Snakes
January 29, 2008Snakes
Description
Herpetology is the study of snakes. There are around 2700 snake species, including around 375 venomous snakes with medical relevance. Of the latter, around 200 are potentially lethal. The biotopes vary greatly: from the arctic circle to the equator, and from sea level to 5000 m in elevation. Venomous snakes are not found in Chile, Madagascar, New Zealand, Hawaii and New Caledonia. In Belgium there are a very small number of indigenous vipers (Vipera berus), ringed snakes (Natrix natrix or grass snake) and smooth snakes (Coronella austriaca). The last two snakes are not venomous.
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Snakes are quasi-cylindrical reptiles without limbs. They move using a concertina movement, rectilinear, curvilinear, via “sidewinding” or by a combination of these methods. Some species possess a vestigial pelvic girdle, sometimes with vestigial external spurs, as with boas and pythons. The heart has one ventricle and two atria. The left lung is atrophic, except in boas. The right lung can have an extension in the throat, which is important for the animal because there is airway compression when it swallows large prey. In general, the length of the lung is about one-half of the total body length. The posterior half of the lung can serve as a reservoir while the front part is compressed. In reptiles, the nostrils come out in the mouth cavity, right behind the teeth. They can breathe through their mouth if it is empty. A full mouth blocks respiration. They can tolerate apnea for a fairly long time, because as poikilothermic animals they have a rather low oxygen demand. By exhaling quickly some snakes can produce a hissing noise (cf. the puff adder). Distinguishing between male and female snakes is not easy. To do so it is necessary to examine the cloaca and determine the presence or absence of two hemipenes. There are both oviparous and viviparous species. After birth, the new-born venomous snakes already possess a supply of venom.
2.2 Description, scales and colour
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The entire skin is covered with scales. Each eye is covered with an immobile, transparent scale. The animals have no eyelids. Blinking snakes only exist in Hollywood. All snakes shed their skin from time to time (ecdysis), e.g. several days before laying eggs or after trauma. Before moulting, the eyes have a somewhat milky appearance, and the snake will be virtually blind. In this condition the snake probably feels quickly treatened and tends to bite more easily. Freshly shed skin has a moist-greasy feel (never slimy). The skin dries after several hours. The scales can be smooth or display a central lengthwise ridge or “keel”. Some snakes (e.g. Echis carinatus) can make a warning noise by rubbing the scales over one another. All snakes have 1 row of ventral scales on the belly. In some species, the scales form small horns on the skull, e.g. Cerastes cerastes (North African desert horned viper with protuberances above the eyes), Vipera ammodytes (European horned viper), Agkistrodon acutus (”sharp-nosed viper”) and Bitis nasicornis (rhinoceros viper). Yet here too there are variations. The colour of the animals can vary within a given species, and sometimes there is sexual dimorphism. A snake can sometimes change colour over the course of its lifetime (juvenile specimens are generally lighter coloured). Colour descriptions are thus always relative. Colours develop through the presence of pigments, through optical interference (iridescence), and through the Tyndall light scattering effect, i.e. dispersion of light by small intracellular particles (iridophores) composed of purine crystals. Albino, anerythristic, melanotic and amelanotic animals are found, but are not very common in nature.
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The colour patterns have a specific function in helping the animal survive. Many snakes are cryptically coloured and their colour corresponds to that of their natural environment, making them less easily noticed by their prey or a predator. Stripes and/or spots can act as a camouflage, breaking up the visual outline against the surroundings. Countershading (belly lighter than back) make the animal more difficult to see. A harmless snake can imitate a venomous one when both live in the same environment, i.e. Batesian mimicry (1861, Henry Walter Bates, English naturalist). In this way predators avoid the snake, if they have learned earlier that an animal with such coloration is dangerous. We find a typical example of this in coral snakes (Micrurus sp., venomous) and some colubrids (e.g. Lampropeltis sp., not venomous). Müllerian mimicry also occurs in animals (1878, Fritz Müller, German zoologist), whereby two species resemble one another, to the benefit of both. However this phenomenon is more common among insects. A variant of Batesian mimicry is Mertensian mimicry, in which a non-venomous animal resembles a moderately poisonous instead of a highly venomous one. The idea is that a predator will more easily survive a contact with a moderately venomous animal than with a highly venomous animal. A learning process is thus stimulated, without being punished by death. Naive predators will then be less common, which benefits the prey.
2.3 Description, heat sensors and Jacobson´s organ
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Most snakes have poor hearing and limited visual acuity. By contrast, in the roof of their mouth they possess an extremely sensitive organ, known as a Jacobson´s organ. It consists of two openings lined with sensory cells. The animal flicks out its forked tongue and brings it back into the mouth, inserting the tips into the two openings of Jacobson´s organ. The tongue brings molecules from the environment into the organ. In this way the snake can sense its environment. Pit vipers possess heat-sensitive sensors in small pits located between nostrils and eyes. Pythons and some boas also have such sensors, located on their lip scales. Snakes are very good at perceiving vibrations, e.g. of the ground. Some people use this as a means of prevention, by regularly beating a stick on the ground in front of them when they walk in an area with venomous snakes.
2.4 Description, eyes
Given that practically all snakes lack a retinal fovea, their visual acuity is generally limited. Some tree snakes have rather good sight. The eye contains a non-deformable lens which can be moved forwards and backwards to bring objects into focus. The pupil can be round, oval or slit-shaped. Slit-shaped pupils are customary in nocturnal predators. During the day the slit keeps most of the light out and the retina is not overloaded. At night the iris dilates. This is done more easily with slit-shaped pupils than with round pupils. A thin slit 5 mm long has a circumference of 10 mm. When it dilates to a circle 5 mm in diameter the circumference (16 mm) has increased by a factor of only 1.6. By contrast, when a round iris of 1 mm in diameter has to dilate to a circle 5 mm in diameter, there is a fivefold increase of the circumference, which is mechanically more of a burden for the small iris muscles. A small iris diameter improves the resolution (perpendicular to the slit). Given that they often hunt small animals which move horizontally over the ground, a vertical slit-shaped pupil may give the snake the best sight to e.g. spot a scurrying mouse. This advantage disappears at night, however.
2.5 Description, food and body heat
All snakes are carnivorous. Because they do not have to continually maintain their body at a constant temperature, their food intake requirement is a good deal lower than that of warm-blooded animals. The diet differs from species to species and includes snails, earthworms, insects, eggs, lizards, frogs, fish, rodents or other snakes. Most snakes defecate only rarely. Because chronic “constipation” is most pronounced among sit-and-wait predators – animals for which body weight is of great importance – some people assume that these snakes make good use of the extra weight (3 to 22% of their body weight is faecal material). These animals lie still on the ground and use their heavy intestine as a counterweight in order to be able to strike more quickly with the mouth. Most snakes drink water from time to time. Snakes are ectothermic and prefer one particular temperature. Since the environment of the snake is so important for the animal, it is not unusual for a snake to lie at night on a path or road, where the temperature is somewhat higher than in nearby vegetation. Obviously this increases the chances of an accidental bite being suffered by a nighttime walker. In order to conserve heat, they can roll themselves up (small surface/weight ratio). This is also important to limit transcutaneous loss of water. In cold regions snakes can hibernate, individually or in a group. Since snakes do not have to use energy to continually generate heat, but only require food for homeostasis, movement, growth and reproduction, they can get by with very little food. Due to their low metabolism, they cannot maintain a major effort (e.g. pursuit of prey) for a very long time. In this case, they rapidly develop an oxygen deficit. Many snakes have a limited territory. After having bitten somebody, a snake can generally be found within a rather small radius around the site of the incident, even after several hours.
2.6 Description, venom gland
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Colubrids have a modified salivary gland (Duvernoy´s gland), which discharges near the fangs at the rear of the mouth. The venom is slowly introduced into the prey via capillary action. Therefore, in order to get sufficient venom into the tissues, a long contact period is necessary. However, this occurs only exceptionally in humans. This explains why most bites by colubrids are harmless. This also explains why occasionally envenomations are described by snakes that traditionally are regarded as non-venomous. In elapids and vipers, by contrast, the venom glands consist of the uppermost labial salivary glands. They can be actively emptied by the musculus constrictor glandulae, so that the animals can actively and very quickly inject venom, or even spit venom (several metres). Accessory venom glands are present in some snakes. Venom evolved before fangs, and even snakes without highly evolved fangs have potent venom. This explains why so many “harmless” snakes can be venomous. They are not necessarily dangerous to humans, but they have enough venom to kill their ordinary prey.
2.7 Description, jaws and teeth
A snake skull is complex. There are numerous small bones and ligaments. The bones of the upper and lower jaw are muscularly and elastically connected with one another and with the skull. The left and right sides of the jaws can move independently of one another. This makes it possible to swallow large prey, yet the animals cannot chew. Snakes have no sternum, so that a large ingested prey does not constitute a mechanical obstacle when it is being swallowed (some prey have a diameter which is greater than the resting diameter of the snake).
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On the lower jaw of a snake there are small teeth on the os dentale (”teeth bone”). In the upper part of the mouth a double row of teeth is present. There is a lateral row on the maxilla and a medial row on the os palatinum and on the pterygoid bone. These small teeth curve backwards, making it more difficult for a prey to escape. In vipers the fangs are joined to the maxillae. The upper jaw of vipers can rotate vis-à-vis the prefrontal bone. This makes it possible for the fangs to be folded backwards when the mouth is closed. When the animal strikes, the maxillae rotate so that the fangs unfold forwards and can be used to bite. In all other snakes the maxillae and the fangs are immobile. Reserve fangs are brought into functional position before the old fangs fall out. Therefore a bite wound can display 1 to 4 fang marks. The puncture wounds are spaced from 5 to 40 mm apart and are 1 to 8 mm deep (even deeper in case of a bite by a gaboon viper).
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In snakes, the teeth are not so firmly attached to the top/inner side of the jawbones (so-called “pleurodont dentition”). This makes it possible for the teeth to be easily replaced throughout a snake´s lifetime. The teeth break off easily. This influences the biting behaviour. Thus vipers bite, inject venom and release again in rapid succession, because a struggling prey could cause injury or break the teeth.
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Note: temporomandibular joint
A temporomandibular joint is a purely mammalian characteristic that is not found in snakes. In snakes, the joint between lower and upper jaw is formed by the os articulare at the bottom and the os quadratum (quadrate bone) at the top. In vipers, this joint is strongly laterally positioned, giving the head a triangular appearance. In the course of evolution, the small bones of this joint have received another purpose. Snakes have only 1 middle ear bone, the stapes (stirrup). Mammals, by contrast, have 3 middle ear bones. The hammer (malleus) and the anvil (incus) are phylogenetically derived from the os articulare and the os quadratum. The difference in origin is also expressed ontogenetically in the mammalian embryo. Embryologically the mandibula, malleus and incus derive from the 1st gill arch and the stapes derives from the 2nd gill arch. Marsupials are phylogenetically primitive compared to placental mammals. In the immediate postnatal period, when the newborn marsupials are still in the pouch, the incus and the malleus still have a role comparable to the articular and the quadrate bone. During this period, the young suck, they do not chew. When the animals leave the pouch, the bones separate from the lower jaw and penetrate into the middle ear.
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Note: Infections transferred via snakes:
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Pythons can be infested by tongue worms (Pentastomida) such as Armillifer armillatus in Africa or A. moniliformis in Asia. These parasites live in the lungs of the reptiles. The eggs in the snake’s sputum can infect human beings, e.g. through contamination of drinking water or when a snake is prepared as food. Porocephalosis (syn. pentastomiasis) is the result. In general, infection leads to asymptomatic crescent-shaped calcifications in the abdomen. Living parasites are rarely found elsewhere (e.g. subconjunctival). Gnatostomiasis (infection with the nematode Gnathostoma spinigerum) can also follow consumption of undercooked snake meat. A larva migrans syndrome or a very serious eosinophilic meningo-encephalitis can then develop. Spirometra sp. can be transferred via snakes (also via frogs) and cause sparganosis, whereby the immature cestode can be found in the eye. These worms can survive for up to nine years in human beings.
3.1 Taxonomy, introduction
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.]
3.2 Taxonomy, vipers (Viperidae)
3.2.1 General
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).
3.2.2 Daboia russelli
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).
3.2.3 Bitis arietans
The puff adder (Bitis arietans, la vipère heurtante) gives rise to considerable uneasiness in Africa. This large snake has a wide diameter and gets its name from the noise that it sometimes produces. It can be recognised by the black-grey chevrons along its back. They can strike very quickly. In Northern Kenya and Somalia there is a particular subspecies (Bitis arietans somalica). The snake is ovoviviparous and once a year gives birth to around 50 young, 15-20 cm long, which are already dangerous at birth.
3.2.4 Bitis nasicornis
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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.
3.2.5 Vipera lebetina
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.
3.2.6 Cerastes cerastes
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).
3.2.7 Vipera berus (Common viper or adder)
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).
3.2.8 Bitis gabonica
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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).
3.2.9 Echis carinatus complex
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.
3.2.10 Causus 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.
3.3 Taxonomy, pit vipers (Crotalidae)
3.3.1 General
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.
3.3.2 Agkistrodon sp.
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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.
3.3.3 Crotalus sp.
Rattlesnakes belong to the genus Crotalus and Sistrurus. [Sistrurus sp. include pygmy rattlesnakes and the so-called "massasaugas"].
3.3.4 Lachesis
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.
3.3.5 Bothrops sp., Lance-head pit vipers
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.
3.3.6 Calloselasma rhodostoma
Malayan pit vipers are feared in endemic areas and are responsible for a large number of bites. It is a very important asian snake.
3.3.7 Trimeresurus sp
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.
3.3.8 Bothriechis sp.
Palm vipers. Dangerous bites by these animals are very rare.
3.4 Taxonomy, burrowing vipers or Atractaspididae
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”.
3.5 Taxonomy, Elapidae
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.
3.5.1 Cobras
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:
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Chinese cobra
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Monocellate cobra
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Naja naja
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Indian spectacled snake
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Central Asian cobra or Oxus cobra
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Northern Philippine cobra
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Andaman cobra
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Southeastern Philippine cobra or Visayan cobra
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Indochinese spitting cobra
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South Indonesian spitting cobra
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Naja sumatrana
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Equatorial spitting cobra
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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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In Africa there are also various cobras. Naja nivea (Cape cobra), Naja melanoleuca (forest cobra), Naja mossambica (Mozambique cobra), Naja nigricollis woodi (black-necked spitting cobra), Naja nigricollis nigricincta (zebra cobra), Naja pallida (African red spitting cobra), are common snakes in Africa. The rinkhals (Hemachatus haemachatus) [watch the spelling!]
3.5.2 Coral snakes
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.
3.5.3 Bungarus sp. : Kraits
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.
3.5.4 Dendroaspis sp. : Mambas
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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).
3.6 Taxonomy, sea snakes or Hydrophiidae
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 1/5th 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.
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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.
3.7 Taxonomy, Colubridae
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.
3.8 Taxonomy, Boidae
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.
4.1 Distribution, general
- As far as native venomous snakes are concerned, only vipers are found in Europe.
- In Africa there are elapids, vipers and colubrids.
- The most important snakes in America are the pit vipers and several coral snakes.
- In Asia, all families are represented (but not all genera).
- A number of elapsids live in Australia.
- Problems with venomous sea snakes are limited to coastal areas of Asia and Australia.
- Imported exotic pet snakes can be responsible for bites, especially in affluent countries
4.2 Distribution, most important snakes
It is useful to have an idea of which major venomous snakes can be found where.
- In Southeast Asia Russell´s viper (Daboia russelli), Echis carinatus, the habu’s and the Malayan pit viper (Calloselasma rhodostoma) are the most important.
- In Africa the saw-scaled vipers (Echis carinatus complex), the puff viper (Bitis arietans) and to a lesser extent cobras and mambas are important.
- In South and Central America the cascabel (Crotalus durissus terrificus), jararaca (Bothrops jararaca) and fer-de-lance (Bothrops atrox) are the most important venomous snakes. Bites by the notorious bushmaster (Lachesis muta) are actually quite rare.
- In North America the various rattlesnakes (Crotalus sp. and Sistrurus sp.) are the most important, with Crotalus atrox (Western diamondback) heading the list. Mocassins (Agkistrodon sp.) and coral snakes (Micrurus and Micruroides) are statistically less important.
- Coastal areas in Southeast Asia and Northern Australia: sea snakes such as Pelamis, Laticauda sp, Enhydrina sp.
- Australia: Brown snake (Pseudonaja sp), black snake (Pseudoechis), death adder (Acantophis), Taipan (Oxyuranus), tiger snake (Notechis).
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The five medically most important snakes in the world are:
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Echis carinatus complex
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Bitis arietans
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Daboia russelli
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Calloselasma rhodostoma
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Bothrops atrox
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4.3 Distribution, simplified classification
ELAPIDAE
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Example
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Eur
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Afr
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NAm
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SAm
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Asia
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Austr
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Category
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Micrurus
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coral snake
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+
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+
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2
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Dendroaspis
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mamba
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+
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2
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Hemachatus
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rinkhals
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+
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2
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Naja
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cobra
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+
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+
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1
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Ophiophagus
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king cobra
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+
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2
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Bungarus
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krait
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2
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Pseudonaja
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brown snake
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+
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1
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Pseudoechis
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mulga
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+
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2
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Notechis
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tiger snake
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1
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Oxyuranus
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taipan
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+
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2
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Acanthopis
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death adder
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+
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1
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HYDROPHIIDAE
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Enhydrina
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beaked sea snake
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+
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1
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Lapemis
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Hardwick’s snake
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1
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VIPERIDAE (vipers)
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Vipera
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European viper
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2
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Sand viper
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3
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Bitis
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puff viper
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1
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gaboon viper
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2
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Echis
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saw-scaled viper
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1
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Cerastes
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horned viper
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3
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Atractaspis
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burrowing asp
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2
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Daboia
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Russell’s viper
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1
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CROTALIDAE ( pit vipers)
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Bothrops
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fer-de-lance
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+
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1
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jararaca
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1
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Lachesis
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bushmaster
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+
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2
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Crotalus
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cascabel
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+
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1
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timber rattlesnake
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1
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diamondback
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1
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