Envenomation

Published on 14/03/2015 by admin

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Chapter 30 Envenomation

Shane Curran, Thomas McDonagh

Australia is home to many venomous creatures. Australian animals that are important in causing envenoming in humans include snakes, spiders, octopuses, fish and other marine creatures. The distribution of venomous creatures is wide, and each region has its own pattern of envenomation. Local knowledge is very important and local expert knowledge can be invaluable.

Resources that are available to you include the local emergency physician or the on-call toxinologist, who can be contacted via Poisons Information (tel. 131126). They are available to discuss management of patients with possible or definite envenomation.

SNAKEBITE

Australia is home to a number of the most venomous snakes in the world. Snake venom is a complex mixture of substances. Australian snakes produce venoms that have a range of clinical effects including neurotoxic (presenting as progressive paralysis), myotoxic (causing rhabdomyolysis and subsequent renal failure) or severe coagulation disturbances and haemolysis (Table 30.1).

Table 30.1 Clinical effects of Australian snake venom

Snakebite is a medical emergency. Patients presenting following possible snakebite should receive urgent assessment and management. Patients who have significant envenomation may initially appear well.

The majority of snakebites will not result in significant envenoming, and so will not require antivenom. This is because many snakebites are ‘dry bites’ where no venom is injected. The amount of venom injected by a snake depends on snake maturity, fang length, venom yield, snake temperament, the number of bites and the time since the snake’s last meal. Overall, fewer than one in four patients require antivenom.

First aid: the pressure-immobilisation technnique

Snake venom spreads via the lymphatics. The pressure-immobilisation method of first aid prevents spread of the venom via the lymphatics and can prevent clinical envenomation. It involves applying a firm broad bandage, commencing at the site of the bite, and then applying the bandage over the entire limb, extending both proximally and distally. The pressure is the same as that used for a sprained ankle. A splint is then applied to the limb, to immobilise the limb and reduce muscle contraction to further reduce lymphatic spread of the venom. The patient is then kept as still as possible and transported to hospital.

The pressure-immobilisation technique can prevent clinical envenomation if applied early and correctly. Both the limb and the patient should be kept still. The bandages are kept in place until facilities are available to treat clinical envenomation. This may mean transporting a patient to a hospital with a supply of antivenom. If deterioration occurs when the bandages are removed, the bandages should be reapplied.

It is important not to wash the site of the bite, as it contains traces of venom that are important for identifying the snake type.

If a patient arrives following a possible snakebite and has had no first aid but is well with no signs of envenoming, there is no need to apply the pressure-immobilisation technique.

If a patient arrives with symptoms of envenomation and does not have a bandage applied, then one should be applied.

Table 30.2 Use of the pressure-immobilisation technique of first aid

Pressure-immobilisation is recommended for	Do not use pressure-immobilisation first aid for
All Australian venomous snake bites, including sea snake bites	Redback spider bites
Funnel web spider bites	Other spider bites, including mouse spiders, white tailed spiders
Box jellyfish stings (if possible)	Bluebottle jellyfish stings
Bee, wasp and ant stings in allergic individuals	Other jellyfish stings
Blue-ringed octopus bites	Stonefish and other fish stings
Cone snail (cone shell) stings	Bee and wasp stings in non-allergic individuals
Australian paralysis tick envenomation	Bites or stings by scorpions, centipedes, beetles

Adapted with permission of the Australian Venom Research Unit

Venom detection kit

The venom detection kit developed by the Commonwealth Serum Laboratories (CSL) is important in the management of the patient with snakebite. The venom detection kit can detect minute amounts of snake venom if present, and is used to determine what family of snake that venom came from.

The venom detection kit does not indicate that a patient has been envenomed. It also does not determine that a patient should be given antivenom.

The venom detection kit does indicate which antivenom to use, once the decision to give antivenom is made. The decision to give antivenom is a clinical decision based on symptoms, signs and pathology testing.

A swab of the bite site is best as this is where venom is present in highest quantities. A window can be cut in the bandages overlying the bite site. When the swab has been taken, the bandage is reapplied.

Urine is the next best for venom detection as Australian snake venoms are excreted in the urine. Blood or serum is not used.

The venom detection kit is used by trained laboratory staff and takes about 20 minutes to complete.

Antivenom

Antivenom is the definitive treatment for a patient with systemic envenomation. The administration of antivenom can reverse the clinical effects of envenomation. The treatment of envenomation following snakebite involves the administration of adequate quantities of the appropriate antivenom.

Antivenom is produced by CSL from antibodies that are harvested from horses that have been injected with subclinical doses of snake venom toxins. As the antibodies are obtained from horse serum there is the risk of anaphylaxis, allergic reaction or delayed serum sickness. Prior to administration, therefore, preparations should be made to treat a possible anaphylactic reaction. Currently premedication with adrenaline is not recommended.

Antivenom is given intravenously and multiple ampoules may be necessary. Monovalent antivenom is indicated if the type of snake is known, when the venom detection kit determines the type of antivenom to be given, or in areas where the occurrence of snakes is limited to specific snake types. Monovalent antivenom is preferred as it is less expensive, is a smaller volume of foreign protein and has fewer side effects.

Polyvalent antivenom contains antibodies to venom of all groups of snakes. It consists of a large volume and is expensive, but is used when treatment with antivenom is indicated and the snake type is unknown and the venom detection kit procedure is negative or will take too long to perform.

In certain areas a mixture of monovalent antivenoms, based on the local prevalence of snake types, may replace polyvalent antivenom.

The dose of antivenom is dependent on the dose of venom injected by the snake, not patient size, so children require the same amount of antivenom as adults do.

Laboratory testing

Laboratory tests are vital to assessing the patient possibly bitten by a snake. Patients with possible snakebite should have blood taken for:

• coagulation—activated partial thromboplastin time (APTT), prothrombin time (PT), fibrinogen level, d-dimer/fibrinogen degradation products (XDP), platelet count

• creatine kinase

• renal function

• electrolytes.

Urine should be tested for myoglobin and blood.

If there is no access to a laboratory, a whole blood clotting time can be performed to determine the presence of coagulopathy. To perform a whole blood clotting time, 5–10 mL of blood are placed in a clean plain glass test tube and allowed to stand. The time to clotting is measured and is normally less than 10 minutes. A prolonged time indicates coagulopathy.

Laboratory tests may be normal initially and will need repeating. If all tests are normal then the first aid bandages should be removed and pathology tests repeated after 2–3 hours. Onset of coagulopathy may be delayed and blood tests should be repeated.

Signs and symptoms

Local signs of snakebite may vary from obvious bite marks, with local pain and swelling, to a trivial puncture or scratch. The absence of bite marks does not exclude significant snakebite.

General symptoms include non-specific symptoms such as nausea, headache, abdominal pain and collapse. Specific symptoms may include muscle pain, bleeding sites and muscle weakness. In an emergency, it may be possible to determine the snake from the clinical and laboratory effects. It is advisable to seek expert advice prior to administering antivenom.

Disposition

Admit and observe all cases of possible snakebite. Patients should be checked frequently for signs of envenomation, and blood tests should be repeated, even if normal initially. Urine should be tested again for blood and myoglobin. The patient should be observed for early signs of paralysis, e.g. ptosis, diplopia and dysarthria.

Patients who receive multiple doses of antivenom should also be given doses of steroids to prevent serum sickness.

All patients with brown snake bite should be admitted for 24 hours.

Pearls

Not all patients bitten by venomous creatures will suffer envenomation. Antivenom is not indicated in all cases. Each case needs to be assessed individually. First aid should not be discontinued if antivenom and treatment facilities are not immediately available. In the envenomed patient multiple ampoules of antivenom are often necessary. For advice on bites and stings, ring Poisons Information on 131126.

SPIDER BITES

Redback spider (Latrodectus hasseltii)

Redback spiders are common throughout Australia. They are common in urban areas, in and around houses, gardens, sheds, rubbish piles and discarded objects. It is the female spider that is responsible for envenoming.

Only one-fifth of bites will result in significant envenoming. Only people with significant envenoming receive antivenom.

The redback bite is usually felt as an initial mild sting, with no signs of a bite at the site. Fifteen minutes to 1 hour later the bite site may become painful. The pain can become severe and extend to the regional lymph nodes. Following this, pain may extend to the whole limb and the abdomen, and headache may occur. There may be nausea, malaise and hypertension. An area of sweating may develop surrounding the bite site. The patient may be sweating profusely and this may be generalised or localised to the bite site, or in an area remote from the bite.

If there is clear evidence of envenoming, then redback antivenom is given. The decision to treat is a clinical one; there are no tests to assist in the diagnosis.

Redback antivenom is given by intramuscular injection. This can be repeated in 1 hour, if there is incomplete reduction of symptoms, or if symptoms return. No premedication is required, but facilities to treat anaphylaxis should be at hand.

Redback antivenom can be useful for relief of symptoms up to days after the initial bite. The need for multiple doses is not uncommon. Advice should be sought if multiple doses of antivenom are needed.

Funnelweb spider (Atrax spp.)

There are at least 35 species of funnelweb spiders, but only one is known to cause death in humans. It is the Sydney funnelweb (Atrax robustus), which has a restricted range that includes the Sydney and Gosford regions. Other types are found over a wider range along the eastern seaboard of Australia.

Following a bite from the Sydney funnelweb, death can occur within 1 hour. The effects are a bite that is usually painful, followed by perioral tingling, fasciculation of the tongue, profuse sweating, lacrimation and salivation, then skeletal muscle fasciculation and spasms. Hypertension and tachycardia usually occur and there may be rapid onset of pulmonary oedema.

First aid is with the pressure-immobilisation technique. Definitive treatment is with Sydney funnelweb antivenom. Multiple vials of antivenom, usually two to four, may be necessary. Resuscitation with airway control and ventilation may be necessary.

All Sydney funnelweb spider bites should be admitted for close observation.

White-tailed spider (Lampona cylindrata)

The white-tailed spider is a common hunting spider that is often found in houses. Definite bites from this spider may cause some local pain and inflammation but are usually mild or moderate. The evidence linking the white-tailed spider to necrotic ulcers is limited, and research has failed to show that its venom causes significant skin damage.

Necrotic arachnidism

Many spiders can cause a bite that may be painful and mildly inflamed locally. In some patients the skin may ulcerate. These small ulcers usually heal without specific treatment. The development of skin bites that ulcerate and develop necrosis is usually only tenuously linked to a spider bite. Often there is a painful area of skin that is found in the morning after a presumed bite, and this may later ulcerate.

A number of overseas spiders, such as the recluse and fiddleback spiders, are implicated in causing necrotic skin ulceration and systemic illness.

There is little evidence to suggest that any single species of Australian spider is the cause of necrotic toxic damage. In many cases, the cause of the tissue damage is a secondary bacterial infection.

Management includes adequate debridement of the wound, microbiological specimens as necessary and antibiotics if required.

MARINE ENVENOMATION

Box jellyfish or sea wasp (Chironex fleckeri)

The box jellyfish is one of the most venomous creatures in the world. It is found in the coastal waters of northern Australia, and is most commonly encountered in the summer months. Its tentacles contain stinging cells called nematocysts that contain venom and a spring-loaded harpoon. On contact with a victim, the nematocyst releases the coiled harpoon, which pierces the skin and releases the stored venom. Victims may further discharge nematocysts as they attempt to remove the tentacles that still contain intact stinging cells.

Clinically, there is severe localised pain, and local skin changes range from painful erythema to full-thickness skin necrosis. Typically, there are linear red welts that may go on to blister and ulcerate. Confusion, agitation and collapse with respiratory or cardiac arrest can occur. Onset of envenomation may be very rapid and death can occur in 5 minutes. The venom contains cardiotoxic, neurotoxic and dermatotoxic compounds.

The extent of envenoming is dependent on the area of skin suffering tentacle contact. An area of tentacle contact of 10% of the total surface area is potentially lethal.

Immediate first aid is vital. Any undischarged nematocysts can be inactivated by applying large volumes of household vinegar (dilute acetic acid). Antivenom is available and can be given by intramuscular injection.

Box jellyfish antivenom is indicated for all but minor stings. One to three vials are administered by intramuscular injection as soon as possible in severe stings. Antivenom is given intravenously in the hospital setting.

Early administration of antivenom may result in reduced pain and reduces skin necrosis and scarring.

Irukandji (Carukia barnesi)

The irukandji is a small jellyfish, about 2 cm in diameter, found in the coastal waters of northern Australia.

The sting of the irukandji is moderately painful, with little skin damage, but within 30 minutes onset of systemic envenoming may occur, with severe abdominal and back pain, nausea and vomiting, and muscle or joint pain. Sweating and agitation occur, and the patient is hypertensive and tachycardic. Symptoms are thought to be due to catecholamine release.

Treatment includes analgesia, and antihypertensive treatment may be required, in which case alpha-blocking drugs (phentolamine) may be used.

No antivenom is available to treat irukandji envenomation. The best form of first aid is currently unclear.

Portuguese man-o’-war or bluebottle (Physalia sp.)

The Portuguese man-o’-war, or bluebottle, occurs throughout Australian coastal waters, where it is often found washed up on beaches. It has a typical bright blue appearance.

The bluebottle causes a painful sting with localised discrete weals and surrounding erythema. A line of redness with raised weals or blisters is typical. Systemic symptoms are uncommon but comprise nausea, vomiting, headache and abdominal pains. Deaths have not been reported following bluebottle sting.

Treatment consists of removing the tentacles with forceps. Vinegar is not recommended. Analgesia is indicated, as are ice packs and local anaesthesia.

Blue-ringed octopus (Hapalochlaena spp.)

A bite from the blue-ringed octopus may be painless and difficult to see, and so may go unnoticed. The bites occur when the octopus is handled, and is in contact with the skin. The victim will usually report handling a small octopus, in or around a coastal rock pool.

The saliva of the blue ringed octopus contains a potent neurotoxin, tetrodotoxin, which causes a rapidly progressive flaccid paralysis, followed by respiratory failure and hypotension.

First aid involves applying a pressure-immobilisation bandage. There is no antivenom and treatment is based on maintaining supportive measures. Respiratory support with ventilation may be required for a few days, until the effects of the toxin wear off.

Sea snakes

Sea snakes are found predominantly in the tropical waters of northern Australia. All are potentially dangerous to humans, but very few sea snake bites of significance occur.

Sea snake venoms contain postsynaptic neurotoxins and myolysins. Victims may therefore develop paralysis, with ptosis and diplopia, or muscle pain, weakness and myoglobinuria and hyperkalaemia, and renal failure may occur secondary to muscle breakdown. Coagulopathy is not a feature of sea snake bite.

Treatment is with the pressure-immobilisation technique of first aid. Sea snake antivenom is available and is used for neutralising venoms of all species of sea snakes. Sea snake venoms are not reliably detected by the venom detection kit.

Stonefish

The stonefish is responsible for a painful sting that usually occurs when wanderers step on it in shallow water around reefs and rock pools.

Most stonefish stings occur when the fish is stepped on and the spines inject venom, which causes instant and severe pain. Local swelling follows, which may be marked. Dizziness, nausea, hypotension and pulmonary oedema may rarely occur.

First aid involves immersing the limb in hot water. The water should not be hot enough to cause a scald. Analgesics are usually required. Local regional anaesthesia may help in providing analgesia.

Antivenom is available if there is significant envenoming, and is indicated if there is severe local pain. The number of ampoules to be given relates to the number of puncture wounds from the venomous spines.

TICK BITES

Ticks most commonly cause a local skin reaction, consisting of irritation and pruritis. Multiple bites may cause a rash. The most dangerous ticks are those that cause paralysis, belonging to the Ixodes group. The common paralysis tick is found along eastern Australia.

Tick paralysis occurs due to a presynaptic neurotoxin that occurs in the saliva of the Ixodes ticks. The toxin causes a progressive flaccid paralysis. Tick paralysis often occurs in children and presents as an ataxic gait with malaise and lethargy. The weakness may progress even after removal of the ticks.

A thorough search should be conducted to find all ticks, including scalp, ear canal and genitals. Ticks are removed by using tweezers on either side of the embedded mouthparts. Care is taken not to squeeze the body of the tick or more of the toxin can be injected. Care is also taken not to break off the mouthparts that may remain embedded in the skin.

The paralysis is treated with tick antivenom along with appropriate supportive care.

CENTIPEDES AND SCORPIONS

None of the species of centipedes or scorpions found in Australia are dangerous to humans. They may cause a painful bite, but the pain does not usually last long. Systemic symptoms are unusual and are not usually severe.

PEARLS AND PITFALLS

The most common pitfall is not relying on the history that is provided to you or underestimating the potential for envenomation.

Unexplained coagulopathies, muscle pain or weakness should include envenomation as a differential.

Children with unexplained irritability or boys with unexplained testicular pain should have redback spider bites as part of the differential diagnosis.

Failure to apply or removal of the pressure-immobilisation bandage in snake or funnelweb envenomation may cause worsening symptoms; a pressure bandage should be reapplied.

The decision to administer antivenom when clinically indicated should not be delayed.

Adequate antivenom should be used as indicated on either clinical or laboratory grounds.

Being bitten does not automatically indicate the need for antivenom administration.

A positive result for a venom detection kit does not indicate the need for antivenom administration.

Toxinologists who are on call do not regard any question as silly if it relates to a patient or presentation you do not fully understand.

Ask for advice and ask early. There are no stupid questions, only stupid people who don’t ask questions.

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