Used for neutralising systemic envenoming by Australian red back spiders, which are members of the black widow spider group found world-wide. It is made from horse IgG. Each ampoule contains 500 units of neutralising capacity against the target venom. Average volume per ampoule is 1-1.5mL. The immunising species is the Australian red back spider.
Although this antivenom is developed purely for use against bites by the Australian red back spider, there is evidence it is effective against bites by any black widow spider. It may also be effective against bites by related Theriid spiders, such as the cupboard spider, Steatoda spp.. Red back spiders usually build webs under objects, with droplines to the ground or other flat surface, thus they may commonly be found under lower shelves, the bottom rails of fence lines, in refuse areas, especially old car bodies, even on the underside of seats left outside and in the recesses of cupboards indoors! Only the female is dangerous to humans. As they tend not to wander, most bites occur when the person comes in contact with the web structure.
Red back spiders are found throughout Australia, in essentially all habitats, from very arid through to tropical, and they are common in urban areas, where most bites occur.
The venom is multicomponent, with a family of protein toxins, the latrotoxins, most prominent. One of these, a-latrotoxin, is effective in mammals, including humans, causing stimulation of neural synapses throughout the body with wide ranging effects.
The clinical features of major envenoming by red back spiders are sometimes Herculean in extent, but rarely lethal. Less than 20% of bites actually result in significant envenoming, and it is only in this latter group that antivenom therapy should be considered. The classic effective red back spider bite is felt as a mild sting only, with little to see at the bite site. Between 10 and about 40 minutes later, the bite site becomes painful. The pain becomes severe, over a variable timeframe, extending proximally and involving draining lymph nodes in the axilla or groin. There is often local sweating. The pain may then spread to the abdomen, chest, neck or head, often associated with profuse sweating, either localised or general, mild to severe hypertension, and malaise with nausea. Many other symptoms or signs may occur, but the forgoing are clinically most consistent and useful for diagnosis. The progression from local to generalised pain may occur within 1 to 3 hours, or take up to 24 hours. In infants, general miserableness, refusal of feeds, inconsolable crying, and sometimes a non specific erythematous rash are key features of red back spider bite. Occasionally the initial bite may not have been noticed and the patient may present with abdominal or chest pain. In such cases, careful questioning will usually elicit a history of possible exposure to spider bite and initial localised pain, pointing to the true diagnosis. In pregnancy, the generalised abdominal pain of red back spider bite may appear similar to onset of premature labour. Usually a clear history of a bite with initially local pain will be available.
As with most antivenoms, CSL Red Back Spider Antivenom should only be given if there is clear evidence of envenoming. See potential contraindications. It should be given intramuscularly in most cases, not IV. Prior to commencing antivenom therapy, make sure everything is ready to treat anaphylaxis, should this occur. Specifically, have adrenaline ready to give.
The use of adrenaline as pre-treatment when using antivenoms is still being debated. The risk of anaphylaxis varies from antivenom to antivenom. People who have had previous exposure to equine derived products may be at greater risk. It is recommended that the Product Information be read before use and if necessary contact be made with a specialist in the field.
Because of its low volume, CSL Red Back Spider Antivenom has a very low incidence of adverse reactions and this has allowed a lowering of the threshold for using it. Any patient with symptoms consistent with significant envenoming by a red back spider, such as local then generalised pain, sweating, ± hypertension, should be considered for a trial of the antivenom, even if no spider was seen. Give 1 vial of CSL Red Back Spider Antivenom IM. If there is a complete or partial resolution of symptoms, the patient's distress has been alleviated and the diagnosis confirmed. If the initial response to antivenom is incomplete or a relapse occurs, give a further vial of antivenom. Occasionally it is necessary to give a third vial, but only rarely more than this, except if treatment has been delayed more than 24 hours, when higher doses are frequently required.
Red back spider bite is one of the few cases where antivenom may be effective days after the bite. If the symptoms fit red back spider bite, it is quite acceptable to try a dose of antivenom a week, or more, after the original bite occurred. Multiple doses may be required.
If, after 3 ampoules of antivenom, either early or late, there has been no improvement at all in symptoms, then reconsider the diagnosis. Patients in whom antivenom treatment has been delayed more than 24 hours occasionally require higher doses of antivenom, but it is often adviseable to extend the time between doses and consider the IV route. CSL Red Back Spider Antivenom has been given to pregnant women on a number of occasions, without detriment to the mother or foetus, with a normal infant delivered at term being the outcome.
If there is either a sudden fall in blood pressure or bronchospasm, after giving the antivenom, give adrenaline by subcutaneous injection, give 100% O2 and IV fluids (Haemaccel®).
If adrenaline is to be given by subcutaneous injection, use a 1:1000 solution. For adults give 0.5mL (0.5mg) initially. For children give 0.01mg/kg initially. Repeat as necessary and try IM injection if no response to SC injection.If bronchospasm is the major problem, try nebulised adrenaline, 2mL of 1:1000 solution.
Any patient who has received antivenom may develop serum sickness, from 4 to 14 days later, but this is quite rare for CSL Red Back Spider Antivenom. Before leaving hospital, they should be advised of the symptoms of serum sickness, such as rash, fever, joint aches and pains, malaise, and told to return immediately for review and probable commencement of oral steroid therapy. If there was major envenoming, organise follow up. This is usually not necessary for envenoming by red back spiders.