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Proceedings of the First Symposium on Marsupials in New Zealand

Topic 1: Anaesthesia & Handling Animals

page 235

Topic 1: Anaesthesia & Handling Animals

CUMMINS. To start this discussion session, I draw your attention to the topic of anaesthesia. Those of us working on marsupials in the laboratory, and those examining them in the field require appropriate means of subduing or anaesthetising them. To start with, at Victoria University we have used a CIBA barbiturate product called Dial quite successfully for longterm anaesthesia of brushtail possums. With a dose rate of 75–80 mg/kg we get deep anaesthesia which can be maintained up to 7 hours. We use it in physiology class demonstrations and certain longterm experiments. It is not suitable for work on the central nervous system.

SUTTON. We do get deaths using this barbiturate, often at quite low dose levels. We think stress might be involved. Premedication with Ketamine-HC1 does not appear to have any effect.

MORIARTY. What route were you using?

CUMMINS. Intravenous.

WHITE. At Ecology Division I have used halothane gas anaesthesia for possums. The animal is placed in a container, and halothane passes in with oxygen as the propellant. The animals are anaesthetised in about a minute, though this varies with size. I do not know the exact percentage of halothane/oxygen mix but the mixed gas is supplied at 5–6 litres down to less than 1 litre per minute. The animals can be maintained in deep anaesthesia in this way. It works well, though a few have died and I agree with Mrs Sutton that it may be due to some nervous reaction. However, it has proved the safest and fastest anaesthetic of some fifteen I have tested. The possums come round within 2–3 minutes and appear normal within 5–10 minutes. There appears to be some addiction of possums to the gas.

CUMMINS. Do you see halothane as a field anaesthetic?

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WHITE. I encountered problems in the field, for instance the gas may not disperse fast enough for the operator's health.

GREEN. We have tried both halothane and ether in the field on possums. Halothane has no great advantage over ether for us, and is far too expensive for the use we require. We have had no deaths from either anaesthetic. With halothane the possums tended to come round very quickly after doing an operation like ear-tattooing; with ether recovery was a little slower and therefore better for our purposes.

CUMMINS. We have noticed some respiratory problems using ether, have you found this in the field?

GREEN. No, we operate fairly close to the 'knock-out' point and measure the animals as soon as the eyes close. We apply artificial respiration if necessary by holding them by the neck and rump and making concertina-like movements with them.

COLEMAN. We also noticed animals under halothane seemed to come out on a "high", which did not happen with ether.

FITZGERALD. How did you administer ether and halothane in the field?

GREEN. By using a 1 ft3 (0.3m2) perspex box that was applied to the end of the trap. Gas was applied using a foot pump.

FITZGERALD. So you had no control over the dose?

GREEN. No, though we can observe the animal's response.

MORIARTY. We have used halothane for many years and rarely had a death. I agree some possums do become addicted to it. The type of anaesthesia you want depends on what you do with the animal. We started using halothane to harvest lymphocytes from possum blood. It proved expensive so we used ether but found that was not suitable for good yields of lymphocytes so went back to halothane.

CUMMINS. Anaesthetics typically function by increasing the cell membrane fluidity so presumably your lympocytes were more fragile with ether.

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HATHAWAY. I have done many hundreds of ether anaesthetisations but I have only lost one possum and that was due to inattention.

SUTTON. We control the level of anaesthesia with a portable anaesthetic machine that can be used in the field. We have used it for halothane and penthrane. Using penthrane the degree of analgesis is much greater and they take longer to go down - some 20 minutes. By using halothane in conjunction with scoline (succinyl choline chloride) as a muscle relaxant we find we can just keep possums down so that the central nervous system is not too depressed. This allows neuro-physiological work to be carried out. As an alternative to scoline we have used intravenous valium - which is not too expensive and since it potentiates the halothane, less halothane is needed.

MORIARTY. Would anyone accept a couple of strong men each with a pair of gloves and dispense entirely with anaesthesia?

JANE. When trapping and marking at Kaingaroa we used such a method.

GREEN. We only anaesthetise possums at the first capture for tattooing and detailed examination; all subsequent recaptures are done without anaesthetic and that includes measuring the head-lengths of pouch-young.

FITZGERALD. We have used a cylindrical retaining device made out of plastic cyclone netting through which wire pegs are inserted to secure the possum.

CUMMINS. I wonder what the field workers feel about using muscle relaxants?

WHITE. In the Orongorongo Valley we have used scoline since the beginning of studies by Dr Crawley. Despite my trials with anaesthetics I've come back to using scoline. Typically it takes only 3–4 minutes to work, the animal stays immobilised for 5–6 minutes and recovers over 5 or so minutes. We use a 10% solution of scoline. We rarely lose animals but on occasions have to apply artificial respiration.

JULIAN. We experimented with various intramuscular anaesthetics and found saffan was satisfactory for the short-term in possums, allowing both tranquilisation and cardiac punctures. We used 1–1.5 ml.

KEBER. With saffan, once the animal is disturbed it may take 15–20 minutes to take effect and there is a long period of recovery. So in the field page 238 I am still using scoline.

MEADOWS. I have used ketamine successfully on wallabies. With a dose of 0.5–1.0 mg/kg they go under in about 30 mins. and stay under for 30–45 mins.

CUMMINS. There seems to be a difference between macropods and possums. We tried ketamine on possums and it is certainly not a long-term anaesthetic for them. Again the research group in the University of West Australia use nembutal for wallabies but in possums I found animals still crawling off the table with 2–3 times the lethal dose when injected intravenously.

FITZGERALD. Direct injection of nembutal into the liver of possums seems to be effective.

HAMILTON. For ether anaesthesia we simply put the possums in a bucket, pour ether onto cotton wool, place on a lid and wait until the animal is anaesthetised. However while ether is quite a good anaesthetic, we shifted to using saffan on advice from Wallaceville Animal Research Centre. With saffan we got a fairly good relaxation, good surgical anaesthesia and we could do cardiac punctures with no recovery problems.

RAMADASS. How do you quieten down the animals before anaesthesia?

SUTTON. We use intramuscular ketamine at 25 mg/kg.

CUMMINS. I am sure our veteran possum researchers will know that a good thick sack for half an hour can be a very effective tranquilliser. The animal can be anaesthetised through the ear merely by pulling it out of a fold in the sack.

MEADOWS. I think many people are trying to use anaesthetics or similar agents in ways which are not suitable. Many of the so-called criticisms of drugs may not be because the drug is no good but that people are not using them in the right way.

SUTTON. That could apply to saffan because it can be absorbed erratically if not used intramuscularly and so it may take longer to take effect.

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WHITE. While the analgesic rompun is used on animals such as goats, on possums it is not satisfactory for they go crazy. A useful gas is nitrous oxide which only takes some 25 seconds to take effect. The animal can be kept under for your examination period and recovery takes only about 5 seconds with apparently no side-effects. However, there is a risk to the operator if the gas escapes.

CUMMINS. Nitrous oxide is quite useful to use prior to the application of halothane if trouble is encountered in inducing halothane anaesthesia.

D.J. BELL To what extent are pouch-young affected when the mother is anaesthetised?

FITZGERALD. We have no records of loss of pouch-young.

HATHAWAY. I have also had no trouble in losing small pouch young from 5g and older.

B.D. BELL. In the field we have carefully monitored survival of pouch-young and there is no evidence of increased losses of young in those females that have been immobilised more often using scoline. So with scoline, at least, there is no clear evidence of detrimental side-effects on the pouch-young.