| Snakebite | |
|---|---|
| Classification and external resources | |
Cobra Naja naja |
|
| ICD-10 | T63.0, T14.1, W59 (nonvenomous), X20 (venomous) |
| ICD-9 | 989.5, E905.0, E906.2 |
| DiseasesDB | 29733 |
| MedlinePlus | 000031 |
| eMedicine | med/2143 |
| MeSH | D012909 |
A snakebite is an injury caused by a bite from a snake, often resulting in puncture wounds inflicted by the animal's fangs and sometimes resulting in envenomation. Although the majority of snake species are non-venomous and typically kill their prey with constriction rather than venom, venomous snakes can be found on every continent except Antarctica.[1] Snakes often bite their prey as a method of hunting, but also for defensive purposes against predators. Since the physical appearance of snakes may differ, there is often no practical way to identify a species and professional medical attention should be sought.[2][3]
The outcome of snake bites depends on numerous factors, including the species of snake, the area of the body bitten, the amount of venom injected, and the health conditions of the victim. Feelings of terror and panic are common after a snakebite and can produce a characteristic set of symptoms mediated by the autonomic nervous system, such as a racing heart and nausea.[4][5] Bites from non-venomous snakes can also cause injury, often due to lacerations caused by the snake's teeth, or from a resulting infection. A bite may also trigger an anaphylactic reaction, which is potentially fatal. First aid recommendations for bites depend on the snakes inhabiting the region, as effective treatments for bites inflicted by some species can be ineffective for others.
The number of fatalities attributed to snake bites varies greatly by geographical area. Although deaths are relatively rare in Australia, Europe and North America,[1][6][7] the morbidity and mortality associated with snake bites is a serious public health problem in many regions of the world, particularly in rural areas lacking medical facilities. Further, while South Asia, Southeast Asia, and sub-Saharan Africa report the highest number of bites, there is also a high incidence in the Neotropics and other equatorial and subtropical regions.[1][6][7] Each year tens of thousands of people die from snake bites,[1] yet the risk of being bitten can be lowered with preventive measures, such as wearing protective footwear and avoiding areas known to be inhabited by dangerous snakes.
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The most common symptoms of all snakebites are overwhelming fear, panic, and emotional instability, which may cause symptoms such as nausea and vomiting, diarrhea, vertigo, fainting, tachycardia, and cold, clammy skin.[4][5] Television, literature, and folklore are in part responsible for the hype surrounding snakebites, and a victim may have unwarranted thoughts of imminent death.
Dry snakebites, and those inflicted by a non-venomous species, can still cause severe injury to the victim. There are several reasons for this: a snakebite which is not treated properly may become infected (as is often reported by the victims of viper bites whose fangs are capable of inflicting deep puncture wounds), the bite may cause anaphylaxis in certain people, and the snake's saliva and fangs may harbor many dangerous microbial contaminants, including Clostridium tetani. If neglected, an infection may spread and potentially kill the victim.
Most snakebites, whether by a venomous snake or not, will have some type of local effect. There is minor pain and redness in over 90% of cases, although this varies depending on the site.[4] Bites by vipers and some cobras may be extremely painful, with the local tissue sometimes becoming tender and severely swollen within 5 minutes.[7] This area may also bleed and blister and can eventually lead to tissue necrosis. Other common initial symptoms of pitviper and viper bites include lethargy, bleeding, weakness, nausea, and vomiting.[4][7] Symptoms may become more life-threatening over time, developing into hypotension, tachypnea, severe tachycardia, severe internal bleeding, altered sensorium, kidney failure and respiratory failure.[4][7]
Interestingly, bites caused by the Mojave rattlesnake, kraits, coral snake, and the speckled rattlesnake reportedly cause little or no pain despite being serious injuries.[4] Victims may also describe a "rubbery," "minty," or "metallic" taste if bitten by certain species of rattlesnake.[4] Spitting cobras and rinkhalses can spit venom in their victims' eyes. This results in immediate pain, ophthalmoparesis, and sometimes blindness.[12][13]
Some Australian elapids and most viper envenomations will cause coagulopathy, sometimes so severe that a person may bleed spontaneously from the mouth, nose, and even old, seemingly-healed wounds.[7] Internal organs may bleed, including the brain and intestines and will cause ecchymosis (bruising) of the victim's skin.
Venom emitted from elapids, including sea snakes, kraits, cobras, king cobra, mambas, and many Australian species, contain toxins which attack the nervous system, causing neurotoxicity.[4][7][14] The victim may present with strange disturbances to their vision, including blurriness. Paresthesia throughout the body, as well as difficulty speaking and breathing, may be reported.[4] Nervous system problems will cause a huge array of symptoms, and those provided here are not exhaustive. If the victim is not treated immediately they may die from respiratory failure.
Venom emitted from some types of cobras, almost all vipers, some Australian elapids and some sea snakes causes necrosis of muscle tissue.[7] Muscle tissue will begin to die throughout the body, a condition known as rhabdomyolysis. Rhabdomyolysis can result in damage to the kidneys as a result of myoglobin accumulation in the renal tubules. This, coupled with hypotension, can lead to acute renal failure, and, if left untreated, eventually death.[7]
Since envenomation is completely voluntary, all venomous snakes are capable of biting without injecting venom into their victim. Snakes may deliver such a "dry bite" rather than waste their venom on a creature too large for them to eat.[15] However, the percentage of dry bites varies between species: 50% of bites from the normally timid coral snake do not result in envenomation, whereas only 25% of pitviper bites are dry.[4] Furthermore, some snake genera, such as rattlesnakes, significantly increase the amount of venom injected in defensive bites compared to predatory strikes.[16]
Some dry bites may also be the result of imprecise timing on the snake's part, as venom may be prematurely released before the fangs have penetrated the victim's flesh.[15] Even without venom, some snakes, particularly large constrictors such as those belonging to the Boidae and Pythonidae families, can deliver damaging bites; large specimens often cause severe lacerations as the victim or the snake itself pull away, causing the flesh to be torn by the needle-sharp recurved teeth embedded in the victim. While not as life-threatening as a bite from a venomous species, the bite can be at least temporarily debilitating and could lead to dangerous infections if improperly dealt with.
While most snakes must open their mouths before biting, African and Middle Eastern snakes belonging to the family Atractaspididae are able to fold their fangs to the side of their head without opening their mouth and jab at victims.[17]
It has been suggested that snakes evolved the mechanisms necessary for venom formation and delivery sometime during the Miocene epoch.[18] During the mid-Tertiary, most snakes were large ambush predators belonging to the superfamily Henophidia, which use constriction to kill their prey. As open grasslands replaced forested areas in parts of the world, some snake families evolved to become smaller and thus more agile. However, subduing and killing prey became more difficult for the smaller snakes, leading to the evolution of snake venom.[18] Other research on Toxicofera, a hypothetical clade thought to be ancestral to most living reptiles, suggests an earlier time frame for the evolution of snake venom, possibly to the order of tens of millions of years, during the Late Cretaceous.[19]
Snake venom is produced in modified parotid glands normally responsible for secreting saliva. It is stored in structures called alveoli behind the animal's eyes, and ejected voluntarily through its hollow tubular fangs. Venom is composed of hundreds to thousands of different proteins and enzymes, all serving a variety of purposes, such as interfering with a prey's cardiac system or increasing tissue permeability so that venom is absorbed faster.
Venom in many snakes, such as pitvipers, affects virtually every organ system in the human body and can be a combination of many toxins, including cytotoxins, hemotoxins, neurotoxins, and myotoxins, allowing for an enormous variety of symptoms.[4][20] Earlier, the venom of a particular snake was considered to be one kind only i.e. either hemotoxic or neurotoxic, and this erroneous belief may still persist wherever the updated literature is hard to access. Although there is much known about the protein compositions of venoms from Asian and American snakes, comparatively little is known of Australian snakes.
The strength of venom differs markedly between species and even more so between families, as measured by LD50 in mice. Subcutaneous LD50 varies by over 140-fold within elapids and by more than 100-fold in vipers. The amount of venom produced also differs among species, with the Gaboon viper able to potentially deliver from 450–600mg of venom in a single bite, the most of any snake.[21] Opisthoglyphous colubrids have venom ranging from life-threatening (in the case of the boomslang) to barely noticeable (as in Tantilla).
Snakes are most likely to bite when they feel threatened, are startled, are provoked, or have no means of escape when cornered. Encountering a snake is always considered dangerous and it is recommended to leave the vicinity. There is no practical way to safely identify any snake species as appearances may vary dramatically.
Snakes are likely to approach residential areas when attracted by prey, such as rodents. Practising regular pest control can reduce the threat of snakes considerably. It is beneficial to know the species of snake that are common in local areas, or while travelling or hiking. Areas of the world such as Africa, Australia, the Neotropics, and southern Asia are inhabited by many highly dangerous species. Being wary of snake presence and ultimately avoiding it when known is strongly recommended.
When in the wilderness, treading heavily creates ground vibrations and noise, which will often cause snakes to flee from the area. However, this generally only applies to North America as some larger and more aggressive snakes in other parts of the world, such as king cobras [22] and black mambas,[23] will protect their territories. When dealing with direct encounters it is best to remain silent and motionless. If the snake has not yet fled it is important to step away slowly and cautiously.
The use of a flashlight when engaged in camping activities, such as gathering firewood at night, can be helpful. Snakes may also be unusually active during especially warm nights when ambient temperatures exceed 21 °C (70 °F). It is advised not to reach blindly into hollow logs, flip over large rocks, and enter old cabins or other potential snake hiding-places. When rock climbing, it is not safe to grab ledges or crevices without examining them first, as snakes are cold-blooded and often sunbathe atop rock ledges.
Pet owners of domestic animals or snakes should be aware that a snake is capable of causing injury and that it is necessary to always act with caution. When handling snakes it is never wise to consume alcoholic beverages. In the United States more than 40% of snakebite victims intentionally put themselves in harm's way by attempting to capture wild snakes or by carelessly handling their dangerous pets—40% of that number had a blood alcohol level of 0.1 percent or more.[24]
It is also important to avoid snakes that appear to be dead, as some species will actually roll over on their backs and stick out their tongue to fool potential threats. A snake's detached head can immediately act by reflex and potentially bite. The induced bite can be just as severe as that of a live snake.[4][25] Dead snakes are also incapable of regulating the venom they inject, so a bite from a dead snake can often contain large amounts of venom.[26]
It is not an easy task determining whether or not a bite by any species of snake is life-threatening. A bite by a North American copperhead on the ankle is usually a moderate injury to a healthy adult, but a bite to a child's abdomen or face by the same snake may be fatal. The outcome of all snakebites depends on a multitude of factors: the size, physical condition, and temperature of the snake, the age and physical condition of the victim, the area and tissue bitten (e.g., foot, torso, vein or muscle), the amount of venom injected, the time it takes for the person to find treatment, and finally the quality of that treatment.[4][27]
Identification of the snake is important in planning treatment in certain areas of the world, but is not always possible. Ideally the dead snake would be brought in with the person, but in areas where snake bite is more common, local knowledge may be sufficient to recognize the snake. However, in regions where polyvalent antivenoms are available, such as North America, identification of snake is not a high priority item. Attempting to catch or kill the offending snake also puts one at risk for re-envenomation or creating a second victim, and generally is not recommended.
The three types of venomous snakes that cause the majority of major clinical problems are vipers, kraits, and cobras. Knowledge of what species are present locally can be crucial, as is knowledge of typical signs and symptoms of envenomation by each type of snake. A scoring system can be used to try to determine the biting snake based on clinical features,[28] but these scoring systems are extremely specific to particular geographical areas.
Snakebite first aid recommendations vary, in part because different snakes have different types of venom. Some have little local effect, but life-threatening systemic effects, in which case containing the venom in the region of the bite by pressure immobilization is desirable. Other venoms instigate localized tissue damage around the bitten area, and immobilization may increase the severity of the damage in this area, but also reduce the total area affected; whether this trade-off is desirable remains a point of controversy. Because snakes vary from one country to another, first aid methods also vary.
However, most first aid guidelines agree on the following:
Many organizations, including the American Medical Association and American Red Cross, recommend washing the bite with soap and water. Australian recommendations for snake bite treatment recommend against cleaning the wound. Traces of venom left on the skin/bandages from the strike can be used in combination with a snake bite identification kit to identify the species of snake. This speeds determination of which antivenom to administer in the emergency room.[29]
In 1979, Australia's National Health and Medical Research Council formally adopted pressure immobilization as the preferred method of first aid treatment for snakebites in Australia.[30] As of 2009, clinical evidence for pressure immobilization remains limited, with current evidence based almost entirely on anecdotal case reports.[30] This has led most international authorities to question its efficacy.[30] Despite this, all reputable first aid organizations in Australia recommend pressure immobilization treatment; however, it is not widely adhered to, with one study showing that only a third of snakebite people attempt pressure immobilization.[30]
Pressure immobilization is not appropriate for cytotoxic bites such as those inflicted by most vipers,[31][32][33] but may be effective against neurotoxic venoms such as those of most elapids.[34][35][36] Developed by medical researcher Struan Sutherland in 1978,[37] the object of pressure immobilization is to contain venom within a bitten limb and prevent it from moving through the lymphatic system to the vital organs. This therapy has two components: pressure to prevent lymphatic drainage, and immobilization of the bitten limb to prevent the pumping action of the skeletal muscles.
Pressure is preferably applied with an elastic bandage, but any cloth will do in an emergency. Bandaging begins two to four inches above the bite (i.e. between the bite and the heart), winding around in overlapping turns and moving up towards the heart, then back down over the bite and past it towards the hand or foot. Then the limb must be held immobile: not used, and if possible held with a splint or sling. The bandage should be about as tight as when strapping a sprained ankle. It must not cut off blood flow, or even be uncomfortable; if it is uncomfortable, the person will unconsciously flex the limb, defeating the immobilization portion of the therapy. The location of the bite should be clearly marked on the outside of the bandages. Some peripheral edema is an expected consequence of this process.
Apply pressure immobilization as quickly as possible; if you wait until symptoms become noticeable you will have missed the best time for treatment. Once a pressure bandage has been applied, it should not be removed until the person has reached a medical professional. The combination of pressure and immobilization may contain venom so effectively that no symptoms are visible for more than 24 hours, giving the illusion of a dry bite. But this is only a delay; removing the bandage releases that venom into the person's system with rapid and possibly fatal consequences.
Until the advent of antivenom, bites from some species of snake were almost universally fatal.[38] Despite huge advances in emergency therapy, antivenom is often still the only effective treatment for envenomation. The first antivenom was developed in 1895 by French physician Albert Calmette for the treatment of Indian cobra bites. Antivenom is made by injecting a small amount of venom into an animal (usually a horse or sheep) to initiate an immune system response. The resulting antibodies are then harvested from the animal's blood.
Antivenom is injected into the person intravenously, and works by binding to and neutralizing venom enzymes. It cannot undo damage already caused by venom, so antivenom treatment should be sought as soon as possible. Modern antivenoms are usually polyvalent, making them effective against the venom of numerous snake species. Pharmaceutical companies which produce antivenom target their products against the species native to a particular area. Although some people may develop serious adverse reactions to antivenom, such as anaphylaxis, in emergency situations this is usually treatable and hence the benefit outweighs the potential consequences of not using antivenom.
The following treatments have all been recommended at one time or another, but are now considered to be ineffective or outright dangerous. Many cases in which such treatments appear to work are in fact the result of dry bites.
In extreme cases, where the victims were in remote areas, all of these misguided attempts at treatment have resulted in injuries far worse than an otherwise mild to moderate snakebite. In worst case scenarios, thoroughly constricting tourniquets have been applied to bitten limbs, completely shutting off blood flow to the area. By the time the victims finally reached appropriate medical facilities their limbs had to be amputated.
Most snakebites are caused by non-venomous snakes. Of the roughly 3,000 known species of snake found worldwide, only 15 percent are considered dangerous to humans.[1][4][50] Snakes are found on every continent except Antarctica.[1] The most diverse and widely distributed snake family, the colubrids, has approximately 700 venomous species,[51] but only five genera—boomslangs, twig snakes, keelback snakes, green snakes, and slender snakes—have caused human fatalities.[51]
Since reporting is not mandatory in many regions of the world,[1] snakebites often go unreported. Consequently, no accurate study has ever been conducted to determine the frequency of snakebites on the international level. However, some estimates put the number at 5.4 million snakebites, 2.5 million envenomings, resulting in perhaps 125,000 deaths.[1] Others estimate 1.2 to 5.5 million snakebites, 421,000 to 1.8 million envenomings, and 20,000 to 94,000 deaths.[1] Many people who survive bites nevertheless suffer from permanent tissue damage caused by venom, leading to disability.[7]
Most snake envenomings and fatalities occur in South Asia, Southeast Asia, and sub-Saharan Africa, with India reporting the most snakebite deaths of any country.[1] In India almost all of these deaths are caused by the Big Four, consisting of the Russell's viper, Indian cobra, saw-scaled viper, and the common krait. In Burma 80 percent of the approximately 1000 deaths each year from snake bite are caused by the Russell's Viper. In Thailand, the monocled cobra is responsible for the majority of snakebite fatalities.[52] Tea plantations sometimes are associated with some elapids such as the common cobras and the king cobra.
In the Neotropics, the lance-headed vipers inflict the majority of fatal bites, although of the many known species, only two, the common lancehead and terciopelo, are responsible for most cases.[7][53] The tropical rattlesnake is another important species.
In Africa, the puff adder is responsible for most fatalities,[54] although there are regional differences, with the saw-scaled viper inflicting more bites in Northern Africa, where the puff adder is not normally found.[53] Most bites occur in industrial plantations, which attract many types of snake prey. Banana plantations are associated with vipers such as night adders, while rubber and palm tree plantations attract elapids, including cobras and black mambas.[6] There are also highly venomous colubrids in Africa, such as the boomslang.
In the Middle East, the snakes responsible for most bites tend to be more venomous than European species, but deaths are rare, with some estimating perhaps 100 fatal bites annually.[6] The Palestine viper and Lebetine viper are the species involved in most bites.[6] Larger and more venomous elapids, such as the Egyptian cobra, can also be found throughout the Middle East.
In Europe, nearly all of the snakes responsible for venomous bites belong to the viper family, and of these, the coastal viper, nose-horned viper, asp viper, and Lataste's viper inflict the majority of bites.[6] Although Europe has a population of some 731 million people, snake bites only kill about 30 people each year, largely due to wide access to health care services and antivenom, as well as the relatively mild potency of many native species' venom.[6]
In Australia, the only continent where venomous snakes constitute the majority of species,[55] the Taipan, tiger snake and Eastern brown snake inflict virtually all reported venomous bites,[6][55] with the latter responsible for perhaps 60% of deaths caused by snakebite.[55] Although Australian snakes are highly venomous, wide access to antivenom has made deaths exceedingly rare, with only a few fatalities each year.
Most of the Pacific Islands are free of terrestrial snakes;[6] however, sea snakes are common in the Indian Ocean and tropical Pacific Ocean, but are not found in the Atlantic Ocean or the Caribbean, Mediterranean or Red Seas.[14] While the majority of species live close to shorelines or coral reefs, the yellow-bellied sea snake can be found in the open ocean.[14] Over 50% of bites inflicted by sea snakes, which are generally not aggressive, occur when fishermen attempt to remove snakes which have become tangled in fishing nets.[14][56] Symptoms may appear in as little as 5 minutes or take 8 hours to develop, depending on the species and region of the body bitten.[14] Although sea snakes are highly venomous, about 80% of reported bites end up being dry.[14][57] The advent of antivenom and advances in emergency medicine have reduced fatalities to about 3% of snakebite cases.[14]
Of the 120 known indigenous snake species in North America, only 20 are venomous to human beings, all belonging to the families Viperidae and Elapidae.[4] However, in the United States, every state except Maine, Alaska, and Hawaii is home to at least one of 20 venomous snake species.[4] Most snakebite related deaths in the United States are attributed to Eastern and Western diamondback rattlesnake bites.[4] Further, the majority of bites in the United States occur in the southwestern part of the country, in part because rattlesnake populations in the eastern states are much lower.[58] The state of North Carolina has the highest frequency of reported snakebites, averaging approximately 19 bites per 100,000 persons.[20] The national average is roughly 4 bites per 100,000 persons.[20]
Worldwide, snakebites occur most frequently in the summer season when snakes are active and humans are outdoors.[1][59] Agricultural and tropical regions report more snakebites than anywhere else.[1][53] Victims are typically male and between 17 and 27 years of age.[4][59][60] Children and the elderly are the most likely to die.[4][27]
Snakes were both revered and worshipped and feared by early civilizations. The ancient Egyptians recorded prescribed treatments for snakebites as early as the Thirteenth dynasty in the Brooklyn Papyrus, which includes at least seven venomous species common to the region today, such as the horned vipers.[61] In Judaism, the Nehushtan was a pole with a snake made of copper wrapped around it, similar in appearance to the Rod of Asclepius. The object was considered sacred with the power to heal bites caused by the snakes which had infested the desert, with victims merely having to touch it in order to save themselves from imminent death.
Historically, snakebites were seen as a means of execution in some cultures. In medieval Europe, a form of capital punishment was to throw people into snake pits, leaving victims to die from multiple venomous bites. A similar form of punishment was common in Southern Han during China's Five Dynasties and Ten Kingdoms Period and in India.[62] Snakebites were also used as a form of suicide, most notably by Egyptian queen Cleopatra VII, who reportedly died from the bite of an asp—likely an Egyptian cobra[61][63]—after hearing of Mark Antony's death.
Snakebite as a surreptitious form of murder has been featured in stories such as Sir Arthur Conan Doyle's The Adventure of the Speckled Band, but actual occurrences are virtually unheard of, with only a few documented cases.[62][64][65] It has been suggested that Boris III of Bulgaria, who was allied to Nazi Germany during World War II, may have been killed with snake venom,[62] although there is no definitive evidence. At least one attempted suicide by snakebite has been documented in medical literature involving a puff adder bite to the hand.[66]
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