Field trials in Norway with SLICE (0.2% emamectin benzoate) for the oral treatment of sea lice infestation in farmed Atlantic salmon Salmo salar.

Four commercial salmon farms on the West coast of Norway were recruited to a programme of field trials in which the efficacy of SLICE (0.2% emamectin benzoate; Schering-Plough Animal Health) was compared with a commercially available product, EKTOBANN (teflubenzuron 2 g kg(-1); Skretting A/S) in treating natural sea lice Lepeophtheirus salmonis infections in Atlantic salmon Salmo salmar L. At each test site, 3 fish pens were treated with each product. In total, nearly 1.2 million first-year-class fish were included in the trial, of which approximately 561,000 received emamectin benzoate at a dosage of 50 microg kg(-1) body wt d(-1), while approximately 610,000 received teflubenzuron at a dosage of 10 mg kg(-1) body wt d(-1). Medicated feed was provided at 0.5% body wt d(-1) over 7 consecutive days. Feed containing emamectin benzoate was generally well accepted by the fish and no problems were encountered in feeding the medicated diet at the desired dose. Lice numbers were counted 2 d before and 1, 7, 14 and 21 d after commencement of treatment. While treatment with both substances rapidly reduced lice numbers, pens treated with emamectin benzoate were found to harbour significantly fewer lice 14 and 21 d post-treatment. Twenty-one days following treatment with emamectin benzoate the lice abundance was reduced on average by 94%. Limited sampling outside the main study period indicated that emamectin benzoate protects against sea-lice infestation over longer periods.

Strengths and weaknesses of different challenge methods.

The development of effective fish vaccines is greatly dependent on reproducible standardised challenge methods. Different test protocols involving bath exposure, intra-peritoneal and intra-muscular injection and co-habitation are all well described in the literature. Ideally, to represent the many facets of infectious diseases in fish, a test challenge method should closely mimic natural exposure to the pathogen and ensure that immune mechanisms located in the body surfaces play their role. Bath and co-habitation challenges best fulfil this requirement. On the other hand, these methods are more difficult to control and standardise than injection methods. The pathogenicity of some strains of bacteria may also be insufficient to induce a satisfactory outbreak of disease in the target species when using bath or co-habitation challenge. A common measure of efficacy of a vaccine is by Relative Percent Survival (RPS), which expresses the proportional relationship between mortalities in a vaccinated group compared to unvaccinated controls. It has been demonstrated that RPS may vary significantly between challenge methods in groups of Atlantic salmon vaccinated with the same batch of vaccine. Also, RPS may vary for the same batch of vaccine when used in identical test systems at different times. Great care should therefore be taken when competitive products evaluated in different tests are compared. As several vaccines have been marketed and shown to have very good efficacy under field conditions, such products could be introduced as reference standards when testing new candidates for marketing authorisation.