Redback spider
Redback spider bites are the commonest cause of significant envenoming in Australia. Severe and persistent pain occurs in a half to two-thirds of cases and may be severe enough to prevent sleep in about a third of cases.
Redback spiders live in dry or dark areas and commonly cause bites when people put on shoes or when they move outdoor furniture, bike helmets, firewood or pot plants. Most bites are by the larger female spider and in most cases the spider is recognised by the patient if it is seen. Redback spider bites occur in the warmer months and peak between January and April.
Envenoming by redback spiders is characterised by local, radiating and regional pain which may be associated with local and regional diaphoresis, non-specific systemic features, and less commonly other autonomic or neurological effects. The bite may not be felt or may only be an initial irritation or discomfort.
Pain increases over about an hour and may radiate proximally to the limb or less commonly the trunk. These spiders are small and rarely leave fang marks or cause local bleeding. Local erythema is common and local diaphoresis occurs in about a third of cases. Common nonspecific effects include nausea, lethargy, malaise and headache. Numerous other systemic effects are reported less commonly. The effects last about 1-4 days with almost all cases resolving within one week. There have been no deaths since the 1950s.
The diagnosis is based on the history, but can be difficult in young children and infants who may present with undifferentiated pain or distress.
Treatment
There has been controversy over the management of redback spider bites, particularly who should be treated with antivenom and the route of administration.
Pressure bandaging is contraindicated in redback spider bites. A recent prospective study has suggested that many patients would benefit from antivenom treatment because untreated patients had persistent pain and many were unable to sleep because of it Although intramuscular antivenom has been recommended and used for over 40 years there are concerns that it is less effective than intravenous antivenom.
A recent randomised controlled trial was unable to demonstrate a difference between intramuscular and intravenous routes, but the trial was small and many patients were lost to follow-up. A larger ongoing randomised controlled trial hopes to determine the more effective route. Despite concern about the safety of intravenous antivenom, diluted intravenous antivenom appears to have a similar low reaction rate to intramuscular antivenom.
Symptomatic relief is probably only effective in the most minor cases and even parenteral opiates are ineffective in many cases. Antivenom is recommended for systemic envenoming and for severe local or radiating pain.
The current recommendation is an initial dose of two vials of antivenom given as an intramuscular injection or as a slow intravenous infusion over 15 minutes. Intravenous antivenom may be preferred for severe envenoming, in children or if there is a poor response to intramuscular antivenom. Antivenom has been safely used in breast-feeding and pregnant women.
The use of antivenom 24-96 hours after the bites is reasonable based on the natural course of envenoming and reported response in these cases.
Adverse effects
Early allergic reactions to redback spider antivenom are rare (less than 2%) and premedication is not recommended. Serum sickness is uncommon, but all patients should be warned about it. For moderate to severe cases of serum sickness a short course of prednisone is recommended.
Patients who do not require treatment with antivenom can be discharged and told to return if they require treatment for the pain or systemic effects.
The following statements are FALSE.
1. Most patients bitten by redback spiders only need analgesia and do not require antivenom.
2. Pregnant women should not be given spider antivenom.