Spider bite

Introduction

Australasia is home to a large variety of arachnids including spiders, scorpions and ticks. Spiders are the most medically important arachnids in Australasia and include redback spiders and funnel-web spiders (FWSs). FWS envenoming occurs rarely in Eastern Australia and can cause severe and potentially life-threatening neurotoxicity. Redback spider envenoming (latrodectism) occurs throughout Australia and causes a local or regional pain syndrome associated with non-specific systemic symptoms and, less commonly, autonomic effects. Other spiders that commonly cause human bites are not associated with major medical effects and include huntsman spiders (Sparassidae) , orb-weaving spiders (Araneidae) , white-tail spiders ( Lampona spp.), wolf spiders (Lycosidae) and jumping spiders (Salticidae) . FWS envenoming has resulted in death prior to antivenom, but still remains a life-threatening condition.

An approach to the patient with spider bite

Initially, a careful history should be taken to determine whether the patient has suffered a definite spider bite or only a suspected spider bite. The diagnosis of definite spider bite requires sighting of the spider at the time of the bite and usually some initial symptoms, such as local pain. If there is no history of bite or no spider was seen, then other diagnoses must be considered first. This is particularly important in persons presenting with ulcers or skin lesions with suspected spider bites ( Box 26.3.1 ). It is important in these cases that appropriate investigations are done and the case treated as a necrotic ulcer of unknown aetiology. In the majority of these cases, an infective cause is found, although less commonly they are a result of pyoderma gangrenosum or a vasculitis.

If the patient has a definite history of a spider bite and has either captured the spider or has a good description of the spider, a simple approach can be taken. Health professionals should not attempt to identify spiders beyond the following simple classification:

• redback spider

• moderate to large black spider that is potentially an FWS in Eastern Australia

• all other spiders.

The majority of redback spiders are likely to be identified correctly and, with supporting clinical features, this diagnosis is usually straightforward.

The second group is only important in regions where FWS are known to occur and cause significant effects (east coast of Australia from Southern Queensland to Southern New South Wales [NSW]). If the spider is large and black, then the patient should be managed as an FWS. A pressure bandage with immobilization (PBI) is the appropriate first-aid measure. The PBI should only be removed with FWS antivenom immediately available (i.e. antivenom present in the emergency department [ED] but not opened). Once in the ED they should be observed for at least 2 hours after the pressure bandage has been released or after the bite in a patient without first aid. If the patient is asymptomatic at this time, they can be safely discharged. No attempt should be made to identify the spider because the distinction between some FWS and the less significant trapdoor spiders is impossible for non-experts.

The third group includes all other spiders. Despite previous concerns about particular spiders, such as the white-tail spiders, all other spiders are very unlikely to cause more than minor effects. Patients can be reassured, their tetanus status confirmed and updated, if required, and symptomatic treatment with ice and analgesia can be offered. These patients do not need to be observed in hospital.