RESUMEN
Naltrexone is used to antagonise etorphine immobilisation, but a safe and effective dose for this purpose has not been objectively determined. Eight domestic goats were immobilised with etorphine (0.07 mg/kg) eight times at ≥13 day intervals. Naltrexone at doses of 0.5, 1, 2, 5, 10, 20 and 40 mg/mg etorphine were administered intravenously 17 minutes after etorphine injection. Effectiveness of antagonism was recorded based on recovery and renarcotisation scores and clinical observations. All doses produced rapid recovery to the point of standing (median 59 seconds, range 33-157 seconds), with no significant differences in recovery times (P=0.44). The lower naltrexone doses resulted in renarcotisation in some goats: 4/8 in the 10-mg dose trial, 7/8 in the 5-mg dose trial, and 8/8 in the 2-mg, 1-mg and 0.5-mg dose trials. Lower doses resulted in more severe signs of renarcotisation. Complications of renarcotisation included increased body temperature; this occurred just before signs of renarcotisation and was greater in animals with high renarcotisation scores (P<0.01). The lowest, safest effective naltrexone dose that we used to antagonise etorphine immobilisation was 20 mg/mg etorphine, which produced rapid recovery to standing with no renarcotisation.