By Jerry G. Walls
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
) 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.
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.
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.
What follows are my suggestions for the five most deadly venomous snakes — snakes that not only can kill mice in a laboratory but can and do kill large numbers of humans each year. You will immediately notice the strong correlation between the snakes on this list and the economic standing of the countries in which they occur. The fact is that snakes in countries that are heavily industrialized — in which agriculture is conducted largely from the seat of a tractor and in which trails through forests have been carefully scoured of snakes — are unlikely to cause many bites and even fewer deaths when rapid transport to hospitals and antivenom are sure to be available. Deaths from snakebites are much more likely where field workers wear minimal footwear and clothing, live in open houses and lack access to antivenoms and supportive care.
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The common lancehead (Bothrops atrox) is widely known as the fer-de-lance. This species utilizes hemotoxic venom, which causes massive swelling and the destruction of the circulatory system.
The listing is organized by region to try to keep things “even” (otherwise Asia could provide all five deadly snake species). It also includes some snakes that are almost as deadly as the five major types. Note that in some cases, my choices are actually groups of closely related, very similar species that often replace each other geographically, rather than a single biological species.
(Bothrops atrox, also known as the fer-de-lance)
The genus Bothrops currently contains about 30 species of heavy-bodied terrestrial pit vipers found (with one exception) in South America. Although all are dangerous, the common lancehead, of central South America, is the most notorious, causing perhaps a few hundred deaths a year even when antivenom is available.
Large Bothrops atrox may be at home near villages and gardens, increasing bite incidence. Identification of Bothrops species is complicated and subject to much dispute, but this species has a broad dark brown stripe back from the eye and reaches at least 4 to 6 feet in length. The common lancehead’s venom is abundant, potent and of the classic hemotoxic type, causing pain and massive swelling followed by destruction of the circulatory system (including the kidneys) and extensive tissue destruction. Bites from this snake, even when not fatal, may cause loss of limbs.
This large (males sometimes attain 5 feet in length), heavy-bodied rattlesnake has a discontinuous range from southern Mexico over parts of Central America and northern South America and then a more uniform range south of the Amazon into Argentina. The neotropical rattlesnake is easily recognized by the sharply defined dark diamonds on the back in combination with a pair of broad dark brown stripes running from the top of the head onto the body, the neotropical rattlesnake in the northern part of its range has a bite typical of any other large rattlesnake, with pain and extensive swelling near the bite and massive destruction of blood and tissues.
In Brazil and Argentina, however, the venom of Crotalus durissus seldom produces swelling and has a dangerous neurotoxic component that causes facial paralysis and sometimes whole-body paralysis. The bite of southern neotropical rattlesnakes often results in death from kidney failure in addition to circulatory collapse, with untreated mortality rates as high as 70 percent. Today, the neotropical rattlesnake still causes perhaps a hundred deaths a year, especially in Brazil.
Ocellated Carpet Viper
Although it appears on few lists of the most deadly venomous snakes, this 2-foot carpet viper of the savannas of western Africa appears to cause more deaths than all other African species combined, perhaps more than 20,000 per year among agricultural workers.
Once considered part of Echis carinatus (now restricted to Asia), the ocellated carpet viper is recognized by details of scale counts as well as small round white spots in regular rows down the back. The ocellated carpet viper’s venom is slow acting and largely hemotoxic in action, causing extensive pain and swelling near the bite, with massive tissue damage and blistering. These local symptoms are followed within a day by bleeding throughout the body and collapse of the circulatory system. Transfusions may help make antivenom treatments more successful. Antivenoms for other carpet viper species may be relatively ineffective.
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In theory, a full bite from a black mamba (Dendroaspis polylepis) contains enough venom to kill 10 humans. This snake is known to stand its ground when cornered and can deliver a very vicious bite.
At 10 feet or more, the black mamba is one of the largest elapids, and it also is considered one of the most deadly snakes. The venom has an LD50 of 0.28 mg/kg, with enough venom in a full bite to (in theory) kill 10 humans. Black mambas are noted for standing their ground when cornered, being vicious biters under such circumstances. They frequent semipermanent territories, often near villages, and are common over much of southern and eastern Africa in savanna to open forest. The venom is neurotoxic, seldom producing more than local pain with minor swelling, but within 15 minutes to a few hours paralysis of the facial muscles and then limb muscles occurs, followed within six or seven hours by an inability to breathe. If a black mamba snakebite victim is not given treatment within hours of the bite, death is not uncommon (there’s nearly a 100-percent chance of death without treatment). Hospitalization with artificial respiration and very large doses of antivenom is necessary for several days, as attacks of paralysis may occur days after the black mamba bite even when antivenom is given.
The common cobra of most of Pakistan, India and Sri Lanka, Naja naja is recognized by typically having a pair of pale spots (the spectacles) on the spread hood. The closely related N. kaouthia of Southeast Asia generally has a single dark-centered pale spot on the hood, but cobra species are variable and not easy to distinguish.
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Because they are known to hunt around human dwellings in rural areas, Indian cobras (Naja naja) often come into contact with people. In the worst cases, a bite victim will die from respiratory failure in from as little as 15 minutes to several hours if supportive care is not available.
Indian cobras may reach 5 to 6 feet in length and are noted for hunting around houses in rural areas, bringing them into constant contact with humans. The bite of both N. naja and N. kaouthia produces a mixture of neurotoxic and hemotoxic signs, starting as severe pain and rapidly spreading swelling and tissue damage, progressing within an hour or less to paralysis of the eyelids (ptosis), mouth area and other facial muscles.
Death may occur from respiratory failure within as little as 15 minutes of the bite from an Indian cobra, but more typically takes several hours. No current numbers are available for deaths due to cobra bites, but certainly several thousand people die each year in areas where antivenom and supportive care are not available.
Although seldom even 4 feet long and relatively slender and not inclined to bite, Indian kraits and the related many-banded krait (Bungarus multicinctus) probably cause thousands of deaths each year across southern Asia. What appears to be a many-banded krait last year bit and killed the noted American herpetologist J. B. Slowinski while he was collecting in Myanmar.
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The Indian krait (Bungarus caeruleus) is a glossy brown to black snake that delivers a potent neurotoxic venom. In its range of Pakistan through India and Sri Lanka, people often get bitten because they roll onto a snake in their sleep after the snake has entered their homes in search of prey.
Indian kraits (found in Pakistan through India and Sri Lanka) are mostly glossy brown to black with pairs of narrow white bands; many-banded kraits (Southeast Asia) are evenly banded in black and white with a mostly white belly. Both species have the characteristic “ridgepole” or keeled back of the genus.
Indian krait bites inject a potent neurotoxic venom that causes little or no pain and swelling near the site of the bite. In fact, many people are bitten while sleeping as a result of their rolling onto the Indian krait after it enters a house to hunt rodents. Although vomiting may occur within an hour of a bite, often there are no signs noted until the morning, by which time the Indian krait snakebite has caused paralysis of the facial muscles and sometimes the entire body. Death from respiratory failure may occur after one to six hours, and Indian krait bites may be 100-percent fatal if not immediately treated.
This large (6 feet or longer), beautifully marked viper may be a complex of species ranging from Pakistan to Sri Lanka and Myanmar to Indonesia and Taiwan. Its color pattern of large dark lozenges in regular rows on a tan body is distinctive in the area, and the snake is well known and greatly feared.
A nocturnal hunter near fields and villages, Russell’s viper often is in contact with humans and may cause several thousand deaths a year. The action of Russell’s viper venom varies at least somewhat with geography and subspecies, but in most areas the venom is hemotoxic, causing pain and swelling along with tissue destruction and bleeding; death often results from cerebral hemorrhage and kidney failure. In Sri Lanka and perhaps other areas, the venom of Russell’s viper has a strong neurotoxic component as well that causes the typical signs of facial muscle paralysis and respiratory distress.
Indian Saw-Scaled Viper
Although small (adults are often only 10 inches long, and seldom more than 30 inches) and slender with a short head, Indian saw-scaled vipers are among the most common snakes of Sri Lanka and southern India, leading to many snakebite accidents among field workers.
Like the other Echis species, the scales of its back are strongly keeled, the keels of the scales on the lower sides having serrations. The Indian saw-scaled viper features a wavy white stripe along each lower side and usually a cross-shaped white mark on top of the head. The bite is painful and causes swelling almost immediately, though the swelling seldom spreads far from the snakebite site. The venom of the Indian saw-scaled viper is strongly hemotoxic, causing massive internal bleeding and circulatory system collapse as little as 10 hours after the snakebite. Recent studies indicate that the Indian saw-scaled viper may cause thousands of deaths each year where immediate antivenom treatment is unavailable.
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Most Australian snakebite deaths in the past 20 years have resulted from the brown snakes: the eastern (Pseudonaja textilis, pictured here) and the western (P. nuchalis). Bites typically result from people attempting to kill or handle these diurnal species.
Eastern Brown Snake
The majority of deaths from snakebites in Australia over the past two decades (perhaps 15 of fewer than 30 total) have resulted from bites of the eastern brown snake and the western brown snake (P. nuchalis). Together, these two snake species span virtually the entire Australian continent, with the eastern brown snake also extending into New Guinea and causing some deaths there.
Both brown snakes are large (5 feet), active, diurnal snake species that seem to invite attacks from thoughtless humans: Most bites occur during attempts to kill or handle the brown snakes. Bites produce a mixture of hemotoxic and neurotoxic symptoms (a feature shared with the taipans). A bite produces immediate pain, swelling of the lymph nodes and bleeding from wounds, the mouth, anus and bladder, as well as fainting spells. The blood fails to clot and there may be circulatory shock, but the cause of death typically is from respiratory failure brought on by neurotoxic elements in the venom of the brown snake. Signs of attacks on the central nervous system appear about 30 minutes after the snakebite and include drooping of the eyelids and inability to open the mouth. Deep unconsciousness may occur in as little as an hour, with death often following if not prevented by artificial respiration and antivenom.
(Notechis scutatus and relatives)
Tiger snakes are highly variable brown to black snakes of southern Australia that can often attain a length of 5 feet. Although well known for the toxicity of their venom, they are considered relatively calm snakes that are not likely to bite unless greatly stressed. During the early days of Australian settlement they caused many accidental deaths, but antivenom is now widely available and deaths from tiger snake bites are rare. Unlike the brown snakes, their bites produce mostly neurotoxic symptoms leading to respiratory collapse without greatly affecting the circulatory system.
There you have my picks for the most deadly venomous snakes today. It is very disturbing to see that even in the 21st century, death from snakebites can still be listed among the leading causes of death in dozens of African and Asian countries, with little chance for improvement in the near future.