MEDICALLY IMPORTANT SNAKES

INTRODUCTION

Snakes are elongate limbless reptiles, with bodies that are covered with scales. They are medically important because the bites of some snakes can cause systemic poisoning. Basically there are two types of snakes found; sea snakes and land snakes. The land snakes include the elapids, vipers, colubrids, earth snakes, water snakes and constrictors. Depending on the degree of toxicity to man, poisonous snakes are classified as deadly venomous, moderately venomous and mildly venomous. Usually sea snakes, elapids and vipers are deadly venomous, but some vipers are only moderately venomous. Colubrids are mildly venomous or non-venomous. Constrictors, water snakes and earth snakes are non-venomous.

POISONOUS OR VENOMOUS SNAKES

All venomous snakes possess a pair of venom producing glands and maxillary teeth called “fangs” which inject venom into the prey. The venom glands lie below and behind the eye, one on each side .Two types of venom glands are found;

1. True venom glands: These are found in elapids, sea snakes and vipers. It is a large gland and the fangs are situated anteriorly.
2. Duvernoy’s gland: These are found some colubrids. It is a small gland situated posteriorly and venom is injected through a posterior fang. Snakes with well developed Duvernoy’s glands and posterior fangs are mildly poisonous.

FANG:

It is a curved maxillary tooth modified to conduct and inject venom, either by having a posterior groove (closed or open) or a central canal. Sea snakes and elapids have anterior fixed fangs (proteroglyphous) while vipers have movable fangs (solenoglyphous) which are kept folded backwards when the mouth is closed. Snakes with Duvernoy’s glands and posterior fans are termed back fanged (opisthoglyphous) snakes.

SCALATION:

The arrangement of scales is termed scalation. This is important in identification of snakes.

Head; Scales on the head may be equal and smaller in size or some may be enlarged. Enlarged head scales are called shield.

Body; Scales on the top midline of the body are called vertebrals and scales on the lateral and under side are called costals and ventrals respectively.

Tail; the segment between the anal verge and the posterior end of the snake is the tail. Dorsal scales of the tail are called caudals and ventral scales of the tail are called subcordals ( uniserial or biserial)

IDENTIFICATION FEATURES AND HABITS

1.SEA SNAKES

All sea snakes are venomous, but reports of envenomation in man are rare. Sea snakes are well adapted for life in the sea. They have a characteristics tail which is laterally flattened and rudder-like (allows movements in the water) while body is dorso-ventrally flattened. Sea snakes are found in coastal waters and river mouths, but they can also be found swimming several miles upstream in rivers.

Pelamis platurus (Yellow bellied sea snake)

This species is thought be the commonest sea snake. It needs to be differentiated from eels, which are gill breathing fish with cylindrical bodies and pectoral fins

2.ELAPIDS

Elapids are slender snakes with long tails. The head is spatula-shaped (spoon shaped) with no distinct neck. Some are oviparous (lay eggs) whilst others are viviparous (lay young). Scales on the dorsum of the head are enlarged. Vertebrals are also enlarged. Subcordals are single (Uniserial). Common elapids are;

I. Cobra

II. Ceylon krait

III. Common (Indian) krait

IV. Ceylon coral snake

2.1) Cobra (Naja naja)

The cobra is one of the largest elapids. Average length is one meter. This snake is commonly found in South Asian countries. It is easily recognized by the characteristics “hood” which is a loose fold of skin below the head. On the dorsal surface, the hood bears a spectacle marking, while there are two black spots on the ventral aspect in the Sri Lankan Cobra. Colour varies from very dark brown, black or olive to light reddish brown. It is a diurnal snake, living near human habitation. These are slow moving and generally not aggressive. On provocation it erects the forebody off the ground, spreads its hood, hisses, sways from side to side and strikes. The striking radius depends on the raised forepart. This is deadly venomous snake. Young cobras are very irritable and aggressive. Female lays eggs.

2.2) Ceylon Krait (Bungarus ceylonicus)

This is the smallest of the known kraits. It measures up to one meter. It possesses the general features of elapids and in addition it has some characteristic markings. It is a dorsally black snake with single wide white band bands on the body which continues onto the belly. Generally they are found in the wet and intermediate zones. They are nocturnal, sluggish and inoffensive. It usually lives in and around human dwellings. The victim is often bitten during sleep. This is deadly venomous snake. The female is oviparous.

2.3) Common (Indian) Krait (Bungarus caeruleus)

It is a fairly long snake (average length about 140 cm).It is a shiny blue black snake with narrow white double bands on the dorsum. The belly is clear white. It is highly poisonous and nocturnal in habits. They are usually found in dry zones and very closely associated with human dwellings. The female is oviparous.

2.4) Ceylon coral snakes (Calliophis melanarus)

It is a small slender snake, which has a head, neck and body of uniform width. Average length is about 30 cm. Head and tail is black in colour. The colour of the body is yellowish brown with orange coloured spot near the vent. This snake is nocturnal in habit. It spends most of the day underneath fallen leaves and tree trunks .Recorded human bites are very low.

3.VIPERS

All vipers possess true venom glands with movable fangs. They have thick bodies with short tails. The head is triangular with a distinct neck (viperine head). Head scales are usually small and equal in size. Generally females lay young.

Vipers can be divided into two groups on the presence or absence of a pit, the loreal pit, which is located on the each side of the head between the nostril and the eye. The loreal pit is a thermoreceptor which can detect warm blooded prey in the dark.

3.1) True vipers (pits are absence)

I. Russell’s viper

II. Saw scaled viper

3.2) Pit vipers

I. Green pit viper

II. Hump nosed viper

3.1) True vipers

3.1.1) Russell’s viper (Vipera russelli)

This is the largest viper found in Sri Lanka, measuring up to150 cm. It is easily recognized by its characteristic markings, viz. three rows of chain like markings along the length of the body. In addition it has an inverted ‘V’ mark on the dorsum of the head. Colour varies from grey-brown to reddish-brown. It is mainly nocturnal in its habits. During the day it remains concealed in the undergrowth, coiled up with its head lying in the centre of the coil. The Russell’s viper strikes man only on provocation. When it is provoked, the tongue quivers, the head is raised, the body sways rhythmically and the snake hisses loudly and continuously. When it strikes it does with force and determination, throwing itself violently at the victim. It is found all climates, especially in the dry zones. The female is oviparous.

3.1.2) Saw-scaled viper (Echis carinatus)

It is a small viper. The average length is less than 50cm. It has a rough appearance due to the presence of saw-edged scales on the body. When the snake moves the scales rub against each other, to produce a sound almost like continuous hiss. It has a characteristic mark on the head which is described as a bird’s foot or a cross. The colour is known to vary to merge with the colour of the surroundings. Usually colour is sandy with dark blotches. It is nocturnal in habits. It is commonly found in sandy costal areas. Fatal bites are not reported commonly. The female is oviparous.

3.2) Pit vipers

3.2.1) Green pit viper (Trimeresurus trigonocephalus)

The green pit viper has a large broad flat triangular head with a loreal pit. It grows up to about 130 cm. It is dorsally blue green with black markings from head to tail. Usually it is arboreal and appears to be active during the day and night. This snake is widely distributed and found up to about 6000 feet elevation.

3.2.2) Hump nosed viper (Hypnale hypnale)

The hump nosed viper has a viperine head with enlarged head scales. Its snout is actually pointed and turned up. Average length ranges from 30 to 40 cm. This snake is highly variable in colour. The most commonly dorsal aspect is brown with darker markings. It is a nocturnal snake which hides under leaves and stones during the day.It are a common snake and widely distributed, but commoner in plantation districts. Fatal bites are uncommon.

4.COLUBRIDS

Colubrids are a commonest type of snakes found everywhere. The members of this family show a wide range of diversity and adaptation to different habitats. The majority are ground dwellers while others are arboreal, aquatic or burrowing. In contrast to the elapids the vertebral are not enlarged. Subcordals are usually divided or biserial. Most of the colubrids are non-poisonous. There are a few which are mildly poisonous. These mildly poisonous snakes are back fanged with grooved fang.

4.1) mildly poisonous colubrids

I. Cat snake

II. Whip snake

III. Water snake

IV. Sri Lanka keel-back

V. Flying snake

4.2) non-poisonous colubrids

I. Rat snake

II. Ceylon wolf snake

4.1) mildly poisonous colubrids

4.1.1) Cat snake (Boiga forsteni)

The head is more or less triangular in shape with a distinct neck. Head scales are enlarged. The cat snake can be wrongly identified as a hump-nosed viper but it can be differentiated by the absence of a loreal pit and a humped snout. Colour varies from light brown to reddish brown. They are nocturnal in habits and mainly arboreal.

4.1.2) Whip snake (Ahaetulla nasuta)

The head is narrow and elongated. The body is log and slender. The tail is long and cylindrical. It grows to a length of 190cm. It is an aggressive, fierce snake that darts at the eye, hence its name (Eye puckers). The common species is dorsally green in colour, with a pale yellow belly. Brown coloured species are also found. It is a diurnal snake, commonly found in low shrubs and hedges. It is found up to an altitude of 15000m.

4.1.3) Water snake (Cerberus rhynchops rhynchops)

It is common aquatic snake, with average length of about 60 cm. Dorsally it is grayish or chocolate brown with short dark bars broken up into spots. It is usually quiet and inoffensive but on provocation hisses, bites and produces a strong odour per anum. It is commonly found in costal areas, especially lagoons, streams and rivers.

4.1.4) Sri Lankan keel back (Ahara kukka)

It is an uncommon snake and is found mainly in hill country. It has a small head with large eyes. It is slow moving snake, found under decaying logs and leaves. It is reddish brown with a white or yellow belly.

4.1.5) Flying snake (Golden tree snake)

It is an uncommon snake. It is arboreal and it can glide and is fast moving. It bites fiercely. It is brightly coloured beautiful snake .It is diurnal in habits. It is found in wet and dry zones up to 460m.

4.2.1) Rat snake (Ptyas mucosus)

These are the largest among the Sri Lankan colubrids, some reaching a length of 8 feet. They are characterized by dark-edged scales, colour variable, dorsally olive green to olive brown. Some have a yellow belly. These are diurnal, fast moving. Commonly found almost anywhere, especially in jungle tracts or paddy fields. It is also found ceilings of houses.

4.2.2) Ceylon wolf snake

It rarely exceeds 2 feet in length. Colour is variable dorsally brown or purplish with a series of whitish to yellow bands. These bands are similar to those of the Ceylon krait, but in the wolf snake. The vertebrals are not enlarged.

5.CONSTRICTORS

They are a group of large snakes and non-venomous. They crush their prey.

I. Python

II. Sand boa

5.1) Python

The python is the largest of Sri Lankan snakes. It has characteristic quadrate markings on the dorsal surface. It feeds mainly on birds and mammals.

5.2) Sand boa

It is smaller than the python, being about 3 feet in length. Body colour is yellowish, brownish or grayish, with a series of large dark brown, black edged dorsal spots, usually confluent with one another.

6.EARTH SNAKES

Primitive snakes live under the soil. They are Timid and harmless. Some are blind and very small (about 4 inches in length) while others like the pipe snake could be 2 feet in length. The body scales are smooth.

7.WATER SNAKES

They are aquatic snakes which inhabit rivers, river mouths and coastal waters. They are completely adapted to aquatic life. They are unattractive and sluggish, with rough body scales. They are nocturnal and feed on fish. If they are handled roughly, the sharp teeth can inflict deep wounds.