Responsible use of antiparasitics in Aquaculture

The purpose of this webpage and associated downloadable document is to examine the parasites which affect the raising of fish for human consumption in commercial aquaculture production systems, how they are monitored, and the current treatments available, as well as other management methods in order to encourage responsible use of parasiticides.

Fish have an anomalous status as far as the Veterinary Surgeons Act 1966 and the Medicines Regulations are concerned. Fish are not covered by the Veterinary Surgeons Act 1966 which means that non-veterinarians can legally diagnose medical conditions in fish, which is not the case in any other livestock. However, the Medicines Regulations stipulate that Veterinary Prescription Only Medicines may only be prescribed by veterinary surgeons and may only be supplied either directly by them or by pharmacists or Suitably Qualified Persons to a prescription issued by them.

Background to the use of parasiticides in fish production

All parasiticides used in the EU have been registered for their current uses on the basis that they are effective and safe to both man and animals including fish. This safety and efficacy is kept under review. Use for the purposes for which they are registered should therefore pose little or no hazard to the treated fish, to the environment, to those who administer them, to workers involved in the preparation of food or to consumers of food originating in treated animals. In the United Kingdom the Veterinary Medicines Directorate issues the marketing authorisation for veterinary medicines where the product has been found to be safe, effective and of acceptable quality. The Veterinary Products Committee advises the VMD in respect of safety, quality and efficacy in relation to veterinary medicinal use of any substance or article. There is a chain of quality assurance from the pharmaceutical manufacturers, through wholesalers, veterinary surgeons and distributors that dovetails with the farm assurance schemes and effectively ensures that medicines are used responsibly.
Anti-parasitic resistance is a natural phenomenon. It can exist in the absence of medication. Particular species and strains of parasites are naturally resistant to certain anti-parasitics. Most discussion of resistance focuses on ‘acquired resistance’ – that which occurs after exposure of the parasite to the antiparasitic. This is an inherent risk associated with any use of antiparasitic medication in any species.
The ability to use antiparasitics provides us with an important tool to reduce disease and suffering in fish. However measures aimed at limiting the development of resistance are important for prolonging the useful life of all antiparasitics in fish medicine. The effectiveness of measures and products needs to be monitored and those which are appropriate today may need to be adjusted in the future in the light of changing resistance patterns and scientific knowledge.
There is continuing concern about the impact of external parasites such as salmon lice on wild fish populations. Maintaining high levels of lice control combined with regular monitoring of lice numbers on farmed fish reduce the potential for salmon cages to act as reservoirs of infection for the wild fish populations. At the same time infected wild fish can act as hosts for parasites and can potentially infect clean populations of farmed fish.
There is a joint responsibility between the veterinary surgeon and the farmer to ensure that antiparasitics are used correctly and for the right reasons. This is essential so that the consumer can be assured that antiparasitic residues will not appear in food. It is important always to assess the efficacy of any treatment to ensure there is a cost benefit but treatment may also be justified in order to improve fish welfare.
The Responsible Use of Medicines in Agriculture Alliance (RUMA) is a coalition of organisations including agricultural, veterinary, pharmaceutical and retail interests. This paper is one of a series of species-specific documents developed by RUMA. Broadly, the objectives of this paper are:
- To review the use of antiparasitics in fish production, and to produce responsible use guidance for farmers.
- To aim to establish and communicate practical strategies by which the need for use of antiparasitics might be reduced without adversely affecting either the welfare of farmed fish, or the viability of the industry.
This paper establishes a framework against which future activities may be evaluated. It also seeks to encourage the involvement of the different organisations and individuals that have a role in achieving these aims.

Legal controls

Animal medicines play an important role in the control and prevention of disease and animal suffering but have the potential to impact on human health if not used properly. In the UK consumers have long enjoyed the benefits of rigorous systems designed to protect them from residues of such medicines in their food which could impact on them. These include the controls on the authorisation, distribution and use of such medicines. Additionally foods of animal origin are monitored for medicinal residues in accordance with EU directives.
It is believed that the general public are not aware of the current controls on the distribution and use of veterinary medicinal products. The industry considers the current controls to be effective safeguards.
The Veterinary Medicines Directorate (VMD) is responsible for the authorisation and control of the manufacture and marketing of animal medicines and for surveillance for residues of animal medicines in meat and other animal products.
All animal medicines in the UK are assigned into one of the following legal categories:
- Prescription-Only Medicines (POM-V) can only be supplied by veterinary surgeons for animals under their care, or dispensed from a pharmacy in accordance with a written prescription from a veterinary surgeon. Authorised medicated premixes can be incorporated into animal feed under the terms of a medicated feedingstuffs prescription signed by a veterinary surgeon for animals under his/her care.
- Some animal medicines (POM-VPS) can only be supplied by veterinary surgeons in respect of animals under their care, or by pharmacists or Suitably Qualified Persons (SQP) from a registered premise.
- Others medicinal products may come under the NFA-VPS classification, which can be supplied by a veterinarian, pharmacist or SQP, to be used in non-food animals only.
- AVM-GSL products can be sold by anyone (this was formerly the GSL group of products).
- SAES (Small Animal Exemption Scheme) – these are products which can be marketed without a Marketing Authorisation for certain pet species.
- Other classifications include ZFA (zootechnical feed additive) and CD (controlled drug) are outside the remit of this publication since they are not used to control parasites in aquaculture.
- The Cascade is included in the European Medicines Legislation to give a veterinarian the opportunity where there is no authorised veterinary medicinal product for the condition to use another product not specifically licensed in order to prevent unnecessary suffering.

Codes of Practice

Ultimately it is the farmer who is responsible for ensuring that aquaculture medicines are used in a safe, responsible and effective way.
Fish farmers and their veterinary surgeons aim to ensure that fish are kept in the best state of health and welfare. This must be viewed against a backdrop of a sound commercial base and the economics of the business but never compromised by it. Antiparasitic use on all fish farms is under the supervision of the veterinary surgeon. It is a legal requirement for farmers to keep a record of the administration of medicines, including in-feed medication, which must be available for inspection. Within this context, some farm assurance schemes may be a good vehicle for auditing compliance with the legislation. Farmers and veterinary surgeons have a shared responsibility to ensure that medicines are used responsibly.
Various codes of practice have been produced which give technical guidance on good practice, including the National Sea Lice Treatment Strategy, Integrated Sea Lice Management, Containment, and a Veterinary Health Plan.

Food Fish Aquaculture in the United Kingdom

The main species of food fish farmed in the UK are Atlantic salmon (Salmo salar) and Rainbow trout (Oncorhynchus mykiss), although other trout species such as brown trout (Salmo trutta) are produced along with some more exotic fish species such as tilapia, Arctic charr and sea bass and Halibut.The methods and systems employed to keep and farm fish will vary not only according to the species, but also even to the age or stage of development of the fish being reared.Atlantic salmon are hatched in freshwater, in purpose-built hatcheries, and the parr are reared in freshwater tanks on land or pens in lochs, until they undergo a major metabolic change which adapts them for life in seawater. This is called smoltification, and is genetically pre-programmed in fish in the wild to take account of the time for the parr to swim from the point of hatch, usually far up a small stream or burn, to the estuary and then to sea for the next stage of their life cycle. In farmed salmon, the time taken from hatch to smoltification is usually about 15 months, and the fish grow to about 80 grams in weight in this time. At this point they are transferred to sea pens, where their most rapid phase of growth takes place, reaching around 3 to 5 kg in weight at harvest after approximately 15 months at sea. The sea pens are large anchored suspended nets, which although providing the fish with a natural environment, at the same time carry the risk of contact with wild fish and aquatic pathogens in the surrounding environment – a challenge fairly unique to a number of methods of fish farming.

Rainbow trout are kept in freshwater (and seawater) in tanks and net pens, ponds or concrete raceways, with the water supply from various sources such as an adjacent river, spring or borehole. Although mechanical filtration is employed to clean the water on entry to the farm, again the challenge here can be the potential proximity of wild fish as well as other hosts such as wild birds.

Recirculation units are becoming more widespread – these may be used for a variety of situations including salmon hatcheries and the rearing of more exotic species such as halibut and even cod. Although they represent a significant investment to start-up, they have the advantage of keeping the fish in a protected environment especially as far as pathogen challenges including parasites are concerned.

Farmed Food Fish – Common Parasites

Farmed fish can be affected by both ectoparasites and endoparasites.

The most important ectoparasites affecting farmed salmon and some farmed rainbow trout are from the parasitic crustacean group which includes sea lice which are divided into two main species, Lepeophtheirus salmonis and Caligus elongatus, as well as freshwater copepods Argulus foliaceous and Ergasilus sieboldi. Common endoparasites include Eubothrium sp. in farmed salmon and Proteocephalus sp. in farmed rainbow trout.