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8.4.5 Use Of Anaesthetic Overdose

If the brain is to be sampled at necropsy then an overdose of anaesthetic is recommended. An overdose of barbiturate anaesthetic is commonly used. The carcass must be burned or buried immediately after the necropsy to avoid poisoning animals that might eat parts of the carcass. A knackery or rendering service may refuse to remove it.

An alternative approach is to administer a maximal dose of xylazine sufficient to anaesthetise the animal. When the animal is unconscious, a saturated solution of magnesium sulphate (also known as epsom salts and one of the first large animal anaesthetics) is injected intravenously to kill the animal. The dose rate of a saturated magnesium sulphate solution required to kill an animal is 2 mL/kg bodyweight administered intravenously. The 1 L required to kill a steer could be administered via a flutter valve fixed to a bottle and a 14 to 16g needle. The 100 mL required to kill an adult wether could be administered via two 50 mL syringes with an 18g needle. Prior anaesthesia with xylazine is always necessary except when the animal is already moribund.

The advantage of the xylazine/magnesium sulphate method is that it is cheap and renders the carcass distasteful. Scavengers, if they should eat the carcass, would be unaffected other than perhaps by temporary vomiting and diarrhoea.