Envenomations

Snake bites.

Make sure you take a good history, and repeat it if necessary. Severe anxiety, particularly in adolescents and children, often limits the description to ‘big’ and ‘black’ but careful review of the events leading up to the bite (was somebody trying to catch it?) and the circumstances of the bite may be informative.

Examine the bite site carefully. Do not dismiss relatively innocuous looking marks, and remember that over anatomically ‘difficult’ areas (for the snake) there may be only one puncture wound if the strike wasn’t symmetrical.

While wondering what to do next, prescribe tetanus toxoid and make sure it is actually given – it tends to get forgotten in the heat of the moment.

If there is a chance this was a significant attack, DO NOT send the patient home.

If there is any chance of a boomslang bite, do an INR/PTT and repeat this at six and twelve hours before reassuring the patient. If specific boomslang antivenom is required (i.e. genuine coagulopathy) you may be lucky and find some locally; if not it will mean phone calls and couriers, but it should always be possible to get it in less than 24 hours.

The other two types of snake bite (puff adders and cobras) are easier to recognise. Puff adders are fat lazy snakes and cobras are thinner and have a hood. Information from the patient about the position of the teeth is a luxury that you all too often have to do without. The fat ones (vipers) are cytotoxic (local tissue necrosis which may lead to a DIC) and the thin ones (elapids) are neurotoxic (ptosis, difficulty swallowing, difficulty breathing.)

If you are struggling to differentiate the hyperventilation of acute anxiety from the hypoventilation of respiratory muscle paralysis, do not assume that because there is no hypoxaemia by pulse oximetry that all is well. Cyanosis is a late sign. Rather check FEV1 or vital capacity (not peak flow!) or a blood gas. A low CO₂ in a tachypnoeic patient favours anxiety, whereas a normal or rising CO2 is concerning.

The polyvalent antiserum dose is related to the amount of envenomation, not the size of the patient – small people need just as much antidote. Read the instructions.

In general, for peripheral cytotoxic bites give 50 ml (5 amps) of the polyvalent antivenom, and for cytotoxic bites of the head and neck and neurotoxic bites, give 100ml (10 amps) and for black mamba bites, give 200ml (20 amps). Dilute it in 200 ml normal saline, and give slowly over first 30 minutes (and stay physically around) as most allergic reactions happen early. If stable after 30 mminutes, try and give the rest more rapidly so that it is all in within one hour.

Perspective – efficacy of anti-venom.

A review on bites in the US suggested that the mortality of crotaline bites (rattlesnakes and copperheads which are pit vipers as opposed to the ‘true’ vipers such as puffadders) had been reduced from 5-25% in the 19th century down to 0.5% recently, and intimated that much of this mortality reduction was due to the introduction of antivenom in the 1950s¹. However the true efficacy of antivenom has been poorly studied, for the unsurprising reason that planning a RCT would be ethically problematic. The same review reported an incidence of acute reactions to the antivenom of 14%, and 16% for later development of serum sickness.

Perspective – prophylactic antibiotics after snake bite.

There doesn’t seem to be much useful evidence either way here. For what it is worth, a prospective observational study² of rattlesnake bites found no documented cases of infection in 50 patients not given antibiotics.

Scorpion and spider bites

In adults, those that do come to hospital attention are usually painful but seldom dangerous. Fatality in adults has not been described from any South African species of spider in the last forty years, although the mortality from button spider bites in children may be as high as 5%.

Spiders³

(Images courtesy News-24.com)

The main source of anxiety is the black widow spider, a shy beast that only gets involved with a huge human if sorely pressed. Latrodectus indistinctus is a black or dark brown spider about 12-15 mm long. Young ones have a red stripe on the dorsum of the abdomen, but the stripe fades with age until there is eventually only a little red spot at the back. L indistictus is otherwise known as the black button or black widow spider, and is found mostly in the Western Cape. (There is a little confusion of terminology here – in other countries L mactans is called the black widow.) Latrodectus geometricus is the brown button spider and is ubiquitous throughout South Africa. It has a pretty geometric pattern on the dorsum of the abdomen, and a red hourglass on the ventral surface of the abdomen. (One common variety of garden spider belonging to the Araneus species also has a red hourglass on the ventral abdomen, but is larger (15-20 mm) and fairly hairy with a boring back and is not poisonous to humans.)

Symptoms of envenomation (latrodectism) are due to the neurotoxin known as alpha-latrotoxin, and can be startlingly severe if due to the more venomous L indistinctus. Muscle cramps (80% of patients attending hospital), abdominal pain and cramps (66%) chest tightness (30%), with profuse sweating (70%) and other autonomic features are described in both adults and children bitten by L indistinctus, but are rare (5-10%) in those bitten by L geometricus. The specific antiserum apparently relieves signs and symptoms within an hour in many patients, although RCTs are lacking. There are case reports of untreated (unrecognised) patients remaining ill for up to a week. Muscle cramps have been reported to respond to calcium gluconate, although the effect wears off after 30 minutes.

The condition may be confused with organophosphate poisoning (check the cholinesterase level) or neurotoxic snake bites, although in the latter the bulbar features of dysphagia and ptosis with visual changes occur early and are unusual with spider bites. Tetanus, delirium tremens, acute abdomen, and myocardial infarction have also caused confusion.

Bites from cytotoxic spiders ⁴ (e.g. Loxosceles, or violin spiders and Chirocanthium or sac spiders) can cause severe pain and leave a lesion that initially looks like localised cellulitis (and often gets treated as such) but then may go on to develop a central eschar which will take weeks to heal. Treat the initial pain with adequate analgesia, and consider secondary infection. Tetanus toxoid may be appropriate.

Scorpions

Scorpion bites usually just cause severe pain, but can cause a very similar syndrome to spider envenomation. The key is that everybody notices the bite, as it is immediately painful. In children Parabuthus species (a fat tail and thin pincers) can cause neurotoxic respiratory paralysis, but adults usually just develop limb paraesthesia and some myalgia⁵. The role of calcium in pain relief is as poorly defined as for spider bites. There is antivenom, but its true value is unclear in adults as it doesn’t provide the dramatic symptom response sometimes found with spider bites.

Severe envenomation causes autonomic features as the toxin ( a variety of peptides known as scorpion alpha-toxins) leave voltage-gated sodium channels in a prolonged state of depolarisation, resulting in neuro-excitation with features similar to organophosphate poisoning, although BP and pulse can be variable. Fatalities are related to cardiogentic shock and pulmonary oedema. The thrust of treatment i supportive; antivenom is commonly recommended for such patients.

Digging deeper – efficacy of treatments for scorpion envenomation.

A systematic review from 2017 ⁶ found no evidence for steroids (single fair sized RCT) and apart from that the evidence for anything was extremely weak. Prazosin may reduce autonomic instability and shorten duration of symptoms in children, and although there was no evidence of efficacy of antivenom against old-world scorpions (basically North Africa and Middle East/Asian studies) a couple of linked trials of very poor quality of antivenom in ‘new-world’ scorpions in Mexico showed reduced duration of symptoms. Efficacy of local antivenom is unclear.

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1. Gold BS, Dart RC, Barish RA. Bites of venomous snakes. N Engl J Med. 2002;347:347-55.  ↩

2. LoVecchio F, Klemens J, Welch S, et al. Antibiotics after rattlesnake envenomation. J Emergency Med. 2002;23:327-8 ↩

3. Muller GJ. Black and brown widow spider bites in South Africa. A series of 45 cases. S Afr Med J. 1993;83:399-405 ↩

4. Newlands G, Atkinson P. Behavioural and epidemiological considerations pertaining to necrotic araneism in Southern Africa. S Afr Med J. 1990;77:92-5. ↩

5. Muller GJ. Scorpionism in South Africa. A report of 42 serious envenomations. S Afr Med J. 1993;83:405-11 ↩

6. Rodrigo, C., Gnanathasan, A. Management of scorpion envenoming: a systematic review and meta-analysis of controlled clinical trials. Syst Rev 6, 74 (2017). https://doi.org/10.1186/s13643-017-0469-8 ↩

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