakebite in about 20 years, nearly a quarter of which resulted from people trying to kill or handle venomous snakes (usually Pseudonaja species, one of the brown snakes), while there may be more than 50 deaths per million people a year in Sri Lanka and western Africa. Curiously, the Sri Lankan and West African snakes that cause most mortality (saw-scaled vipers, Echis spp.) are absent from the usual lists or way down on the bottom. There even is doubt that the inland taipan (“the world’s most venomous snake”) has caused any human fatalities in the past several decades.

Many lists also emphasize the very venomous nature of bites from sea snakes (now usually treated as two subfamilies of the cobra family, Elapidae) while failing to mention that most sea snakes have venoms specialized to kill eel prey and produce very little venom in a bite, and that the records of bites are largely due to fishermen carelessly handling the snakes.

Venoms
In snakes, venoms are modified salivary gland secretions that may have developed to help digest prey before it was swallowed. The venom glands are located behind the upper jaw and vary in complexity and size with species. They feed their usually yellowish liquid venom through ducts leading into the bases of externally grooved (family Elapidae) or hollow (family Viperidae) fangs located near or at the front of the upper jaw. The distinction between grooved and hollow teeth is not complete, however, and many larger elapids, such as tiger snakes (Notechis) and taipans (Oxyuranus), have fangs that are essentially as hollow as those of vipers, with the external groove being almost unnoticeable. In elapids, the fangs are fixed to the bone or nearly so; in vipers, the fangs can be rotated out of the way when the mouth is closed.

Venomous snakes can be divided into two major groups: neurotoxic types, such as cobras, coral snakes and sea snakes (family Elapidae), and hemotoxic types, such as the vipers and pit vipers (family Viperidae). Although this may be generally accurate, it falls far short of expressing the true complexity of snake venoms.

Snake venoms are combinations of literally dozens of complex biochemicals, their presence or absence and proportions varying both by species and individually. As a rule, proteolytic enzymes capable of breaking down proteins and myotoxins destroying muscle tissue are widely present in snake venoms. Different neurotoxins that block or destroy parts of the nervous system are typical of elapids, while hemorrhagic toxins preventing blood clotting and containing proteins that attack the circulatory system are typical of vipers.

Many exceptions occur, however. Some vipers (such as the Mojave rattlesnake, Crotalus scutulatus) have significant amounts of neurotoxins in their venom, while some elapids (such as black-necked spitting cobras, Naja nigricollis) seldom produce neurotoxic effects, instead destroying tissue much like a viper.

Other Factors
Detailed research on the incidences and causes of snakebites around the world has identified many problems that make constructing a listing of the most deadly venomous snakes exceedingly difficult.

First might be the time factor — snakebites were more widely reported in many parts of the world during colonial times than after independence and may have more accurately identified the snakes concerned. In many countries today, politics and economics prevent the gathering of accurate data.

Changing taxonomy also makes many identifications of death-causing snakes doubtful. It is now known that venom toxicity and even envenomation symptoms sometimes vary over the range of what is currently treated as a single species of snake.

Simple reports of snakes biting humans may be misleading, as even the most deadly snakes often (perhaps 15 to 50 percent of the time) do not inject venom when they bite humans if restrained or attacked, and even if venom is injected it may be released in minimal quantities. Individual humans also react differently to the venom of a single species, and it has been suggested that in at least some cases the serious effects of a bite are due as much to allergic reactions (anaphylactic shock) as to the tissue-destroying effects of the venom. Next Page>>

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