red back spider bites / Latrodectism

introduction

the red back spider ((L hasselti)) belongs to the widow spider family (Latrodectus sp.) and are a common cause of spider bite in Australia with 3,000-5,000 bites per annum which result in Latrodectism which is primarily due to neurotoxins such as alpha latrotoxin
most bites occur when people accidentally put their hand in the web and the spider feels threatened, or put on clothing containing a spider

the most consistent and prominent symptom is pain at the bite site which generally increases over the 1st hour and may spread proximally up the limb
this may be associated with other local features such as sweating, local erythema, piloerection, and systemic features such as:
- nausea, malaise
- headache
- abdominal pain
- tachycardia
- hypertension
- diaphoresis elsewhere (eg. bilateral legs)
in some cases, there may also be:
- paraesthesiae
- fever
- muscle spasms and even some patchy paralysis
- priapism
it is rarely lethal (none since 1950 in Australia) although bites to babies may be a significant risk
effects generally last 1-4 days and nearly all resolve within 1 week

unlike snake bites:
- first aid principles of compression and immobilisation are NOT indicated and may increase the pain
- there is generally NO need for blood tests nor IV cannulation, unless you also suspect snake bite
- there are NO diagnostic tests - the diagnosis is clinical
- antivenom may NOT be needed even in those with envenomation
- consider using an ice pack locally
it is rare for victims to require any Rx other than analgesia (although these analgesic regimes tend to be poorly effective in these bites) such as:
historically most victims were given im or iv red back antivenom, however, a recent study by Isbister et al suggests that the benefits of antivenom in reducing pain, systemic symptoms or sequelae in those over 8 years of age may not outweigh the 3-4% risk of allergic reaction ¹⁾
older children and adults generally do not need observation in hospital, nor anti-venom
it may be prudent to given anti-venom to younger children and consider admission for observation, particularly in the very young

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