How Do You Medicate Fish?

ROUTES OF ADMINISTRATION

TOPICAL

This usually involves the application of a paste/gel onto an external

lesion (e.g. applying dilute Betadine over a bacterial skin

infection).

PARENTERAL (i.e. via injection)

Intramuscular (IM)

This is the most commonly used method for antibiotic injection. Aim

for the 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 caudo-rostral angle. Inject slowly to prevent reflux. 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).


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 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 harm the flesh. It is also good for

administrating "painful" 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.

Intravenous (IV)

Fish must be anaesthetised. Insert the needle midline and at 90 to 45

degrees, needle pointing towards the front, 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.

ORAL

Per os

This is a common delivery method for antibiotics on large farms for

ease of 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. 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%.

Gastric intubation

If fish are not eating, they can be anaesthetized and force-fed via a

stomach tube. Use a 3mm outer diameter catheter and a 5ml syringe (for

larger fish, use a 6mm outer diameter with a 20ml syringe). Administer

1.0-1.15 ml/kg body weight. Due to the high pressure needed to deliver

the suspension, the tube may need to be glued to the syringe with

cyanoacrylate (e.g. super glue).

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.

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.

Prolonged immersion / permanent bath

This is yet another variation of medicated water where very low

concentrations of the drug is used, but the length of exposure is

increased. This 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 2-5mg/L of salt (sodium chloride) is added, in

conjunction with other forms of treatment. It is also the most

frequently used method where client compliance may be low as it is the

easiest option.

This article taken from: Loh & Landos (2011) Fish Vetting Essentials.

Richmond Loh Publishing, Perth.

http://www.thefishvet.com.au

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