Snakebite

Introduction

Australia has a number of venomous snakes with some of the most potent venoms in the world. All the medically important snakes are elapids (front-fanged), although bites rarely occur from colubrids and non-venomous snakes. New Zealand has no snakes of medical importance. The risk of significant coagulopathy and uncommonly death, even after apparently trivial contact with Australian snakes, remains and must be appreciated by health care workers.

Epidemiology

It is thought that approximately 3000 suspected snakebites occur annually in Australia, but this figure is difficult to estimate and depends on how many suspected bites, non-venomous bites and non-envenomed cases are included. The number of envenomed cases is far less and probably in the order of 100 to 200 each year; the majority of which occur in rural and regional areas. Snakebite deaths continue to occur (about 1 to 5 per year) and are usually a result of early cardiac arrest in brown snakebites or major haemorrhage in coagulopathic patients.

The commonest clinical manifestation is coagulopathy, which occurs in about three-quarters of envenomed cases (the majority in brown snake bites). Neurotoxicity and myotoxicity are now uncommon, and mechanical ventilation is rarely required for treatment. The types of snakes causing major envenoming differ across Australia. Bites in snake handlers remain an important problem, with about 10% of all bites being in snake handlers. However, they are almost all bites from Australian snakes, albeit the more uncommon and interesting snakes and exotic snakebite is very rare. Although snake handlers often want to avoid antivenom, they should be treated like anyone else because there is little evidence to support they are at higher risk of antivenom reactions. Snake handlers and people working with snake venoms can develop systemic hypersensitivity reactions to venom itself, so venom anaphylaxis must be a differential diagnosis in these patients.

Prevention

Most snakebites are preventable and result from snake handling or interference with snakes in the wild, sometimes in the setting of alcohol consumption. Ideally, snakes should be left alone, and those working with or keeping snakes should have appropriate training and licences. Simple precautions, such as wearing thick long pants and boots when walking in the bush or when working with snakes, can prevent most bites due to the short length of Australian elapid fangs. Snake handlers should carry and maintain first-aid kits that include at least four broad elastic bandages (15 cm; e.g. ACE), and they should practise applying the bandage. If exotic snakes are being held, including Australasian snakes out of their geographical distribution, appropriate antivenoms should be available.

Clinical features

Systemic envenoming results when venom is injected subcutaneously and reaches the systemic circulation. Whether or not a snakebite results in systemic envenoming depends on a number of factors, including fang length, average venom yield of the snake, effectiveness of the bite and bite site. Recent studies have suggested that only a small amount of the injected venom actually reaches the systemic circulation. Most snakebites do not result in envenoming because either insufficient venom reaches the systemic circulation or the snake is non-venomous.

Envenoming is characterized by local and systemic effects, although Australasian elapids rarely cause major local effects, such as necrosis and local haemorrhage. The clinical features of envenoming depend on the particular toxins present in each snake’s venom, but non-specific systemic symptoms (nausea, vomiting, headache, abdominal pain, diarrhoea and diaphoresis) occur in many cases. The major clinical syndromes are coagulopathy, neurotoxicity, myotoxicity and acute kidney injury. Severe envenoming can result in early collapse associated with dizziness, loss of consciousness, apnoea and hypotension. In the majority of cases, there is spontaneous recovery over 5 to 15 minutes, but in some cases, this does not occur, and multiorgan failure and death ensue if resuscitation is delayed.

The medically important Australian snakes and their associated clinical effects are listed in Table 26.1.1 .