Anaesthesia - Fish

Introduction

Anaesthesia is a useful tool for the fish veterinary surgeon and enables various tasks to be performed.

Fish often struggle when restrained or handled, which can result in damage to the animal and also sometimes to the handler.

Anaesthesia greatly facilitates examination, transport and diagnostic sampling and reduces stress to the fish.

Surgery requires anaesthesia, and with appropriate techniques, the fish can be maintained out of the water for extended periods. This is especially important in ornamental fish medicine, where veterinary involvement is increasing.

Pre-anaesthetic Preparation

If possible, fish should be starved for 12-24 hours prior to anaesthesia, as regurgitated food will decrease water quality and can become lodged in the gills.

Baseline behavioural parameters should be obtained, such as ventilation, caudal fin stroke rate and overall activity rate.

The anaesthetic and recovery baths should contain water from the aquatic system the fish is used to. All water parameters should be in an acceptable range and the temperature should be constant. The baths can be aerated using an air diffuser or air stone.

If the procedure is going to be completed out of the water, arrangements must be in place to prevent drying out of the skin, fins and eyes. This may include coverage with clear plastic drapes and regular wetting of tissues with a syringe or an atomiser.

Gloves should be worn when handling fish to prevent damage to the skin and transmission of zoonotic diseases.

For preparation prior to surgery, the scales should be removed along the incision site, and the skin can be cleaned with a mild antiseptic.

Immersion Anaesthesia

The most common anaesthetic technique in fish is to add the anaesthetic agent in the water. It is similar to inhalation anaesthesia in terrestrial animals.

An artificial ventilation system is necessary in procedures of more than 10 minutes or in all but the shortest out-of-water anaesthetics. A bifurcated pipe or mouth piece should be placed in the buccal cavity to deliver aerated anaesthetic solution across the gills. Nonrecirculating and recirculating systems are available.

Adjustement of the drug concentration in response to depth of anaesthesia is difficult with immersion anaesthesia. Different concentrations of anaesthetic solutions can be prepared in different bags that can be exchanged if necessary. Small amounts of anaesthetic fluid can be delivered directly to the gills via a bulb syringe without disconnecting the fish from the system.

Common products used for immersion anaesthesia include:

Benzocaine

Clove oil (note this only immobilises the fish so is not suitable for painful procedures)

Halothane

Isoflurane

Lignocaine

MS222

Phenoxyethol

Parenteral Anaesthesia

Anaesthetics can also be administered orally, intramuscularly, intravenously or intracoelomically. IM injection is the preferred route, usually in the dorsal saddle area. Supplementary immersion anaesthesia is often required, and ventilatory support is essential, particularly if recovery is prolonged.

Parenteral products include:

Ketamine

Ketamine and medetomidine

Saffan (alphaxolone - alphadolone)

Monitoring

Anaesthetic depth can be determined by assessing activity, reactivity to stimuli, equilibrium, jaw tone, muscle tone and respiratory and heart rates.

Induction usually takes 5-10 minutes and is marked by a decrease in caudal fin strokes, swimming, respiratory rate and reaction to stimuli. There is also a loss of equilibrium and the righting reflex is poor.

With surgical anaesthesia there is loss of muscle tone and further respiratory depression. No response to a firm squeeze at the base of the tail confirms a loss of reaction to stimuli and general anaesthesia. Opercular movements usually show that respiration is occurring, but in deep anaesthesia these movements may cease.

There may be a short excitement phase during immersion induction and some fish may traumatise themselves.

The heart rate can be monitored by directly observing cardiac movement, or via ultrasonography, Doppler flow probes or ECG. Gill colour may be a useful indicator of oxygenation.

Venous blood gas samples can be obtained from large fish to periodically monitor trends in oxygenation, ventilation and pH.

Water quality should be monitored to prevent anaesthetic morbidity and mortality. Partial water changes may be necessary.

Recovery

With immersion anaesthesia, recovery involves placing the fish in an anaesthetic-free tank. Reversal agents are available for some injectable anaesthetics (atipamezole). Fresh water can also be administered directly to the fish through the circuit.

The recovery water should be aerated and the fish's mouth should be directed towards water flow. The fish can also be pulled forward through the water with its mouth open.

Recovery is usually complete within 5 minutes when immersion anaesthesia was used. Respiration rate increases, ataxia disappears and muscle tone reappears.

An excitement phase may occur and fish should be prevented from escaping from the tank and from injuring themselves and others.

Antibiotics may be necessary following surgery.

Anaesthesia - Fish Learning Resources
Flashcards Test your knowledge using flashcard type questions	Ornamental Fish Q&A 16

References

Longley, L. (2008) Anaesthesia of exotic pets Saunders

Hall, L. (2001) Veterinary anaesthesia Elsevier Health Sciences

Neiffer, D. (2009) Fish Sedation, Anesthesia, Analgesia, and Euthanasia: Considerations, Methods, and Types of Drugs ILAR journal

WikiVet® Introduction - Help WikiVet - Report a Problem