Redback Spider

The Australian Red Back Spider (Latrodectus hasselti) is a close relative of the American Black Widow Spider (the main difference being the presence of a red stripe on the spiders back). It is widely distributed throughout Australia in essentially all habitats and is common in urban areas. The spider typically builds webs under objects and in dry, sheltered spaces (seats / shelves / fence lines etc) and has drop lines to the ground. Only the female Red Back Spider is dangerous to humans. The female spider does not wander far from its web and unlike the funnel web spider, is not aggressive with most bites occurring during summer months when people make contact with the web.

Red Back Spider venom

Of the multiple toxins that make up the female red back spider venom, the component most responsible for symptoms in humans is the latrotoxin. This toxin causes excessive stimulation of nerve pathways throughout the body with a wide range of effects sometime referred to as ‘latrodectism’.

Symptoms of envenomation

Although the bite is frequently very painful, it is rarely lethal.

• Localised symptoms. Due to the small fangs, the spider bite is often painless and puncture marks may not be visible. Intense burning pain is usually felt within 5 minutes at the bite site and the characteristic finding of a raised welt with small drops of overlying sweat will confirm the diagnosis. The pain will frequently extend up the limb with resultant tenderness over lymph glands in the groin or arm pit
• Generalised symptoms. The pain may spread to the abdomen or chest and if this occurs is often associated with profuse sweating, nausea and occasionally vomiting. In very severe cases it may cause muscle weakness and tremor and is frequently responsible for elevated blood pressure and rapid and irregular heart beat

Treatment

In the Field

The principles of general First Aid should be followed. The pressure immobilisation technique should not be applied as this will not inhibit spread of the venom and may cause an increase in pain at the bite site. Ice may be applied to the bite site to reduce pain.

If the spider has been caught (ensuring no risk involved) it should be transported to the hospital with the victim to assist with identification.

Antivenom

CSL Red Back Spider Antivenom is for neutralising the venom of the Australian Red Back Spider but has shown to be effective against most of the black widow group of spiders. It is produced from Horse serum.

The antivenom should only be administered if there is clear evidence of severe symptoms of envenomation that fail to respond to simple treatment measures (pain relief / ice etc). It may be administered in the field but must be administered by experienced medical personnel and all victims of envenomation require transport to hospital. Although uncommon, anaphylaxis to the antivenom can occur and the capacity to manage anaphylaxis must be available.

The antivenom may be administered as intravenously or intramuscularly push. Recent evidence suggests IM and IV administration are equally effective.  The initial dose is one ampoule. The dose should be repeated as required according to response to treatment. It is uncommon to need to give more than three ampoules.

There are many case reports of the antivenom having dramatic effect when given up to a week or more following the bite.  However, there is some concern that the antivenom may be no more effective than placebo.  There is a clinical trial comparing the effects of IV redback antivenom with placebo currently underway in Australia.

Following antivenom

The tetanus immunisation status of the victim should be checked and updated if necessary.

The victim should be observed for a minimum of 6 hours following antivenom administration to ensure no side effects

The victim should receive advice on the potential to develop serum sickness if multiple doses of antivenom were required.