Fish Diseases and Management

7.2.2 Route of drug administration

Water medication

The commonest method by which drugs have traditionally been administrated to fish is by medication of water which the fish inhabit.

Merits and Demerits

In addition to its simplicity water medication has the advantage that it is adaptable to mass medication of large numbers of fish.

Immersion or Dipping

This term is used to mean the preparation of a small volume of medicated water in a separate container from that holding the fish. The fish, usually held in a net, are immersed in it for a short period of time and then returned to their normal environment. Dipping has disadvantage compared to water medication that the fish are exposed to the stresses of chasing, handling and netting. Dipping has a particular advantage in the use of certain antibacterial drugs in aquaria where filters are used to effect bacterial oxidation of ammonia to nitrites and nitrates.

Hyperosmotic Infiltration

Hyperosmotic Infiltration (HI) is a development of immersion designed to accelerate the absorption of macromolecules or even of particles such as antigenic bacteria. The procedure as originally devised consisted of two separate immersions. The first was in a pharmacologically inert solution, hypertonic to fish plasma; 10% urea and 5.23%sodium chloride, both being1650 mOsm/l, have been used. This immersion was for 3 minutes and was followed immediately by the solution to be absorbed.

Flushing

Where fish are kept in running water which is not recirculated, for example in a raceway, immersion can be achieved by flushing, or, as the process is sometimes called, a California flush. Flushing is more wasteful, and hence more environmentally polluting, than dipping; and it may be difficult to obtain a homogenous distribution of the medication in water. An adaptation of this procedure is commonly used in hatcheries for the control of fungal (saprolegnia) infection.

Bath treatment

In bathing the bottom of the net cage is raised, typically to 2 meters, thus limiting the volume of water to be medicated. This reduces the weight of drug required and hence reduces both the cost and degree of environmental contamination. Bath treatment is wasteful and environmentally contaminating, and additionally is labour-intensive.

In-feed medication

In-feed medication or the provision of medicated feed is a much less wasteful method of administration than water medication. In-feed medication is standard practice for a large number of diseases but it is actually prophylactic not therapeutic.

Pelleted Medicated Feed

The ideal way to medicate feed is to add the medicinal product to the feed mix prior to pelleting.

Surface-coating pelleted feed

This process is suitable to the medication of small batches of feed and can be used for drugs which are heat-liable. It is therefore the normal means of medicating feed on fish farms.

Spray-medication of pelleted feed

Sex hormones are important examples of a class of drugs which are, for practical purposes, insoluble in water and which are used in very small doses.

Leaching

Leaching of drug into the water occurs with all forms of in-feed medication but is a particular problem with surface-coated feed. The extent of leaching varies according to the solubility of the active ingredient in water and the time for which the feed is in the water. A further factor affecting the rate of leaching from medicated pellets is the size of pellets and hence the ratio of surface area to weight. The smaller the pellets the faster will be the leaching.

Micro-encapsulation of drugs

One micro-capsule structure which has been well researched has a calcium alginate core into which the drug is mixed and a chitosan-alginate shell. Whether a micro-capsule is retained in the stomach or not is largely a question of its size.

Artemia enrichment

To enrich artemia cysts they are decapsulated and placed in sea water which has been pre-sterilized by UV light and is well oxygenated. Artemia enrichment has the disadvantage over other forms of in-feed medication that only healthy fish can be treated. In addition it is extremely wasteful.

Gavage

Gavage is a form of oral administration extensively used in experimental work because the dose can be known accurately. It is rarely used in routine fish management as it is labour intensive and stressful to the fish. Gavage is a useful technique where a few fish such as brood fish or valuable ornamentals have to be dosed orally.

Injection

Manual Injection

A prerequisite for injection is that the fish should be anaesthetized; without this precaution injury is likely to be caused to the fish and possibly also to the operator.

Intramuscular injection

Intramuscular injections are given into the epaxial musculature, normally approximately mid-way between the mid-dorsal line and the lateral line.

Intraperitoneal injection

Intraperitoneal injections are made into the mid-ventral line just cranial to the vent. This is a widely used route of injection but it can sometimes cause peritoneal adhesions. In consequence intraperitoneal injection should be avoided; the dose is likely to be made into a viscus or the lumen of the gut.

Injection into the dorso-median sinus

The route of administration which is  recommended for salmonids is  the dorso-median sinus (DMS). The point of insertion of the needle is in the mid dorsal line in the angle at the caudal margin of the cranial dorsal fin. This route is unsuitable for most fishes other than salmonids because of their different anatomy.

Automatic Injectors

Multiple dose syringes such as are available for injecting drugs into mammals rarely deliver sufficiently accurate doses of the volumes used in fish. However automatic injectors of the type used for vaccinating poultry can be used injecting fishes.

Machine injection

Machines are available for the rapid injection of large numbers of fish. In practice this means vaccination by the intraperitoneal route. Anaesthesia is not needed with the machine, but it was found to be safer for the operator than by using manual procedures with anaesthosia. 

Implantation

Where there is a need for prolonged medication with a drug which, for either economic or biochemical reasons, can only be administered by injection, it is sometimes formulated as a pellet or capsule for implantation. As with other injections, implantations may be intramuscular or intraperitoneal.

Topical application

Topical application of drugs of fish is rare; where it is done usually for  treatment of skin ulcers on valuable ornamental fish.