Introduction

The routes of administering medicines are many and are similar to application of medicines in other animals, though the quantities of drugs that the practitioner would deal with would be drastically different from the regular cat/dog. For example, giving an injection of long-acting oxytetracycline to a 200-g fish will entail drawing up 0.025 ml of the 200 mg/mL injectable. Or if treating a 10-kL fish pond with oxytetracycline in-water would require as much as 200–400 g of the medicine! Thus the practitioner will need to take great care when calculating dose rates since there is no average size to fish and tanks. Selecting the best mode of administration is dependent on the pathogen, fish, client compliance, environment and cost.

Topical

This usually involves the application of a paste/gel onto an external lesion. Generally, topical application of medicines is problematic since the drug washes off as soon as fish are returned to the water. But now with the relatively new polyoxamer bandage product with mucoadhesive qualities, various drugs can be incorporated into the powder, up to 10% by weight.

Parenteral

This is the best method to ensure the patient receives the full dose of medicine. When choosing a needle type, factors to consider include fish size and muscling, depth of injection and viscosity of medication. The bore diameter is identified as "gauge", with the smaller value equating to thicker diameter. These have standardised colour codes. Needle lengths vary from 0.5–1.5 inches (these may be given in mm).

Intramuscular (IM)

This is the most commonly used method for antibiotic injection. Medicine is often injected into the epaxial muscles (muscle block just ventral and caudal to the dorsal fin). The needle should be positioned between the scales and aimed to come in from a caudorostral angle.

Inject slowly to prevent reflux and then place digital pressure on the injection point after withdrawing the needle. The medication is rapidly dispersed by the rich blood supply. This technique is recommended for gravid females and not recommended for small fish and fish with little muscle bulk. The disadvantage of this method is that the injection site may discolour (aim for dark-coloured areas on the fish so that melanisation will not show). Antibiotic injections are also sometimes given peri-lesionally. Other intramuscular injection sites include the areas at the base of the fins, and these are particularly useful to administer drugs to species with large scales (e.g., Arowana, Saratoga, Arapaima, etc.).

When giving intramuscular injections to sharks, there is usually considerable back pressure from the compact tissues forcing the injected material to exude from the site. This can be prevented by firmly applying surfboard wax to the site which will form a temporary seal sufficient to allow the drug to dissipate.

Intraperitoneal (IP)

Place the fish on its side or on its back to allow the peritoneal organs to gravitate away from the body wall. Insert the needle into this space at a shallow angle to avoid penetrating the internal organs. This is a common route of administrating vaccines, as it is done on small fish and will not damage the flesh. It is also good for administrating potentially irritating medications such as enrofloxacin.

The disadvantages of this method are:

 Damaging the peritoneal organs or,

 If injected into the ovary, the drug will not redistribute to the rest of the body, and

 If injected into the intestinal lumen, the drug will be excreted too quickly.

Intravenous (IV)

Fish must be anaesthetised. Insert the needle midline and at the angle as shown, just caudal to the ventral fin. Stop just short of the spine. This technique is used more commonly for drawing blood than for administering drugs. The anaesthetic propofol has been used experimentally as an IV injection.

Per Os

When medication is incorporated into fish food, it allows for medicating large numbers of fish and also minimises handling stress. However, the delivery of the correct doses to each individual fish is difficult (sick fish are usually inappetent, and it is these that should actually be receiving the medication). Medication is either sprayed onto the food, impregnated into the food, or prepared with food coated with medicated gelatin, agar or oil (fish oil or vegetable oil). In the field, powdered medicine is mixed with pelleted fish food, and then oil is added as the second step.

Dosages are based on the assumption that fish eat 1–2% of the body weight per day. The total body weights or biomass of the tank can be estimated by multiplying the average weight of the fish by the number of fish.

Depending on the drug, this medicated food may sometimes be less palatable. It is thus recommended to reduce the amount of food fed per day by 25–50%. The palatability of food can be enhanced by additives such as garlic or aniseed oil.

Capsule Sizes

Alternatively, medications can be presented in capsules to deliver measured doses to large individual fish. However, medicated tablets are not often available for fish. This is where compounding pharmacists come in handy. Once the medicines are packed into the capsules, these can then be 'hidden' inside bogue or the cavity of the fish food (e.g., in the head of squid or coelom of baitfish) and then target-fed to the patient.

Gastric Intubation

If fish are not eating, they can be sedated and force-fed via a stomach tube. Use a suitable-sized catheter and needle (e.g., for small fishes, a 3-mm outer diameter catheter and a 5-ml syringe would be suitable, and for larger fish, a 6-mm outer diameter catheter with a 20-ml syringe). Measurements for the different catheter sizes are provided in the table* below. Administer 1.0–1.15 ml/kg body weight.

For small patients, the best food to use is newly hatched, decysted Artemia nauplii because the particle size is very small (allowing easy flow through the feeding tube) and they are packed with nutrients. A slurry of baby fish food is a great option because it has a high protein content of > 50% and is already a fine granulated form that is easy to make into a paste by adding water. Alternatives include flake food or blended granulated feed that is mixed in a small amount of water. Additives such as peas, magnesium sulfate (Epsom salt), vitamins, fish oil and others can be mixed in at this stage. The food should be sufficiently moist to pass through the bore of the syringe easily.

For larger patients, the best food would be vitamised whole fatty fish, fish oil, vitamins. If these are unavailable, some veterinarians may use Hill's a/d® Canine/Feline Critical Care food that contains 44% protein and 30% fat; however, much of the proteins/fats are terrestrial-based, and it would not be the best choice for the long run. An alternative is to prepare a blended mix of 8 parts seafood gruel, 2 parts fish oil, 1 part dextrose and add vitamin C (750 mg per litre or kilogram of food).

In-Water Medication

Dip

This is useful for external parasitic infections and involves using relatively high concentrations of drugs dissolved in the water of a treatment tank. Fish are held in nets and briefly immersed in the treatment solution and then placed into a hospital/quarantine tank. This is often used for new arrivals. Make sure dips are refreshed regularly and properly aerated to keep dissolved oxygen levels up.

Bath

This is a variation of the 'dip' treatment. It uses lower drug concentration and involves allowing the sick fish to swim in the medicated water for a period ranging between 2–60 minutes. These are useful for external infections including protozoal, bacterial and fungal. Commercially, this method is used for vaccinating fish against bacterial infection. It is also useful for fish in display aquaria where they cannot tolerate high concentrations of drugs and are housed in multi-species tanks with creatures that cannot tolerate the treatment.

Prolonged Immersion/Permanent Bath

This is yet another variation of medicated water where a very low concentration of the drug is used, but the length of exposure is increased. It is the most frequently used method where client compliance may be low as it is the easiest option. Medicine is mixed into a watering jar and then broadcasted across the pond.

This method is particularly useful if the parasites have stages in their lifecycle when they are refractory to treatment (e.g., white spot disease) - the longer exposure time in the medicated water will increase the chances of killing the parasites when they reach the susceptible stage. This method is also used to reduce osmoregulatory stress, when 2–5 mg/L of NaCl is added, in conjunction with other forms of treatment. This method, however, is not ideal for administering antibiotics since much larger quantities have to be used and it has disastrous impact on the biofilter.

Conclusion

There are many ways to medicate fishes. The decision is influenced by type of drug, type of fish, type of aquarium/pond, type of pathogen, client compliance, environment and cost. With greater familiarity and understanding, the choice becomes intuitive.

* VIN editor: Table was not available at the time of publication.

References

1.  Loh R, Landos M. Fish Vetting Essentials. Perth, Australia: Richmond Loh Publishing; 2011.

2.  Loh R, Landos M. Fish Vetting Medicines: Formulary for Fish Treatments. Perth, Australia: Richmond Loh Publishing; 2012.

3.  Loh R, Landos M. Fish vetting techniques and practical tips: instructional video. Perth, Australia: Richmond Loh Publishing; 2014.