SLICE® (emamectin benzoate), manufactured by Schering-Plough for the control of sea lice in farmed salmon, is an avermectin compound derived synthetically from avermectins which are produced by fermentation of the soil organism Streptomyces avermitilis. When fed to fish emamectin benzoate is absorbed from the gut and distributed to the tissue of the fish. Emamectin benzoate is metabolized to inactive compounds and excreted slowly by fish resulting in long-lasting protection. Sea lice are considered a potential vector for the transmission of Infectious Salmon Anemia virus (ISAV) and therefore sea lice populations should be controlled. Elevated sea lice infestation is associated with increased stress, poor growth performance, increased susceptibility to opportunistic pathogens and an overall decrease in animal welfare.
The product SLICE® was submitted for approval in the US and Canada in 1999; the approval process has not been completed. This drug can be sold to veterinarians on a case-by-case basis through Health Canada's Veterinary Drugs Directorate's (VDD's) Emergency Drug Release Program (EDR). A maximum residue limit (MRL) of 42 parts per billion (ppb) has been established by VDD which compares to established MRLs in the European Union of 100 ppb. SLICE® is a registered product in all major aquaculture producing countries throughout the world, with the exception of the USA and Canada. Registration of SLICE® in Canada and the US is likely to continue for at least another 2 years.
In addition, the withdrawal period in Canada has been recently revised from 25 days to 68 days from the latest treatment when used at a water temperature of 5°C or greater. This lengthening of the withdrawal period seriously constrains marketing flexibility for the salmon producer. More importantly it prevents salvage slaughter of SLICE® treated Atlantic salmon which test positive for ISAV before the 68 day withdrawal has elapsed. Consideration of these factors are causing producers to delay treating sea lice infestations, or not treating at all. Sea lice have been shown to carry the ISAV so that delaying the treatment of sea lice infestations has the potential to put more farmed and wild Atlantic salmon at risk of being infected with ISAV. It has been accepted by other jurisdictions world wide that the minimum residue limit for SLICE® is 100 ppb with a withdrawal time of 0 days (UK/Chile) to 7 days (Norway). The USA has not established a MRL for SLICE® but has placed a 60 day withdrawal time under their Investigational New Animal Drug (INAD) policy (equivalent to Canada's Emergency Drug Release (EDR)).
Both Health Canada and CFIA are prepared to look at the data provided from this study in order to adjust withdrawal times for producers using SLICE®.
In an attempt to alleviate the problems linked to the 68 day withdrawal period, based on data generated at 5°C, the aquaculture industry needs to determine the appropriate withdrawal interval at a water temperature which is closer to the warmer water temperatures when sea lice infestations become a more serious problem. This laboratory and field based project will help to fill in the knowledge gaps concerning SLICE® residues in salmon using both controlled and natural conditions, in water that is warmer than 5°C.
In the current study, the depletion of SLICE® will be investigated in Atlantic salmon maintained in seawater at 10 ± 1°C following administration of SLICE® orally in the diet at a dose rate of 50 µg/kg body weight per day for 7 consecutive days.