Classification

The suborder of snakes (Ophidia or Serpentes) comprises over 3,000 species. Together with lizards (Sauria), they form the order of scaled reptiles (Squamata). The classification of snakes is controversial, and there exists a variety of proposals for their classification. In older systems, 10–12 families are distinguished; more recent classifications tend to divide these into additional families or subfamilies (Smith et al. 1977, McDowell 1987). Not all of the families are described here, as we have focused only on venomous snakes.

It is possible to distinguish between snakes that possess neither venom glands nor modified teeth and those that have true venom glands and venom fangs. Amongst the non-venomous snakes are the underground-dwelling Typhlopidae and Leototyphlopidae, the boas and pythons (Boidae) and several other, smaller families. The large colubrid family (Colubridae, around 1,700 species) represents an intermediate form, as many species have venom glands in varying preliminary forms ("Duvernoy's glands") and sometimes also simple modified teeth for venom injection.

The true venomous snakes comprise over 660 species, but of these, only around 200 are known to have been associated with cases of fatal envenoming or permanent damage in humans (Warrell 1987).

Among the true venomous snakes, the following families and subfamilies can be distinguished (O'Shea 2005):

• Elapidae:
  • Elapinae ("elapids", including the "Australian elapids"; approx. 250 species)
  • Hydrophiinae (sea snakes, including Sea kraits; 64 species)
• Viperidae:
  • Viperinae (vipers, adders, "viperids"; nearly 90 species)
  • Crotalinae (pit vipers, "crotalids"; approx. 180 species)

• Atractaspididae: (Mole vipers, 18 species)
  

The Mole vipers (Atractaspididae) are placed in the "vipers" group here in the VAPAGuide. 

On the basis of their morphological characteristics and venom composition, sea snakes (Hydrophiinae) appear to be closely related to the Australian elapids (Elapinae), which are likewise placed in the subfamily of the Hydrophiinae in recent years. Australian elapids are presented separately in the VAPAGuide.

The Hydrophiinae are discussed separately as a distinct group of venomous animals. The geographical distribution of the Elapinae, Viperinae and Crotalinae is shown in maps 20–22 below. Note that there are no vipers in the New World, that in Africa and practically the whole of Europe there are no pit vipers, and that in Australia and New Guinea the only terrestrial venomous snakes are elapids.

  Map 20 Worldwide distribution of the Elapinae (including the Australian elapids; excluding the Hydrophiinae -> see Map 9 & 10).

  Map 21 Worldwide distribution of the Viperinae

  Map 22 Worldwide distribution of the Crotalinae

The medically most significant genera among the terrestrial venomous snakes are:

• Elapinae: Naja (Cobras; Africa and Asia), Bungarus (Kraits; Asia), Dendroaspis (Mambas; Africa), and to a lesser extent Micrurus (Coral snakes; New World). The most important genera of the Australian elapids are: Oxyuranus (Taipans), Notechis (Tiger snakes), Pseudonaja (Brown snakes), and to a lesser extent Pseudechis (Black snakes) and Acanthophis (Death adders).
• Viperinae: Echis (Carpet vipers; Africa and Asia), Bitis (Africa) and Vipera (European and Asian vipers).
• Crotalinae: Bothrops (Lance heads; Central and South America), Crotalus (Rattlesnakes; North, Central and South America), Agkistrodon (Moccasins; North and Central America), Deinagkistrodon (eastern Asia), Calloselasma (Southeast Asia) and in some cases also Trimeresurus and allied genera (Asian lance-headed vipers and others; Asia).

Morphological characteristics and identification criteria

Nowadays it is assumed that snakes developed from a group of lizards that specialised in hunting small, secluded or burrowing mammals. In conjunction with their burrowing activity, the limbs regressed in the course of development and the body, as well as some of the internal organs, underwent elongation. Limb rudiments are only present in some of the primitive species, such as the Boids, in the form of cloacal spurs. Although there are also lizards that lack outwardly visible limbs (e.g. the blind worm, Anguis fragilis), in contrast to snakes, they possess movable eyelids. In snakes, the eyelids are fused into a transparent and rigid protective covering over the eyes. A further difference between snakes and lizards is that the former no longer possess outer and middle ears.

Snakes are covered with scales over their entire surface. The head scales of various snakes show great variability and may sometimes occur in the form of larger shields (Table 4.10a and b). The upper surface of the body and the sides are covered in smaller scales arranged in diagonal rows, while on the belly there is always a row of wide ventral scales (Table 4.10c and d).

The configuration of the skull in snakes displays several features that have developed in connection with their specialisation in large prey. Due to their delicate teeth and fragile skull, they cannot tear apart their prey, as does the crocodile, for example, but have to swallow it whole. For this purpose snakes possess a hinged skull which is not very robust and in which the bones of the lower jaw are only loosely held together by muscles and elastic ligaments. By means of a second jaw joint, the bones of the lower jaw can be moved apart, thus allowing the mouth to open very wide. This enables snakes to swallow prey whose diameter is often far greater than that of their own heads.

An approximate taxonomic identification can be made solely with the aid of readily identifiable characteristics.

Differentiation of venomous from non-venomous snakes essentially relies on the presence of venom fangs (Table 4.10e–h). In most cases, the different families of venomous snakes, including the colubrids, can be distinguished on the basis of the nature of the venom fangs. As vipers and pit vipers have the same type of dentition, it is the presence of heat-sensing pit organs in the pit vipers that distinguishes them from the vipers. These heat-sensing organs are located between the eye and nostril on each side of the head and represent a distinctive feature of the pit vipers (Table 4.10k).

Further distinction at the species level is essentially based on the number or form of particular body scales (see also: "Morphological identification"). Colubrids and elapids possess several large shields on the top of their head (Table 4.10a). Such shields are also present in some vipers and pit vipers, but in the majority of species they have become small head scales (Table 4.10b). In addition, in vipers and pit vipers the head is often clearly distinct from the body, whereas in colubrids and elapids the head generally merges with the neck (Table 4.10a and b). Colubrids and elapids tend to have a slender and long body, whereas vipers and pit vipers are generally sturdy and stout (Table 4.10i and j). Pit vipers often have dorsal markings in the form of repeated patterns that stretch along the upper body. Generally, however, colouring and markings are poor features for identification, as they are highly variable not only between different species, but also within species.

For precise herpetological identification at the species and subspecies level, characteristics that are not easily accessible are often relied upon, such as the male reproductive organs. The use of immunological identification methods for the systematic classification of snakes is becoming increasingly common.

  Table 4.10 Morphological characteristics of snakes

a Head shields on an elapid snake (Naja sp.), with labelling of the individual shields. Colubrids and elapids typically have 9 large shields. Only a few genera among the vipers and pit vipers, such as Agkistrodon, Gloydius (see k) or Sistrurus, have head shields of this type.

b Head scales on an Asian lance-headed viper (Trimeresurus sp.). The presence of numerous small scales instead of the 9 large shields is typical for many vipers and pit vipers, but may also be present in harmless snakes (e.g. in many boas and pythons). In contrast to the elapids and colubrids, the head is usually quite distinct from the neck (compare with a).

c A row of enlarged ventral scales stretches along the entire length of the belly. These scales are not as wide in water snakes, particularly the Hydrophiinae. Posterior to the anal scale (A), the scales may be divided or undivided.

d Stretched snake skin to show the pattern of the dorsal scales. The ventral scales are indicated at the edges.

e–f Various types of dentition (see text also):

e Harmless colubrid. All the teeth are of approximately the same length.

f Opisthoglyphous colubrid. Enlarged, grooved teeth located in the posterior region of the upper jaw.

g Proteroglyphous dentition, as possessed by Elapinae and Hydrophiinae. The venom fangs are firmly anchored in the upper jaw.

h Solenoglyphous dentition of the vipers and pit vipers. The extremely large venom fangs are shown here in the folded-back position, covered by a fold of mucous membrane.

i Appearance of an elapid snake (Pseudonaja textilis). In many colubrids and elapids the body is slender and appears long. The colouring is usually uniform, without patterns along the body (exceptions are the american and asiatic coral snakes, aswell as some krait species, Tiger snakes and Death adders).

j Appearance of a viper (Vipera palaestinae), with a sturdy body which appears short. In viperid snakes there are often patterns along the body.

k The heat-sensing pit organs of the pit vipers are located between the eye and nostril. Vipers do not possess such organs. This illustration shows Gloydius halys. Note the large head shields, which are not typical of vipers and pit vipers.

Venom apparatus

In order to reduce the risk of injury, prey that defend themselves must first be immobilised before they are swallowed. For this purpose, boas and pythons as well as many colubrids bite firmly into their prey, rapidly coil their body around it and thereby strangle it. The hunting method of venomous snakes is even more refined. They strike extremely quickly, inject their venom and then let go again. The prey is rapidly paralysed and can no longer escape very far; the snake picks up its scent with flickering tongue and can then safely ingest the immobilised animal via the lengthy process of swallowing it whole.

The venom apparatus consists of a pair of venom glands that open at the base of the venom fangs, which are enlarged and modified to varying degrees in different snakes and are located in the upper jaw. A single venom gland is located in the region behind each eye. They represent modified supralabial salivary glands and are surrounded by musculature with the help of which the snake can squeeze out venom at will. In many colubrids, the supralabial salivary glands produce a mixture rich in enzymes that displays considerable toxicity in some specimens. These so called Duvernoy's glands open into the region of the posterior teeth in the upper jaw. They are considered to aid in digestion, in that proteolytic secretions are injected into the prey during the act of swallowing, thus causing the prey to start disintegrating. In a number of colubrids, the teeth are elongated in the area of the mouth in which the venom glands open at the base of the teeth (Table 4.10f). In this regard, colubrids can be divided into those with smooth, ungrooved teeth (aglyphs) and those in which the teeth possess a longitudinal furrow, i.e. grooved teeth (opisthoglyphs). By channelling the venom along the grooves, opisthoglyphous teeth enable more efficient injection of venom than aglyphous teeth, where the venom flows slowly down the entire surface of the tooth. However, because these teeth are located in the posterior region of the upper jaw, both types of colubrids must hold on to their prey after biting them and inject venom by means of chewing motions.

In the true venomous snakes the venom fangs are located at the very front of the upper jaw (Table 4.10g and h). The anterior groove that is present in opisthoglyphous teeth is closed to form a venom channel which opens out close to the tip of the tooth. In the Elapidae, a fine commissure is still present on the anterior surface of the venom fangs (→ proteroglyphs). In viperids and crotalids this is not visible (→ solenoglyphs). In general there is only one functional venom fang on each side, but behind it there might be several substitute teeth. Elapidae have smaller venom fangs, generally under 1 cm in length. In some snakes, for example many Coral snakes (Micrurus sp.) or sea snakes, they can only be seen upon closer observation. In contrast, viperids and crotalids possess exceptionally long venom fangs, up to several centimetres in length, which are folded back when not in use and covered by a fold of mucous membrane. They are only erected when the snake strikes (Table 4.10h).

The structure and position of the venom fangs as well as the fact that they are frequently associated with strongly toxic secretions make the venom apparatus in true venomous snakes not only a valuable tool for catching prey, but also an effective means of defence. Many snakes have additionally developed certain warning signals which make a defensive strike unnecessary in the majority of confrontations with potential enemies.

Warning signals can be of a visual nature, such as the conspicuous warning colouring of the Coral snakes or the Cobra's threat display of raising its upper body and flattening its neck to form a hood. Others produce clearly audible warning sounds, created by various means or can release badly smelling odours Such mechanisms often succeed in deterring enemies and the snake is able to save its precious venom for hunting prey, as it can take days or weeks to "re-fill" empty venom glands.

Range of effects of snake venom

See the Clinical flowcharts for the various regions.

Way of life

Snakes are widely distributed on all the continents, but the greatest variety of species exists in the subtropical and tropical regions. Snakes inhabit the most varied habitats on land and in water. The terrestrial habitats range from desert and semi-desert regions, many different types of forests, including rainforests, open territory and cultivated areas or mountain regions, where some species are found far above the tree line. Most snakes are ground-dwelling, but there are several species that have adapted to living under the ground or above the ground in trees and bushes. They are generally solitary and come together most notably during the reproductive season. In cooler distribution areas a number of snakes will congregate to hibernate in protected hollows or crevices.

As snakes are cold-blooded animals and thus cannot produce their own body heat, they have to regulate their body temperature by seeking a cooler or warmer place depending on the ambient temperature. Therefore tropical snakes are predominantly active in the evening or at night and stay in their hiding places during the day. In cooler regions, snakes are mostly active during the day and sun themselves in warm places.

The main source of food for many species of snakes are rodents. Snakes are drawn to areas in which rodents are numerous. Thus there are large numbers of snakes in particular in agricultural zones and in living areas in rural regions of tropical countries. These are the places where confrontations between venomous snakes and humans most frequently occur.

Epidemiology

From a global perspective, snakes are the most important venomous animals both epidemiologically and medically. Envenoming is particularly prevalent in rural regions in the tropics. The most common victims in these regions are farm labourers, who make up a considerable proportion of the population, as well as a number of indigenous hunter-gatherer communities. It is difficult to estimate the incidence of venomous snakebites, particularly as with many bites, only small amounts of venom are injected or none at all. It is also difficult to determine the actual mortality and morbidity rates in tropical countries, as in these regions, many incidents are not even reported. Hospital statistics give a false picture, as the patients who reach hospital represent a select group. Many victims are unable to reach hospital in time for treatment to be useful because of the great distances involved. In addition, in certain regions many victims rely on traditional treatment methods ("traditional healers") and avoid hospitals due to superstitious beliefs.

In the 1990s it was estimated that between 50,000 and 100,000 people died from snakebites each year and that many more were left with permanent damage, in particular in the form of deformity or loss of limbs (Warrell and Fenner 1993). A literature analysis and modelling based on regional estimates of envenoming and deaths due to snakebite was published by Kasturiratne et al. in 2008. There it is estimated that, globally, at least 421,000 cases of envenoming and 20,000 deaths occur each year. These figures may actually be as high as 1,841,000 cases of envenoming and 94,000 deaths. On the basis of the fact that envenoming occurs in about one in every four snakebites, between 1.2 and 5.5 million snakebites could occur annually. The vast majority of the estimated burden of snakebite is in South and Southeast Asia, sub-Saharan Africa, and Central and South America. India had the highest estimated number of bites and deaths for a single country (see below).

The mortality rate in the industrial nations is low. Quite a number of the dangerous snakebites in these countries are caused by snakes in terraria. In Europe it is relatively easy to obtain a permit to keep dangerous exotic species of snakes. In many countries in Central Europe, the last-recorded fatalities due to native vipers date back to several decades ago. In the 1950s in the USA, 6,000–7,000 snakebites were treated annually, with an average of 15 cases of envenoming ending fatally. In more recent times, fewer than 10 people die of snakebites each year in the USA (Minton 1980). In Australia it is estimated that there are around 3,000 bites a year, of which 300 need to be treated with antivenom and an average of 4.5 patients die. However, the development of methods that can detect venom post mortem showed that in 1978, at least 10 people died as the consequence of a snakebite (Sutherland 1980). There were no deaths reported in a prospective study from the Northern Territory, including 348 suspected snakebites between 1989 and 2003 (Currie 2004). Nearby Papua New Guinea, with a snake fauna similar to that of Australia, shows a totally different epidemiological situation. In Central Province and the National Capital District the overall snake bite incidence is 215.5 per 100,000, but may be as high as 526 per 100,000 locally (Kairuku subprovince), which is amongst the highest in the world (Lalloo et al. 1995). Most of the patients were bitten in daylight, and the mortality in Central Province is 7.9 per 100,000.

In India the incidence of bites is estimated to be 5.5 per 100,000 inhabitants, with a case fatality rate of 3–4% (Gaitonde and Bhattacharya 1980). In light of the extremely high population density there, these numbers take on a tragic significance. In the most recent estimates India had the highest estimated number of bites and deaths for a single country, with approximately 45,900 deaths per year (95% CI 40,900–50,900) (Mohapatra et al. 2011).

In Burma the incidence of snakebites is estimated to be 3.3 per 100,000 inhabitants, with around 1,000 fatalities a year (Aung-Khin 1980). In Bangladesh the mortality each year is estimated as high as 6,000 (Rahman et al. 2010).

Of the 6,050 estimated cases of snakebite envenoming that occur yearly in Sri Lanka, 10.5% are believed to end fatally (De Silva 1980).

It is difficult to assess the incidence of snakebites in the majority of countries in Africa. More detailed epidemiological studies are only available for Nigeria. The numbers they reveal are alarming, with a snake bite incidence in some areas of the savanna region of 602 per 100,000 inhabitants, and a case fatality rate of over 12%! (Pugh and Theakston 1980).

Epidemiological data for Central and South America are also scarce. In Brazil around 20,000 snakebites are recorded yearly (Ribeiro 1990b, Cardoso 1990). The case fatality rate is estimated to be under 1% and is thus much lower than the above-mentioned numbers for Asia and Africa.

In the period 1993-2006, 48 fatalities due to snakebite were reported in Costa Rica (Fernandez and Gutierrez 2008).

Prevention

Tourists are generally not at risk in tropical and subtropical countries with dangerous venomous snakes, and it is very unlikely that they will even see one of these shy animals. In marked contrast, the local population in rural regions are the common victims of snakebites in these countries. Their living and working areas often overlap with the areas in which snakes live. Moreover it is precisely this group of people who are insufficiently protected. Wearing sturdy, ankle-high footwear and long trousers would be a simple and efficient means of drastically reducing the problem of snakebites, but this kind of comprehensive preventive measure is impossible for large segments of the population in the developing world due to economic, traditional, work-related or other reasons.

The fundamentals:

As snakes are extremely sensitive to ground-borne vibrations, they are able to sense approaching footsteps at a distance of several metres. Most species will then flee before they are even seen. In terrain where it is difficult to see the ground clearly, it is important to always tread firmly. If taken by surprise or unable to flee, many venomous snakes will draw attention to themselves through specific warning behaviours. If a snake is encountered unexpectedly, it is important to stop and stand still and quietly, to give the snake time to make a retreat. In such a situation, never try to scare the snake away or provoke it in any way.

Further advice:

• Never touch a snake, even if it appears to be dead.
• Always remain on the path in terrain where it is difficult to see the ground.
• Always use a torch at night.
• Exercise caution when picking up stones, firewood etc. or when picking berries. Larger objects such as stones, tree trunks or planks should always be lifted in such a way that if a snake is hiding underneath, it is able to escape.
• When climbing, always look where you are putting your hands.

In living quarters (particularly in the tropics and subtropics):

• Ensure there are no unnecessary hiding places, such as clutter, woodpiles etc., around the living quarters. Furthermore, the vegetation around the house should be kept short and the lawn should be mowed regularly.
• Do not dispose of kitchen waste in or around the living quarters. It attracts rodents which in turn attract snakes.
• Hens, peacocks and cats can be useful for controlling snakes around living quarters.
• Make sure that windows and doors can be tightly closed.
• Sleep under a mosquito net.

For snake keepers:

It is inadvisable to keep venomous snakes. If nonetheless insisted upon, the following must be kept in mind:

• A permit is required to keep venomous snakes.
• Use secure terraria. The terrarium must have a hide box installed. It must be able to be closed from outside, by means of a long pole that can be introduced into the terrarium through a small hole. 
• Never touch venomous snakes. Before working in the terrarium, always first maneuver the snake into the hide box.
• In practice it is observed that snake keepers are often bitten when inebriated. It is therefore strongly recommended not to work with the terrarium after consumption of alcohol or drugs.
• Snake keepers are also often bitten after having touched prey animals (eg. mice) and and then working in the terrarium afterwards.

Literature

General overviews
Engelmann and Obst 1981, Gumprecht et al. 2004, Kochva 1987, Mallow and Nilson 2003, McDiarmid et al. 1999, Minton and Minton 1969, Minton et al. 1965, O'Shea 2005, Phelps 1981, Seigel et al. 1987

The Reptile Database

Regional overviews

• North America: Behler and King 1991, Russell 1983, Campbell and Lamar 2004
• Central and South America: Campbell and Lamar 1989, 2004
• Europe: Brodmann 1987, Gruber 1989, Steward 1971
• West Africa: Villiers 1975
• Southern Africa: Broadley 1983
• East Africa: Pitman 1974
• Near and Middle East: Joger 1984, Leviton et al. 1992
• India: Whitaker and Captain 2008
• Southeast and East Asia: Gopalakrishnakone and Chou 1990
• Australia: Cogger 1986, 1987, 2000, O'Shea et al. 1996

Further overviews (individual countries)