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Dental overgrowth and trimming in a pufferfish
R. Rees Davies
Lansdown Veterinary Surgeons, Clockhouse Veterinary Hospital,
Wallbridge, Stroud, Gloucestershire GL5 3JD
Case history
A tropical freshwater pufferfish (Tetraodon palembangensis) was presented
with overgrowth of the upper teeth which appeared to interfere with
prehension. It was part of a mixed collection of eight fish, comprising this
species and T somphongsi. All the fish were around four centimetres long
and kept in a 120 litre tank with filtration, aeration and sand substrate. The
fish were fed mainly daphnia and bloodworm with no coral, clams or other
hard molluscs in their diet. All the other fish appeared healthy.
The affected fish was anaesthetised by immersion in a dilute solution of
benzocaine. This was added to the water at an initial dose of 25 mg/litre and
then incrementally to a final concentration of 75 mg/litre. Satisfactory
anaesthesia allowed the teeth to be clipped to approximately the correct
length with iris scissors, followed by smoothing and contouring with a
dental burr. Post-operative recovery was uneventful.
Discussion
Biology
Pufferfishes belong to the family Tetraodontidae, a name given because of
the presence of four teeth at the front of the mouth. Their skin is scaleless
but lined with small bony plates, or spines in some species. The oesophagus
of these fish contain a number of sacs which can be inflated with water or
air, causing the body to swell to an enormous size, thus either deterring
predators or becoming physically too large to be attacked. Their flesh
contains various neurotoxins, including tetrodotoxin, and these are secreted
when the fish is stressed. The toxins produce fatal cardio-respiratory effects
on nearby fish (Malpezzi and others 1997), and in aquaria these can be fatal
to both the pufferfish and the other inhabitants of the tank. Members of the
genus Tetraodon live in tropical seas, estuaries and rivers of Africa and
Asia. They will eat almost anything that can be removed from rock with
their sharp teeth, including seaweed, coral, sponges and invertebrates (Frank
1970).
Captive management
In general pufferfish are aggressive and best kept alone. The lack of
aggression between members of this group may have been due to their small
size and immaturity. They require a large tank and will destroy any plants or
furniture and so a plain sandy substrate is advised. They thrive at a temperature
of around 27°C. They require excellent water quality and often require
weekly partial water changes in addition to filtration. They can be fed on a
variety of foods including frozen brine shrimp, bloodworm, glass-worms,
dried shrimp, dried krill, dried plankton, and when larger they will take tiger
shrimp, lean beef and frozen smelt. Snails, mussels, clams and other
molluscs provide one of the main sources of food and the action of breaking
the shell is believed to keep the teeth to the appropriate length. Apple snails
(Ampullaria species) should be avoided due to risk of spreading disease.
Dental overgrowth
In the wild, pufferfish feed on various invertebrates including corals and
hard-shelled molluscs. However, in captivity, they are rarely given access to
material that is hard enough to keep the teeth worn down. Consequently,
dental overgrowth and difficulty with food capture and prehension develop.
In the long-term, the provision of harder molluscs such as clams and snails,
or corals should allow normal dental wear. The reason why only one in this
group of eight fish developed the problem is unknown.
Anaesthesia
Anaesthesia in ornamental fish has been described using a number of agents
(Sedgewick 1986, Potts 1987, Brown 1988, Brown 1992). The two
anaesthetic agents recommended for use in fish are tricaine methane
sulfonate (MS222, Thomson & Joseph) and benzocaine (Scott 1991, Brown
1992). Both are readily available, but only MS222 is currently licensed in
the UK and US for fish anaesthesia. Both substances are acidic, especially
MS222, and can act as an irritant and physiological stressor to the fish
(Scott 1991, Brown 1992). Benzocaine has been reported by some to be
more effective than MS222 (Potts 1987). MS222 dissolves directly in water
whereas benzocaine must first be dissolved in acetone, methanol or ethanol
(Scott 1991). The stock solution can be stored for up to three months if
protected from light to prevent build-up of toxic breakdown products,
principally chlorine (Potts 1987, Brown 1992). Both agents are used by
immersion.
In this case, anaesthesia was induced using benzocaine on the grounds of
reduced pH alteration and also cost. A stock solution was prepared by
dissolving four grams of benzocaine powder in 100 mls of acetone. The bag
containing the fish was weighed to give an estimation of the volume of
water, and an initial dose of 0·6 ml stock solution per litre added to produce
a level of approximately 25 mg/litre of benzocaine (Scott 1991). The fish
was gently encouraged to move around the bag to increase the flow of
anaesthetic solution over the gills. Incremental doses of benzocaine/acetone
solution were then added a few drops at a time until the desired depth of
anaesthesia was reached. This finally required a dose of 2·7 mls of stock
solution in 1·5 litres of water: an approximate level of 75 mg/litre giving
surgical anaesthesia. This level of anaesthesia took 15 minutes to reach, due
to the cautious incremental approach used. This contrasts to 50 mg/litre as
quoted by Scott (1991) and an induction time of between 60 and 90 seconds
described by Brown (1992).
As anaesthesia progressed, the fish was observed to reduce movement and
reaction to stimuli, reduce respiration, lose balance control and then fail to
respond to external stimuli. This closely follows stages one to three of the
?stages of anaesthesia? described by McFarland (1959). The fish was
removed from the anaesthetic solution and placed on a paper towel
moistened in the anaesthetic solution whilst the procedure was carried out.
Recovery from anaesthesia was achieved by returning the fish to a bag of
fresh water from its own tank, with balance control returning in 30 seconds
and full reactivity to stimuli restored within 5 minutes. To allow full
recovery from the stress of surgery and to minimise risks of toxin release
from the stressed fish, the owner isolated the fish for several days.
Procedure
Once anaesthetised, the upper teeth were cut back to slightly longer than
their normal length using a pair of iris scissors. Contouring and smoothing
of the teeth was attempted using a jeweller?s file but stabilising the fish?s
head proved difficult. Intermittent filing with a dental burr overcame this
problem and allowed a near normal finish with no appreciable thermal
trauma to the tooth stubs.
One week later, the fish was successfully reintroduced to the rest of the
group, was eating well and difficult to distinguish from its conspecifics. The
inclusion of harder shellfish in the diet was advised. The owner began
raising snails as live-food for the fish and had no further problems in the
subsequent 12 months.
References
Brown, L.A. (1988) Anesthesia in fish. In: The Veterinary Clinics of North America:
Small Animal Practice: Tropical Fish Medicine (ed M.K. Stoskopf).
W.B. Saunders Company, Philadelphia. 18, (2), 317?30
Brown, L.A. (1992) Restraint, handling & anaesthesia. In: Manual of Ornamental
fish (ed R.L. Butcher). British Small Animal Veterinary Association,
Cheltenham. pp135?139
Frank, S. (1970) Tetraodontidae. In: Pictorial Encyclopaedia of Fishes. Hamlyn,
London. pp523?529
Malpezzi, E.L., de Freitas, J.C. & Ranting, F.T. (1997) Occurrence of toxins other
than paralysing type in the skin of Tetraodontiformes fish. Toxicon 35, (1),
57?65
McFarland, W.N. (1959) A study of the effects of anaesthetics on the behaviour and
physiology of fishes. Publications of the Institute of Marine Science. 6,
23?55
Potts, G.W. (1987) Marine Fish. In: The UFAW Handbook on the Care and
Management of Laboratory Animals (6th edition) (ed T. Poole). Longman,
Harlow. pp824?847
Sedgewick, C.J. (1986) Anesthesia for fish. In: The Veterinary Clinics of North
America: Food Animal Practice: Anaesthesia (ed J.C. Thurmon).
W.B. Saunders Company, Philadelphia. 2, (3), 737?742
Scott, P.W. (1991) Ornamental fish. In: Manual of Exotic Pets (eds P.H. Beynon &
J.E. Cooper). British Small Animal Veterinary Association, Cheltenham.
pp272?285
