The classification is important because a certain correlation exists between snake family and pathology. This correlation is not absolute. Studying the fangs in the mouth of a dead snake which has been brought in can help determine the treatment. However, it is better to be cautious when doing this (the bite reflex can continue for over 1 hour after death even after decapitation). It can be useful to have on hand a number of photos or a poster illustrating most of the snakes in the surrounding area. On the basis of these pictures, a patient can sometimes indicate which animal has bitten him or her. [Other characteristics such as the scale structures are also useful for identification, yet fall within the area of the specialist. Thus, in the Colubridae the eye scale touches the upper lid shields, while in Viperidae the eye is separated by at least one row of scales from the upper lid shields.]

Vipers and pit vipers have very long hollow fangs in the front of the mouth. These animals are so-called solenoglypha ("solen" = tube; "glypha" = tooth). When the mouth is closed the fangs lie folded up against the roof of the mouth. Behind the fangs is a diastema (space without teeth). Vipers are slow, heavy snakes and are generally "sit-and-wait" predators. In order to move they generally push themselves flat over the ground. Venomous European vipers have vertical pupils. Non-venomous snakes in Europe have round pupils. There are no native vipers in the New World (however, there are pit-vipers).

Russell´s viper (Daboia russelli = Vipera russelli = "tic-polonga") is one of the most dangerous Asian snakes. It can hiss loudly through its large nostrils. It is quite long (up to 150 cm), has a heavy, muscular body with a thin tail and a characteristic colour pattern ("chain viper"), composed of oval-shaped rings on the back and flanks. This nocturnal animal is often lethargic and will avoid dense jungle. Five subspecies can be distinguished: D.r.russelli in India, D.r.pulchella in Sri Lanka, D.r.siamensis in Southeast Asia (i.e. Burma, Thailand and continental China), D.r.formosensis in Taiwan and D.r.limitis in Indonesia. This is important, because antivenom from another country is often not effective on the local subspecies. The symptomatology too will depend on the subspecies: pituitary haemorrhages and chemosis in Burma and southern India, anticholinesterase-resistant neurotoxicity in India and Sri Lanka, haemorrhages with all subspecies. Sometimes the animals are confused with harmless snakes such as Python sp., Eryx conicus, Spaleropsis diadema in India and Boiga multimaculata and Oligodon cyclurus in Thailand. Females produce 20-60 live young around June-July (India and Burma). The young snakes measure 11-25 cm and are cannibalistic. Bites by these animals display strikingly few local signs, yet can give rise to neurotoxic effects (this is exceptional among vipers).

Bitis nasicornis ("Rhinoceros viper") is an African viper with a beautiful scale pattern and two characteristic small horns on the tip of its snout. The animal often has hues of carmine, olive green, violet, brown and purple. On the skull there is often a black triangular spot with the point directed forwards. The snake measures an average of 70 to 80 cm.

This snake is sometimes called the "levantine viper" or the "blunt nosed viper". The animals are found in the Mediterranean area, the Middle East and in northern Africa. Antivenom against these animals is sometimes included in polyvalent antisera. The venom contains among other substances one that causes a specific activation of blood coagulation factor V. A similar substance is found in the venom of Russell´s viper. Other vipers in the Middle East include the V. xanthina, V. palestinae and Cerastes species.

Cerastes cerastes is also known as the Desert horned viper. This viper often has very typical small horns above the eyes. They sometimes lie burrowed in the sand and they are quite well camouflaged. They can produce a rasping warning noise by rubbing their scales over one another. Side-winding is a typical way of moving. A related species is Cerastes vipera (Avicenna viper).

The common viper is known as Vipera berus. A nocturnal animal with a zigzag band running from the neck to the tip of the tail. Variable colour. To be distinguished from Vipera ammodytes (sand viper or long-nosed viper) by the fact that the latter has a protuberance on the snout. It can be mistaken for two other European vipers, Vipera aspis and Vipera latastei (Lataste´s viper).

The gaboon viper is a long (up to 150 cm), powerful African snake with a wide diameter and often with white-black geometrical scale patterns. The head is white on the top and the snake often has one or two black triangles on the side of the skull. A West African subspecies (Bitis gabonica rhinoceros) has small horns and might be confused with Bitis nasicornis. The snake chooses a wooded environment and is generally exceptionally well camouflaged in the leaves on the ground. It is not a particularly aggressive animal, yet it poses a considerable risk given its size (averaging 5 kg) and the length of its fangs (easily 4 cm).

The saw-scaled vipers are among the most important venomous snakes in the world, since it is estimated that they are responsible for 50% of the global mortality caused by snakes. Echis carinatus actually forms a species complex: Echis coloratus (carpet viper), E. ocellatus, E. leucogaster, E. pyramidum, E. multisquamatus). These are small, thin creatures. The snakes generally measure only 50-60 cm, rarely up to 80 cm. They can be red, brown, grey or olive-coloured with small light spots on the back. There are chevrons (V-shaped marks) on the flanks. An arrow is sometimes visible on top of the head. Dasypeltis sp. (egg-eating snakes) can look a great deal like Echis sp.

Causus rhombeatus, C. maculatus and other related species known as night adders cause severe pathology, although bites are quite rare. Antivenom against these animals is included in some polyvalent antivenom-cocktails.

The pit vipers or Crotalidae get their name from the presence of two pits at the front of the head, about halfway between the eyes and the nostrils. These contain infrared sensors with which the animal can better locate its prey. Besides the heat-sensitive pits in the maxillae, a triangular head, vertical pupils and simple subcaudal scales are characteristic for the Crotalidae. By contrast, coral snakes (North American elapids) and non-venomous North American snakes have a double row of scales behind the anal plate.

Agkistrodon sp. are found in both the New and the Old World. For example, there is a high risk of bites from Agkistrodon halys in Iran and the small snake Agkistrodon blomhoffii is known as mamushi in the Far East. Agkistrodon piscivorus is the North American semi-aquatic "cottonmouth water moccasin", a pit viper. This snake intimidates a potential enemy by opening its characteristic white mouth. Agkistrodon contortrix or copperhead is an American pit viper. Its bite can have serious consequences. Fortunately, cases are fairly infrequent.

The notorious South American bushmaster or Lachesis muta borrows its name from one of the three Moirai or Greek Fates, all three daughters of Zeus and Themis (Clotho who spins the thread of life, Lachesis who determines its length, and Atropos who cuts it). Lachesis muta is a rather rare, long (over 2 metres is nothing exceptional), often grey-brown snake with a diamond-shaped pattern on the back and flanks. Reddish-brown and yellow-brown forms are also found. Often there is a dark stripe from the eye to the corner of the mouth. The tip of the tail can be quickly shaken back and forth, but the animal does not have a real rattle like a rattlesnake. The animal has a characteristic, very rough median dorsal row of scales. It is the only oviparous pit viper in South and Central America.

Bothrops atrox also known as the Fer-de-lance. Its name comes from the sharp triangular head (like the point of a lance). Sometimes other Bothrops sp. are also called "Fer-de-lance", leading to confusion. Bothrops asper, a related snake, sometimes receives the popular name terciopelo (Sp. "velvet"). Bothrops atrox is found in Central and South America. The colour is often grey-brown or reddish-brown, there are often dark cross bands and the tail can be yellowish. Full-grown animals are around 150 cm long. It is responsible for numerous bites in Latin America.

Malayan pit vipers are feared in endemic areas and are responsible for a large number of bites. It is a very important asian snake.

These snakes are also known as habu's, asian lanceheads, green pit vipers or as bamboo pit vipers. Often they live in trees, which can be important information for the doctor (Russell´s vipers do not live in trees). They are found in Asia. Popular names are often used, but can be confusing. So is Agkistrodon rhodostoma known as the Malayan Green Pit Viper. Trimeresurus popeiorum is known as Pope's Pit Viper. Trimeresurus albolabris is known as the White Lipped Tree Viper. Trimeresurus gramineus and T. stejnegeri are other well known species.

Palm vipers. Dangerous bites by these animals are very rare.

These animals (mole vipers or burrowing vipers) were earlier classified among the Viperidae, but currently form a separate family with over 50 species. They are primarily found in Africa. They are rather small animals, although some individuals can be as long as 1 metre. They live primarily underground. They are oviparous and lay 2-11 eggs. Bites are rare, but can have serious consequences. In Atractaspis sp. the fangs are joined to the very short maxilla, but the other teeth are largely atrophic. The maxillae, the frontal bone and prefrontal bone are connected via a complex articulation and the hollow fangs can be moved sideways, even without opening the mouth. In Africa they are imitated by Calamelaps, a harmless colubrid. The venom of Atractaspis engaddensis contains an extremely powerful cardiotoxin, the so-called "sarafotoxin", a word deriving from the Hebrew name of the animal, "Saraf ´En Gedi".

This family includes the cobras, mambas, kraits and coral snakes. The venom produces primarily local necrosis and paralysis. Elapids have moderately short, immobile fangs on the maxillae, at the front of the mouth (proteroglypha) ["protero" = in front]. They cannot be folded backwards as in vipers. Often these snakes have small teeth behind the fangs and sometimes there is a small diastema.

A cobra often raises its head and neck when it is threatened. The animals are characterised by the typical "hood", the widening of the neck caused by spreading its cervical ribs when threatened. With the Indian cobra (Naja naja naja) the typical dorsal "glasses" thus become visible (spectacled snake). Another cobra is the "monocellate cobra" (Naja kaouthia) which displays just a single circle on its neck. The false cobra (Malpolon moilensis) is a harmless colubrid and mimics the hood of a cobra. The small snake Heterodon platyrhinus ("hog-nosed snake") also imitates the spread neck of a cobra when it feels threatened. The king cobra (Ophiophagus hannah) is a very large Asian elapid, often grey or black, with transverse white or yellow stripes on the back. Some call this animal the "hamadryad" (Gr. "tree nymph"). This snake typically eats other snakes. Cobras have religious significance in India and in some other countries. There is some confusion about the taxonomy of the Asian cobras. Within a given population the snakes can vary widely in appearance. Taxonomists seek to solve this through multivariate analysis of morphological (phenotypical) characteristics and via mitochondrial DNA analysis. The modern nomenclature of these animals:

The geographical distribution zones of a number of these animals overlap with one another, yet in large areas only a single type is found, which facilitates "field work". Overlapping is found with:

• Naja kaouthia and N. siamensis in Thailand, Cambodia, Vietnam
• Naja kaouthia and N. sumatrana in northern Malaysia and southern Thailand
• Naja kaouthia and N. naja in northeast India
• Naja naja and N. oxiana in northwest India and Pakistan

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Elapids also live in the New World: the coral snakes (Micrurus and Micruroides). They often have a beautiful colour pattern. E.g. Micrurus fulvius, generally a black snout followed by yellow, black and red bands. Some other snakes (such as Lampropeltis sp.) mimic this pattern. See also above "Batesian mimicry". A mnemonic device for the colour bands in North America: "red on yellow, kill a fellow; red on black, venom lack". This phrase does not work in other geographical areas, however.

Kraits (Bungarus sp.) are found in Asia and have a triangular (cross-section) or a laterolateral flattened body, typical hexagonal median dorsal scales and often a white-black or yellow-black banded pattern. The best known are B. caeruleus (Indian or common krait), B. candidus (Malayan krait), B. multicinctus (Chinese krait) and B. fasciatus (banded krait). It is important to distinguish the species. For example, the antivenom against B. fasciatus (alternating yellow and black bands) is completely useless against bites by B. candidus (black saddle-shaped markings and white belly). Often the animals are distinctly passive during the day. At night, however, they are active and they sometimes enter houses and bite. People with krait bites generally experience remarkably little local pain.

Mambas are only found in sub-Saharan Africa. These venomous snakes are notorious. They belong to the genus Dendroaspis: D. polylepis (black mamba), D. viridis (Western green mamba), D. angusticeps (Eastern green mamba) and D. jamesoni (Jameson´s mamba).

The taxonomical classification is controversial, but these animals can be classified among the Elapidae or be grouped in their own family. Taxonomically they are broken down into the Hydrophiinae (real sea snakes) and the Laticaudinae (sea kraits). In some taxonomic diagrams these groups get the status of family: Hydrophiidae and Laticaudae. Species belonging to other groups (Homalopsinae, Natricinae, Acrochordidae) do not pose any medical problems. Species belonging to the Laticaudinae lay their eggs on land, but Hydrophiinae are viviparous in the water. [Several less important snakes from other groups have also adapted to living in water: fresh water in ponds and rivers, brackish water in lagoons and estuaries, mangrove forests and seacoasts]. Adult sea snakes vary in length from 50 cm (Hydrelaps darwinensis) to more than two metres (Astrotia stokesii, Aipysurus laevis, Hydrophis elegans).

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Identification is difficult for non-herpetologists. They have immobile fangs in front, just as cobras. Often these fangs are small and cannot penetrate a neoprene diving suit. The animals are morphologically adapted to their environment. The tail is laterally flattened. Laticaudae have broad ventral scales ("gastrostega"), in contrast to the other sea snakes, which have very small fine scales that overlap little or not at all with one another and therefore facilitate swimming backwards and forwards. The spinal column in Hydrophinae is quite weak (they do not use their body to move on land). Snakes have a preference for specific depths and prey. Species that eat all kinds of fish have the same "classical snake" morphology whereas specialised eel eaters, for example, have a small head and a heavy posterior (shaped like a plesiosaurus). The cloaca is hermetically closed when diving. The nostrils have valves to keep the water out. These valves contain spongy, erectile tissue. The nostrils are on top of the snout in the Hydrophiinae, while in the Laticaudae they are more lateral. The position is important and enables the snake to take quick breaths without raising its head out of the water (a dangerous moment, because various birds are major enemies of sea snakes). Sea snakes can easily remain under water for 30 minutes, sometimes for several hours. They have a diurnal cycle, and some snakes sleep underwater. The lung extends to the cloaca and has both a respiratory and a hydrostatic role. It is estimated that around ¹/₅^th of the oxygen demand is absorbed through the skin and that virtually all of the CO₂ can be eliminated by this route. Since they are cold-blooded their oxygen demand is 7 times lower than that of a mammal or bird of the same weight. In normal circumstances there is no evidence of lactate acidosis after a long dive. The lung is thin and elongated and displays regional specialisation. The tracheal lung has dense vascularisation for regional gas exchange. It issues into the bronchial lung, which also contains many blood vessels. The terminus is the saccular lung, which has very little vascularisation and is used for storing air. The wall of the latter structure is very muscular. Many animals dive deeper than 50 metres, sometimes even to 100 metres. They avoid diving through the thermocline and generally remain above the sea water isotherm of 18°C. In order to avoid the bends when rising rapidly, the snakes often dive again quickly after having drawn air, so that nitrogen does not have enough time to form gas bubbles in the blood. They also excrete a part of the nitrogen via cutaneous respiration and there is a significant shunting of the blood around the lungs: up to 75% of the blood that is pumped from the heart into the pulmonary artery does not go through the lungs.

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Since the animals live in salt water and their body is hypotonic vis-à-vis sea water, they absorb excess salt. They have to excrete this, but the kidneys produce hypotonic urine (relative to the plasma). The salt gland is located in the lower jaw (posterior sublingual gland) and discharges into the tongue sheath. Surplus salt water is expelled when the animal sticks out its tongue. This is a different mechanism from turtles (salt removal via tear glands), sea crocodiles (via the tongue) or some iguanas (via the nasal gland). Some snakes excrete salt via premaxillary glands. The skin of the snake permits a slight net influx of water, yet is virtually impermeable for salt.

Pelamis platurus can be recognised by its dark top and white-yellow belly, but most sea snakes strongly resemble one another with regard to colour and very often display cross stripes. Some fish such as certain sea eels mimic the form and the zebra stripe pattern of sea snakes almost perfectly (e.g. imitation of the sea snake Laticauda colubrina by the fish Myrichthys colubrinus, Ophichthidae; order of the Anguilliformes). The gills and fins of these fish can only be seen on close inspection. As might be expected, all kinds of algae, Bryozoa, barnacles etc. attach themselves fairly quickly to the skin of sea snakes. The snake rids itself of these by shedding its skin frequently. In the open sea Pelamis cannot rub against the ground to facilitate the removal of the skin. Therefore the animal literally twists itself into a knot and rubs away the old skin with its own body.

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The sea snakes which are most relevant to medicine are Enhydrina schistosa ("Beaked sea snake"), Lapemis hardwickii ("Hardwick´s sea snake"), Laticauda colubrina ("sea krait"), Hydrophis sp. and Pelamis platurus ("yellow-bellied sea snake"). In the coastal waters of Southeast Asia and Australasia they can cause local problems. Laticauda sp. prefer to live in coral reefs, where they seek their prey in caves and crevices. Enhydrina schistosa prefers the turbid waters of estuaries and river mouths as biotope, swimming slowly over the bottom. Sometimes these animals swim great distances upstream in rivers. Pelamis platurus is a real pelagic snake and can sometimes be found in groups composed of enormous numbers in the open ocean, covering large areas. These snakes primarily choose areas where ocean currents converge or where upwelling occurs. These are zones with a great deal of detritus, organic material and many fish which serve as prey. The area of distribution ranges from the western coasts of America to the east coast of Africa. They are not found in the Red Sea, the Mediterranean or the Atlantic Ocean. The depth of the 18°C isotherm in seawater is a major parameter in limiting the distribution of this snake. All the other sea snakes have a much more restricted area of distribution. In 1932 in the Strait of Malacca millions of Astrotia stokesii ("Stoke´s sea snake") were observed in a 3 metre wide band stretching over 100 km. Accidents are sometimes suffered by fishermen (accidental catches) or hunters of these animals (leather industry). Swimmers are sometimes bitten. The local pain is generally minimal, but neurotoxicity, rhabdomyolysis and kidney problems do occur. Blood coagulation is generally normal.

The name derives from the Latin "coluber", which means snake. This group includes more than 50 species distributed over 30 genera which have caused clinically significant venomous bites. Yet, only a few are genuinely dangerous. They have short small fangs on the maxillae at the back of the mouth (Opisthoglypha) [opistho = at the back], so that they have to open their mouth very wide (170 to 180° ) to inject venom. They also require a long contact period to introduce enough venom into the bite wound. Colubrids are often kept as pets, e.g. Elaphe sp. (rat snakes) or Lampropeltis sp. (king snakes, milk snakes). Some colubrids strangle their prey (e.g. Lampropeltis sp.). Thelothornis kirtlandii (vine snake) is a moderately dangerous, very thin snake with horizontal, keyhole-shaped pupils. These animals often slide over the ground with the front part of the body somewhat raised. The boomslang (Dispholidus typhus) in southern Africa is another dangerous colubrid, yet bites by this animal are quite exceptional. Haemorrhages are the most obvious symptom after a bite by a boomslang. Both Rhabdophis tigrinus (Japanese garter snake or yamakagashi) and Rhabdophis subminiatus (red-necked keelback) can inflict fatal bites.

The Boidae include boas and pythons. Constrictor snakes such as the anaconda, boas and pythons are not venomous. Boas are viviparous snakes from the New World and pythons are oviparous snakes from the Old World. Popular names can sometimes cause confusion. Because they must be able to hold their body in small-diameter loops, they have short vertebrae. When they are wrapped around their prey, what makes them so deadly is not that they squeeze so hard, but rather that they can very effectively resist attempts to stretch. Every time the unfortunate prey exhales, the snake contracts a little bit more, and prevents the prey from inhaling. After this has been repeated a few times, the prey simply suffocates.