Australian antivenoms are amongst the safest in the world. Many patients, even those with a past history of reaction to equine proteins, such as snake handlers, have had minimal or no problems with repeat antivenom therapy after premedication. They are, however, foreign protein, and the possibility of allergic reactions, including life threatening anaphylaxis, should always be considered and facilities be made available for dealing with such complications before the administration of antivenoms. Adrenaline is the treatment of choice for anaphylactic and anaphylactoid reactions, in conjunction with bronchdilators, fluid replacement etc.
Most antivenoms are given by the intravenous route, although redback spider antivenom and box jellyfish antivenom are more often used intramuscularly. Skin testing for allergy to antivenom is not recommended, as it is unreliable and may delay urgent therapy.
Antivenoms that are given intravenously should be diluted in at least 100ml of N. Saline, 5% dextrose or Hartmann’s solution immediately prior to administration. It should initially be administered slowly while the patient is observed for signs of allergic reaction. If no reaction is observed, then the rate of infusion may be increased. If the patient reacts to the antivenom, the rate may be slowed or the infusion ceased temporarily. If the reaction is more severe, then treatment with adrenaline, plasma volume expanders and beta agonists should be undertaken as required. The decision to recommence antivenom should be based on the clinical state of the patient. In the case of the patient with a known allergy to antivenom or to horse serum, the decision as to whether to withhold antivenom should again be clinically based, bearing in mind the resuscitation facilities available. The initial doses of antivenom recommended are based on the average amount of venom injected by each of the snakes concerned. There is evidence, however, that these doses may be insufficient to reverse coagulopathy associated with the bites of several Australian venomous snakes, notably the brown snake and the taipan. Larger initial doses should be considered if there is evidence of severe envenomation (multiple bites, rapidly progressive symptoms, large snakes).
The dose of antivenom for children should not be reduced according to their weight, since the amount of venom injected by the snake is independent of the victim’s size.
Note: the antivenom requirements of patients will vary considerably. Some patients with minimal envenoming will required no antivenom, whereas others may require multiple doses of antivenom.
Detailed information regarding the use of antivenoms is packaged with the individual antivenoms. If you require additional advice, contact the Information Centres on 13 11 26 (Australia wide) or CSL Ltd on (03) 9389 1911.