Choice of antivenom: Snake Identification
Identification of the offending snake will aid in the choice of the appropriate antivenom and alert clinicians to particular features characteristic of envenomation by that type of snake. In cases of snakebite involving zoo staff, herpetologists or other experienced snake handlers, the snake’s identity may be known (although this cannot always be relied upon, particularly in the case of enthusiastic amateurs). Identification of snakes by the general public or by hospital staff is frequently unreliable. Sometimes, the snake is not seen, or is only glimpsed in retreat.
Australia is the only country in the world that has snake venom detection kits.They consist of a rapid two step enzyme immunoassay in which wells are coated with antibodies to the various snake venoms. A swab from the bite site, blood, or urine helps to select the type of snake antivenom which may have to be used. Note that the primary purpose of the venom detection kit is not to decide whether envenomation has occurred (i.e. whether antivenom is indicated), but to help to choose the appropriate antivenom if required.
About Snake Venom Detection Kits (VDKs)
- A positive VDK result is not an indication for antivenom.
Venom detected at the bite site may be present in insufficient quantity in the circulation to cause significant illness.
- A negative VDK result does not mean that envenomation has not occurred.
The venom may have been washed off or diluted at the bite site, or may not have reached the urine, but still be present in the circulation.
- Bite site swabs are considered to be the most reliable sample for use in venom detection kits.
Venom may also be obtained from clothing, or even from the fangs of dead snakes.
Note that very high concentrations of venom in the sample may cause inaccuracies in the venom detection kit result. If this occurs, the sample should be diluted and the test repeated.
- Blood and urine samples may also be used in the venom detection kit, but are generally thought to be less reliable than bite site swabs. Urine in particular may be used if there has been some delay in presentation, or if no bite site can be identified.
- Venom Detection Kits should be used in conjunction with other information (such as clinical presentation, knowledge of snakes in the geographic area, identification of snakes brought to hospital with the patient) to determine which antivenom to use if the patient is significantly envenomed. If a reliable identification cannot be made,then polyvalent snake antivenom (or the appropriate combination of monovalent antivenoms in Victoria and Tasmania) should be used.
Administration of antivenoms
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.