A Tour around Snake Bite

(Date of publication 14 February 2005)

We in the UK are lucky; our only poisonous snake is not much larger than an animated bootlace, and even a severe bite is very unlikely to prove fatal. Other countries, however, are not so fortunate. Seven thousand people are bitten every year in the US, although only 15 of those will die, putting the survival rate at something like 99.7%. India has the highest reported death rate (15,000-20,000 victims annually), although it is likely that many fatal bites, particularly in Third World countries, go unreported. Estimates have placed the true number as high as 5 million per year world-wide. The majority of bites occur during the late afternoon and evening, the snake’s time of peak activity, and most affect limbs or feet.

Of the 2,500-3,000 species of snake, approximately 500 are poisonous. However, it is not only venomous bites (or ophitoxaemia) that can be dangerous. The saliva of several so-called non-venomous snakes, such as hognose or garter snakes, contains similar combinations of toxins to the venom of their more dreaded cousins; what they lack is specific venom glands and equipment for injecting it into their victims. Despite this, there have been recorded cases of them killing people.

Both venom and saliva are mostly water, combined with various enzymatic proteins that provide toxic properties. Poisonous snakes have paired venom glands below the eye. Hollow fangs in the upper jaw (which can grow to 20 mm in large rattlesnakes) enable them to inject it into their victims, immobilizing smaller prey and beginning the digestive process. Heat-sensitive nostril pits enable the snake to vary the amount of venom delivered, depending upon the time elapsed since the last bite, the degree of perceived threat, and the size of the prey. The gaboon viper has proportionately the longest fangs, an example of which (and the chance to purchase your very own example!) is provided here. The percentages and concentration of enzymes differ according to species, geographical location, season and age. A detailed breakdown of the pathophysiology of various venoms can be found here.

Depending on the precise combination of toxins, snake venom can be loosely categorised into neurotoxic (particularly cobras and kraits), haemotoxic (vipers) and myotoxic (sea snakes), although the rigidity of such definitions has been to a certain extent discredited, as venom from every poisonous species has the same components in varying proportions. All snake venom also has a direct cytolytic action, causing local necrosis and secondary infection, a common cause of death. Direct neurotoxic action, as in cobra venom, can lead to paralysis and respiratory arrest, while cardiotoxic effects can induce cardiac arrest. Altered coagulation can also cause bleeding which may be severe enough to kill the victim.

Symptom onset is not necessarily immediate, although delayed reactions are rare. Myotoxic poisoning almost inevitably produces symptoms within two hours, so can be ruled out once sufficient time has elapsed. Often the first symptoms are pain, swelling and a small reddish weal, followed by blebs, oedema and discolouration which may extend from the wound as far as the trunk. Those who are not at all squeamish may like to visit the graphic photographs at this site. Signs of systemic toxicity may include regional lymphadenopathy, hypotension, haemoptysis, paraesthesia and the appearance of petechiae. The onset of paralysis and coma is often signalled by repeated vomiting, blurred vision, headache, dizziness and signs of autonomic hyperactivity.

The species most likely to attack depends on the specific country. Those planning a long haul holiday might (or might not!) like to visit this site if contemplating Australasia, this one for the Indian subcontinent, or this one for the US. Coral snake bites are investigated in great detail here.

The right course of action to take if bitten is also region-specific. An international guide is available here, while the University of Maryland provides species-specific advice for Americans. Anyone in Australia (where the native snakes’ fangs are shorter, so that poison spreads closer to the surface of the skin through the lymph system) should check out this site. Much advice is common sense and many poorly substantiated treatment plans may do more harm than good; for example, making an incision over the bite, mouth suctioning, tourniquets, ice packs, or electric shock. The affected area should be immobilized, walking minimised, and the patient taken to an appropriate medical centre as soon as possible.

Clinical treatment initially involves oxygen therapy and the administration of antivenins. Patients should be closely monitored in case intubation becomes necessary. Occasional complications may arise from hypersensitivity to horse serum, the basis for many antivenins, and severe reactions such as anaphylaxis can occur. Methods of treating this are discussed at eMedicine.com.

It’s not all bad news, however. Recent research at the University of South Australia suggests that snake venom’s capacity to target only the growth of endothelial cells could provide the basis for a novel cancer treatment.

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