The management of snake bites

Fortunately, most snakes have no fangs and are not poisonous. When a snake does have fangs, these can be at the back, or the front of its mouth. If the fangs are at the back, it is usually harmless, an important exception being the African boomslang (tree snake). If the fangs are at the front it is usually poisonous.

Apart from a few islands, there are venomous snakes almost everywhere; but only a few are medically important. Being bitten by one, usually on the leg, is one of the risks of being a tropical villager. People who deliberately handle snakes are usually bitten on their hands or arms. Although many people are bitten, less than half are poisoned, even by a poisonous snake, and of these few die. The danger is that a snake will inject its full dose of poison, in which case the patient has about a 50% chance of death, unless he is treated. Many snake bites are treated by traditional practitioners, and of the patients who do arrive in hospital, many arrive late. If necessary, teach your paramedical staff to treat snakebite, and supply them with antivenom.

Snake toxins have a variety of effects, depending on the species. These include local swelling, capillary oozing, and necrosis; generalized damage to muscles (sea snakes), damage to the heart or kidneys, interference with blood clotting, haemolysis, and various neurotoxic effects.

If a patient is treated symptomatically, he usually recovers, but if his symptoms are severe he needs the specific antivenom, ideally in monovalent form, or failing this as a polyvalent mixture suitable for the common snakes of the area. Theoretically, a monospecific antivenom is better, because smaller volumes are required, and this reduces the incidence of serum reactions. In practice, a polyvalent antivenom is more satisfactory in the rural tropics. Antivenom can occasionally cause reactions which are fatal, so don't give it unless it is indicated, and always have adrenalin ready. For sources of specific sera for your local snakes, see below.

In view the potential dangers of antivenom, its cost, and the difficulty of stocking it in sufficient quantities in remote places, controllled ventilation, by any of the methods in ''Primary Anaesthesia' (A 13.1), is the main method of preventing death in a patient with severe neurotoxic symptoms following the bite of an elapid snake.

You should know which snake bit him; but a patient often does not know this, particularly if he was bitten at night. Globally, the carpet or saw-scaled viper, is the most dangerous snake, because of its wide distribution (particularly West Africa, Pakistan, and North West India), its abundance in farming areas, its good camouflage, its irritability, and its toxicity.

Manson-Bahr PEC, and Apted FC, ''Manson's Tropical Diseases'. Bailli[gr]ere Tindall.[-3] page596Reid HA and Theakston RDG, ''The Management of Snake Bite'. Bulletin of the World Health Organization 1983;61(6):885[nd]895.[-3] ''Progress in Characterising Venoms and the Standardisation of antivenoms', 1981 WHO Offset Publication No. 58. TREAT THE PATIENT NOT THE SNAKE Fig. 34-8 SNAKE BITES. A, blood-stained blisters after a viper bite. B, extensive superficial necrosis after a viper bite. After HA Reid.

SNAKE BITES FIRST AID. A snake bite is frightening, so reassure the patient. Move the bitten part as little as possible. If at any time he vomits or loses consciousness, turn him into the recovery position (A 4-5).

CAUTION ! (1) If the snake has been killed, keep it for examination, but don't try to look for it. (2) Don't handle snakes, even dead ones; decapitated heads can bite for some hours. (3) Don't inject antivenom as part of first aid treatment. (4) Wipe, but don't incise the bite.

CREPE BANDAGING. The traditional arterial tourniquet is now outmoded, because of its dangers. Instead, apply a firm cr[ci]epe compression bandage over the whole length of the bitten limb, to slow the spread of venom. Remove it after about 8 hours.

IN HOSPITAL Admit him. Don't panic or rush to inject antivenom. If he demands an injection, give him tetanus toxoid (which he needs anyway). Raise a bitten leg, and put a bitten arm in a sling. Clean the site of the bite and leave it open. Keep the number of injections to a minimum, because he may bleed from the injection sites, if the venom (particularly from a viper) alters blood clotting. Set up a drip. If there is local necrosis, give him a broad-spectrum antibiotic; some surgeons give them routinely. Avoid aspirin, because of its adverse effects on platelets, and morphine, which may mask respiratory depression.

CAUTION ! (1) Don't incise the bitten area, or apply ice or dressings early on. (2) Don't give heparin, fibrinogen, or neostigmine. (3) Steroids are useful for delayed (not immediate) serum reactions.

HAS THE SNAKE INJECTED POISON? If his symptoms are immediate, he is probably only frightened; symptoms of poisoning rarely appear before half an hour, although they can appear in the first 5 or 10 minutes. Fang marks are of little help; there may be marks and no poisoning, or no obvious marks and poisoning. Local pain can be severe when there is no poisoning, and be absent when there is poisoning. Three early non-specific signs suggest poisoning: (1) vomiting, (2) a polymorph leucocytosis and (3) hypotension. If this is accompanied by a slow pulse, suspect a strong vasovagal component to the symptoms, and thus a good prognosis. Other early ]]non-specific signs are headache, abdominal pain, explosive diarrhoea, and collapse with an unrecordable blood pressure. These symptoms usually resolve spontaneously within an hour. After a viper bite a patient's blood fails to clot, his gums bleed, blood appears in his sputum and he may bleed from an old wound. After an elapid bite his eyelids droop (ptosis).

If there is local swelling which starts a few minutes after a viper bite, venom has been injected. This is not a criterion for giving antivenomn: wait for signs of specific poisoning.

If there is no swelling a few minutes after a viper bite, you can be sure that no venom has been injected.

If there is no pain and swelling and no general signs 2 hours after the bite, it is very unlikely that the snake is dangerous. However, signs can occur after a few minutes or be delayed for 12 hours, so observe him carefully.

WHAT KIND OF SNAKE WAS IT? Identifying the snake can be important, because it will allow you to give specific monovalent antivenom (if you have it); but not being able to identify it should never delay treatment. Here are some of the typical syndromes.

Vipers usually cause massive local swelling, due to exudation from injured capillaries; abnormal bleeding; and blood which fails to clot; later there is necrosis and gangrene.

Elapids are the cobras, mambas, kraits, and coral snakes. Their effects are mainly neurotoxic. Paralysis varies, and may be extensive and fatal. He has difficulty breathing due to respiratory paralysis, and cannot swallow. Ptosis is the earliest sign.

Sea snakes are myotoxic, see below.

LOCAL SWELLING starts minutes after the bite of a viper, and may be massive after 72 hours. The swollen area may be bruised, blistered, and ultimately necrotic. Wet gangrene may develop rapidly over days (cobras), or dry gangrene slowly over weeks (vipers). If there is no necrosis, recovery is rapid, provided that the fluid lost into the tissues is replaced. Swelling and necrosis are not usually dangerous in themselves, although they may result in permanent scarring, or occasionally in the need for amputation.

If there is any swelling, expose his limb to reduce its temperature. Raise his arm in a roller towel (75-1), and his leg by raising the foot of his bed.

Leave his blisters undisturbed. If his tissues necrose, excise sloughs and graft early. Necrosis is usually confined to the subcutaneous tissues, without involving his muscles and tendons. Apply saline dressings, and graft the raw areas (57.1). Antibiotics are not helpful, unless and until there is local necrosis. If he presented very late, you may have to amputate.

Very rarely, fasciotomy may be necessary (81.14).

SHOCK, starting later, is the main cause of death in viper bites. It can be late or early, sometimes within a few minutes of the bite, with abdominal pain, explosive diarrhoea, collapse and an unrecordable blood pressure. If it is due to hypersensitivity to the venom, rather than to its toxicity, these symptoms may resolve spontaneously in half an hour.

If he is in hypovolaemic shock give him saline. This is cheaper and more readily available than plasma, and less likely to infect him with HIV. If he shows signs of hypovolaemia, give him 2 litres of saline quickly initially. If he shows signs of circulatory insufficiency and extensive bruising, give him blood. Blood is particularly useful if he was anaemic before he was bitten, or if antivenom is not available.

BLEEDING may occur from the bite, into injection sites, in his vomit, from his gums, under his skin, or into his brain. It is often lethal, and may be delayed for several days. Hess's test may become positive in 30 minutes.

Hess's test Blow up a blood pressure cuff to 80 mm Hg and leave it on for 5 minutes. If a crop of purpuric spots appears below the cuff, the test is positive.

The clotting test gives warning of bleeding to follow. Keep some blood in a tube horizontally for 10 minutes and then tilt it. If it fails to clot, he needs antivenom. In Africa non-clotting blood is a useful indicator of Eichis envenoming, for which there is a specific antivenom.

If he bleeds extensively, and you have no antivenom, transfuse fresh blood, and if necessary give him fibrinogen intravenously.

NEUROTOXIC EFFECTS are characteristic of elapid poisoning, and are the result of a selective neuromuscular block. Ptosis is the earliest sign (don't confuse this with sleepiness). Other effects include: inability to cough, protrude the tongue, smile, or move the lower jaw; drooling, dysphagia; partial, flaccid limb paralysis, more marked proximally (painless, except in the case of sea snakes, when the paralysis is painful), generalized fasciculations, vertigo, convulsions, unconsciousness, respiratory failure from intercostal paralysis, squint, speech incoordination, and generalized paraesthesiae.

If his neck and trunk muscles are involved, he is unable to lift his head or sit up. The important fatal effects are respiratory failure, and failure of his swallowing reflex. Respiratory failure may manifest itself as increasingly shallow respirations, or as a vigorous struggle for air with laboured breathing.

If necessary, intubate (A 13.2) and ventilate (A 13.1) him in the theatre, while you wait for the antivenom to become effective. For the signs of incipient respiratory failure needing artificial ventilation, see A 19.4.

CAUTION ! (1) Intubation and ventilation are the first priority. Antivenom comes second, once he is breathing. (2) The inhalation of secretions, or stomach contents, can cause sudden death at any time (A 16.3).

MYOTOXIC EFFECTS are characteristic of sea snakes. Early symptoms are muscle pain and stiffness anywhere, but particularly in his neck, tongue, and throat. This is soon followed by tenderness, pain on passive movement, muscle weakness, and myoglobinuria. His muscles may take months to recover.

ANTIVENOM INDICATIONS. (1) Undoubted clinical symptoms of systemic poisoning. Antivenom may still be effective hours or days after the bite. It is never too late to give it.

CONTRAINDICATIONS. Enquire for a history of allergy; this increases the risk of an allergic reaction. A known allergic history contraindicates antivenom, unless the risk of death from envenoming is high. There is no point in doing a sensitivity test, because it is unreliable, and if he is seriously poisoned, he will need antivenom anyway.

Give all patients promethazine. This will promote sleep and relieve apprehension, and may minimize sensitivity reactions (rarely severe).

METHOD. Ideally, the venom should be monovalent, but some broad-spectrum polyvalent antivenoms are sufficiently active. Even if an antivenom is out of date, it may still be effective. If it is opaque when you make it up, discard it, because there will be a greater risk of reactions.

Give him 20 to 50 ml of antivenom, diluted in 3 volumes of 0.9% saline. Give it by intravenous infusion diluted in a drip, or by slow bolus injection. The dose depends on the type of antivenom, and the type of snake; you may need much more than 40 ml, especially after an elapid bite. In severe poisoning (especially with neurotoxic envenoming) give 100 to 150 ml. In children give the same dose as in adults.

Start the drip slowly (15 drops a minute). If there is no initial reaction, increase the speed of administration, so that you can complete the infusion in 1 to 2 hours. If there has been little improvement, give more.

If there are signs of sensitivity to antivenom (pallor, hypotension, dyspnoea, laryngeal oedema), temporarily stop the drip and inject 0.5 ml of 1:1000 adrenalin subcutaneously, over some convenient part of his chest. This is almost always effective, and you can then restart the drip.

If he has an allergic history, give him small doses of adrenalin (0.5 ml of 1:1000 adrenalin) subcutaneously, before you give the antivenom, and repeat it if a reaction occurs. Also, give him promethazine or hydrocortisone 100 mg to protect against sensitivity reactions (rarely severe).

If more than 2 hours have elapsed since the bite of a known front-fanged (elapid) snake, and life-threatening signs are obvious, give the antivenom intravenously without delay, and without testing for hypersensitivity, under steroid or antihistamine cover.

In sea-snake poioning, give 3000 to 10,000 units of antivenom. In mild poisoning 1000 to 2000 units should be enough.

CAUTION ! (1) In all patients, have adrenalin ready in a syringe, and give it at the first sign of anaphylaxis (pallor, etc., see above). (2) Don't give antivenom to all patients routinely.

MONITORING. Record his pulse, blood pressure, peripheral circulation, and respiration hourly, and the circumference of his bitten limb at the site of the bite. Monitor his urine output and his blood urea. Watch for specific signs, and don't discharge him for at least 12 hours.

DIFFICULTIES [s7]WITH SNAKE BITES If, immediately after the bite, he appears SEMICONSCIOUS, and cold with clammy skin, a feeble pulse and rapid breathing, this may be due to fright (try a placebo injection), or to venom effects. In some venoms these pass off spontaneously in half an hour.

If he is MENTALLY CONFUSED, suspect respiratory failure.

If he shows signs of GLOSSOPHARYNGEAL PALSY (difficulty swallowing), nurse him in the recovery position.

If confusion, stupor, or RESPIRATORY FAILURE develop, intubate him, and aspirate his secretions. Atropine may diminish them.

If he develops signs of RENAL FAILURE (vipers, elapids or sea snakes), they usually appear towards the end of first week. Avoid drugs which are excreted in the urine (except for penicillin when necessary); treat him as in Section 53.3.

If a spitting COBRA SPAT INTO HIS EYES, wash them out with a large quantity of water, and tell him to blink. The venom is quickly diluted and causes no harm.