Abstract

Safe and efficacious immobilization of marine mammals continues to be an area of intensive investigation. Particularly challenging are large, free-ranging otariids. They are not easily manually restrained and do not have readily accessible blood vessels allowing intravenous administration of short-acting agents that can be safely titrated to effect. For these animals, administration of anesthetic agents continues to be best accomplished by the intramuscular (IM) route. However, many currently used IM agents are associated with prolonged recovery times that can create problems in field conditions. The recent introduction of the α2-agonist medetomidine to North America may provide a distinct advantage over previously used agents because of its reversibility by the α2-antagonist atipamezole. The purpose of this study was to evaluate two medetomidine-based immobilization combinations in California sea lions (Zalophus californianus).

From May 1997 to August 1998, 24 male and 27 female California sea lions, varying in weight from 13.5 to 145 kg, were immobilized for a variety of medical procedures at a rehabilitation center in northern California using a combination of medetomidine and ketamine. Atropine (0.02 mg/kg) was given IM to each animal at least 10 minutes prior to administration of the medetomidine/ketamine combination. Each animal was given 140 µg/kg medetomidine and 2.5 mg/kg of ketamine IM by either hand injection (n = 47) or blowdart (n = 4). Both drugs were administered together in the same syringe. Sites of injection included muscle immediately surrounding the pelvis, femur and tibia and muscle overlying the scapula.

Time to maximal effect (mean ±SD) for hand injection was 8 ±5 min and was significantly (P < 0.01) lower than for those animals immobilized by blowdart (17 ± 6 min). An adequate plane of anesthesia was not achieved in 11 (22%) animals, 2 of which had been blowdarted, and additional ketamine (1/2 of the original dose) was given to 4 of the animals. The remaining 7 animals were physically restrained to complete the desired procedures. Sixteen animals were intubated during the procedure. Six (37%) of these were intubated after being given medetomidine/ketamine only, while the remaining 10 were masked down with isoflurane to allow intubation.

Total immobilization times varied from 9 min to 57 min with a mean of 25 ±12 min for animals that were given only the initial dose of medetomidine/ketamine and were not placed on gas anesthesia. Recovery times for these animals were 9 ± 7 min after being given 200 µ/kg atipamezole IM. Prolonged recoveries (greater than 20 min) were noted in 5 (10%) of the animals. No animals died during the study.

Disadvantages of medetomidine/ketamine use in sea lions include a moderate variation in induction time and plane of anesthesia. Since there was a significant difference in these parameters between animals anesthetized by hand injection and by blowdart administration (where penetration into muscle may not have been reliably achieved), it is thought that some of the variation may be due to injection into poorly vascularized sites such as blubber. In addition, the commercially available 1.0 mg/ml solution of medetomidine requires that very large volumes be used at the recommended dose for sea lions. However, the availability of more concentrated solutions of 10 and 20 mg/ml has resulted in a much more manageable injection volume. The high dose and relatively high price of medetomidine, though, result in a high cost for immobilizing each animal. The advantages of medetomidine/ketamine include safe and reversible immobilization that can be easily administered by the IM route and that produces a plane of anesthesia that is sufficient to carry out most routine diagnostic procedures. From May to November 1998, 8 male and 15 female California sea lions varying from 15 to 220 kg, were immobilized by hand injection of 70 µg/kg medetomidine with 1.0 mg/kg of Telazol® (a commercially-available 1:1 solution of tiletamine and zolazepam). These animals were anesthetized for similar reasons and using the same methods as described above. Time to maximal effect in this group of animals was 5 ± 2 min and was significantly less than for animals immobilized using medetomidine and ketamine. Adequate plane of anesthesia was not accomplished in only 1 (4%) of the 23 animals. Sixteen animals were intubated during the procedure and 7 (44%) were intubated after being given only the initial dose of medetomidine and Telazol®. The remaining animals were intubated after masking with isoflurane for a short duration.

Total immobilization time for animals given only medetomidine and Telazol® varied from 25 to 62 min with a mean of 39 ±15 min. Recovery time was 8 ± 6 min for these animals, which was not significantly different from animals given medetomidine and ketamine. However, prolonged recovery times or rough recoveries characterized by muscle fasiculations or shaking were noted in 6 (26%) of animals. Additionally, one (4%) sea lion died during the procedure. The cause of death was not immediately apparent from gross post mortem and histopathology results and was probably related to the anesthetic procedure. Disadvantages of the use of medetomidine and Telazol® in California sea lions include a moderately prolonged recovery time and a potentially increased risk in comparison to that noted with medetomidine and ketamine. Advantages include a significantly lower time to maximal effect, a more reliable plane of anesthesia, smaller injection volume and lower cost.