The estuarine or Indo-Chinese crocodile (Crocodylus porosus) is a large, semi-aquatic reptile, which inhabits marine, estuarine and freshwater environments in the tropical and subtropical regions of India, southeastern Asia, Indonesia, Papua New Guinea, islands of the south Pacific and northern Australia.1 Historically, crocodiles have been physically restrained. This is potentially dangerous even under controlled conditions, and may require a team of 6–8 people. Chemical restraint has been used for more than 35 years, but has relied predominantly on administration of paralysing agents such as gallamine or more recently pancuronium, which effectively immobilise animals but result in very prolonged recoveries, and occasionally unexpected deaths. Recent research has focussed on agents that are safe, reliable and humane in large animals under varying environmental conditions.
Prior planning is critical to the success of safe anaesthesia, taking into consideration the prevailing environmental conditions, facilities and personnel available for monitoring, and the purpose for which the anaesthesia is required. Part of planning the procedure involves determining how, where and when the drug of choice is to be administered to the animal. Injection site may affect the reliability and efficacy of the drug being used.
Suboptimal environmental temperatures may affect the onset, duration and action of immobilising agents.2 The conditions during induction and maintenance of the animal are as important as during the recovery period, which could be between one and many hours. A constellation of signs should be monitored routinely, while constantly remembering that animals can suddenly change from appearing comatose to being very responsive. If prevailing conditions are extreme, postpone the procedure.
Midazolam, a benzodiazepine, provides good muscle relaxation for about 40–60 minutes, and is an effective anxiolytic. It provides some analgesia and can be used for minor procedures in combination with local anaesthetic, as a premedicant to enable intubation or for restraint and transport.3
Medetomidine is an effective and potent muscle relaxant and sedative with good analgesic properties. Recent research in estuarine crocodiles indicates an allometric relationship between body mass and dose rate of medetomidine.4,5 It produces immobilisation for 90 minutes or longer, and intubation is possible in large animals. It is readily reversible with atipamezole administered at equal dose by volume.2,4,5 Reversal is recommended since medetomidine produces sedation and can affect normal behaviour for up to 2 days following induction if unreversed.
Pancuronium in combination with midazolam or medetomidine has been used in large crocodiles. Neostigmine reverses pancuronium at preferred environmental temperatures, but appears highly unreliable at suboptimal temperatures.
In conclusion, safe anaesthesia of large crocodiles is achievable within certain limitations. However, the importance of appropriate planning, a competent team leader, experienced handlers and appropriate post-anaesthesia monitoring is absolutely critical. Chemical restraint should be chosen as an adjunct and not an alternative to good management by experienced handlers. It is important for veterinarians to be capable of providing humane, effective and safe immobilisation and anaesthesia for large crocodiles when required.
The author wishes to thank Dr. David Phalen of the University of Sydney, Mr. and Mrs. Peter Freeman of Hartleys Crocodile Adventures and North Queensland Wildlife Trust, and Mrs. Nick Stevens and David Leyden of Hartleys Creek Crocodile Farming Company.
* Presenting author
1. Thorbjarnarson J. Crocodiles: An Action Plan for their Conservation. Gland, Switzerland: International Union for the Conservation of Nature and Natural Resources; 1992: 136 pages.
2. Olsson AR, Phalen DN. The effects of decreased body temperature on the onset, duration and action of medetomidine and its antagonist atipamezole in juvenile farmed estuarine crocodiles (Crocodylus porosus). Vet Anaesth Analg. 2013a;40:272–279.
3. Olsson AR, Phalen DN. Comparison of biochemical stress indicators in juvenile captive estuarine crocodiles (Crocodylus porosus) following physical restraint or chemical restraint by midazolam injection. J Wildl Dis. 2013b;49:560–567.
4. Olsson AR, Phalen DN. Preliminary studies of chemical immobilisation of captive juvenile estuarine (Crocodylus porosus) and Australian freshwater (C. johnstoni) crocodiles with medetomidine and reversal with atipamezole. Vet Anaesth Analg. 2012a;39:345–356.
5. Olsson AR, Phalen DN. Medetomidine immobilisation and atipamezole reversal in large estuarine crocodiles, Crocodylus porosus, using metabolically scaled doses. Aust Vet J. 2012b;90:240–244.