Snake Venoms contain more than 20 different constituents, mainly proteins, including enzymes and polypeptide toxins. The following venom constituents cause important clinical effects:
Procoagulant enzymes (Viperidae) that stimulate blood clotting but result in incoagulable blood. Venoms such as Russell’s viper venom contain several different procoagulants which activate different steps of the clotting cascade. The result is formation of fibrin in the blood stream. Most of this is immediately broken down by the body’s own fibrinolytic system. Eventually, and sometimes within 30 minutes of the bite, the levels of clotting factors have been so depleted (“consumption coagulopathy”) that the blood will not clot.
Haemorrhagins (zinc metalloproteinases) that damage the endothelial lining of blood vessel walls causing spontaneous systemic haemorrhage.
Cytolytic or necrotic toxins - these digestive hydrolases (proteolytic enzymes and phospholipases A) polypeptide toxins and other factors increase permeability resulting in local swelling. They may also destroy cell membranes and tissues.
Haemolytic and myolytic phospholipases A2 - these enzymes damage cell membranes, endothelium, skeletal muscle, nerve and red blood cells.
Pre-synaptic neurotoxins (Elapidae and some Viperidae) - these are phospholipases A2 that damage nerve endings, initially releasing acetylcholine transmitter, then interfering with release.
Post-synaptic neurotoxins (Elapidae) - these polypeptides compete with acetylcholine for receptors in the neuromuscular junction and lead to curare-like paralysis.
Quantity of Venom Injected at a Bite
This is very variable, depending on the species and size of the snake, the mechanical efficiency of the bite, whether one or two fangs penetrated the skin and whether there were repeated strikes. The snake may be able to control whether or not venom is injected. For whatever reason, a proportion of bites by venomous snakes do not result in the injection of sufficient venom to cause clinical effects. About 50% of bites by Malayan pit vipers and Russell’s vipers, 30% of bites by cobras and 5-10% of bites by saw-scaled vipers do not result in any symptoms or signs of envenoming. Snakes do not exhaust their store of venom, even after several strikes, and they are no less venomous after eating their prey. Although large snakes tend to inject more venom than smaller specimens of the same species, the venom of smaller, younger vipers may be richer in some dangerous components, such as those affecting haemostasis.
It is difficult to answer this question because many snake bites and even deaths from snake bite are not recorded. One reason is that many snake bite victims are treated not in hospitals but by traditional healers.
Bangladesh – a survey of 10% of the country in 1988-9 revealed 764 bites with 168 deaths in one year. Cobra bites (34% of all bites) caused a case fatality of 40%.
Bhutan – (no data available)
Cambodia – (no data available)
India – estimates in the region of 200,000 bites and 15-20,000 snake bite deaths per year, originally made in the last century, are still quoted. No reliable national statistics are available. In Year, a thousand deaths were reported in Maharashtra State. In the Burdwan district of West Bengal 29,489 people were bitten in one year
with 1,301 deaths. It is estimated that between 35,000 and 50,000 people die of snake bite each year among India’s population of 980 million.
Indonesia – no reliable data are available from this vast archipelago. Snake bites and deaths are reported from some islands, eg Komodo, but fewer than 20 deaths are registered each year.
Lao DPR – (no data available)
Malaysia – bites are common, especially in northwest peninsular Malaysia, but there are few deaths.
Myanmar (Burma) – snake bites and snake bite deaths have been reliably reported from colonial times. Russell’s vipers are responsible for 90% of cases. In 1991, there were 14,000 bites with 1,000 deaths and in 1997, 8,000 bites with 500 deaths. Under-reporting is estimated at 12%. There are peaks of incidence in May and June in urban areas and during the rice harvest in October to December in rural areas.
Nepal – there are estimated to be at least 20,000 snake bites with about 200 deaths in hospitals each year, mainly in the Terai region. One survey suggested as many as 1,000 deaths per year. Among 16 fatalities recorded at one rural clinic during a monsoon season, 15 had died on their way to seek medical care.
Pakistan – there are an estimated 20,000 snake bite deaths each year
Philippines – there are no reliable estimates of mortality among the many islands of thearchipelago. Figures of 200-300 deaths each year have been suggested. Only cobras cause fatal envenoming, their usual victims being rice farmers.
Sri Lanka – epidemiological studies in Anuradhapura showed that only twothirds of cases of fatal snake bite were being reported to the Government Agent Statistical Branch. However, the Registrar General received reports of more than 800 deaths from bites and stings by venomous animals and insects in the late 1970s and
the true annual incidence of snake bite fatalities may exceed 1,000.
Thailand – between 1985 and 1989, the number of reported snake bite cases increased from 3,377 to 6,038 per year, reflecting increased diligence in reporting rather than a true increase in snake bites; the number of deaths ranged from 81 to 183 (average 141) per year. In 1991 there were 1,469 reported bites with five deaths, in 1992, 6,733 bites with 19 deaths and, in 1994, 8,486 bites with eight deaths. Deaths reported in hospital returns were only 11% of the number recorded by the Public Health Authorities. In a national survey of dead snakes brought to hospital by the people they had bitten, 70% of the snakes were venomous species, the most commonly brought species being Malayan pit viper (Calloselasma rhodostoma) 38%, white-lipped green pit viper (Trimeresurus albolabris) 27%, Russell’s viper (Daboia russelii siamensis) 14%, Indo-Chinese spitting cobra (Naja siamensis) 10% and monocellate cobra (N kaouthia) 7%. In an analysis of 46 fatal cases in which the snake had been reliably identified, Malayan kraits (Bungarus candidus) and Malayan pit vipers were each responsible for 13 cases, monocellate cobras for 12 and Russell’s vipers for seven deaths.
Viet Nam – there are an estimated 30,000 bites per year. Among 430 rubber plantation workers bitten by Malayan pit vipers between 1993 and 1998, the case fatality was 22%, but only a minority had received antivenom treatment. Fishermen are still occasionally killed by sea snakes but rarely reach hospitals.
Following the immediate pain of mechanical penetration of the skin by the snake’s fangs, there may be increasing local pain (burning, bursting, throbbing) at the site of the bite, local swelling that gradually extends proximally up the bitten limb and tender, painful enlargement of the regional lymph nodes draining the site of the bite (in the groin – femoral or inguinal, following bites in the lower limb; at the elbow (epitrochlear) or in the axilla following bites in the upper limb). However, bites by kraits, sea snakes and Philippine cobras may be virtually painless and may cause negligible local swelling. Someone who is sleeping may not even wake up when bitten by a krait and there may be no detectable fang marks or signs of local envenoming.
Clinical Pattern of Envenoming by Snakes in South-East Asia
Symptoms and signs vary according to the species of snake responsible for the bite and the amount of venom injected. Sometimes the identity of the biting snake can be confirmed by examining the dead snake; it may be strongly suspected from the patient’s description or the circumstances of the bite or from knowledge of the clinical effects of the venom of that species. This information will enable the doctor to choose an appropriate antivenom, anticipate the likely complications and therefore take appropriate action. If the biting species is unknown, recognition of the emerging pattern of symptoms, signs and results of laboratory tests (“the clinical syndrome”), may suggest which species was responsible.