Emamectin Benzoate in Salmon: Sea Lice Control and Resistance
How emamectin benzoate, an in-feed avermectin, controls sea lice in farmed salmon, why resistance is widespread, and the integrated pest management needed to preserve it.
Overview
Emamectin benzoate is an avermectin, a macrocyclic lactone derived from abamectin. In salmon aquaculture it is given in medicated feed (commonly under the trade name SLICE) to control sea lice. It is absorbed and distributed in the fish so that lice feeding on the host are exposed to the drug.
Mechanism and targets
Emamectin benzoate acts on glutamate-gated chloride channels (and influences GABA signaling) in the parasite, causing hyperpolarization, paralysis and death. It is used against the sea lice Lepeophtheirus salmonis and Caligus species and acts against the host-attached life stages, preventing maturation to reproductive adults.
Administration and dosing
Emamectin benzoate is delivered in medicated feed over a short course. The following sourced figures describe the standard regimen.
| Item | Value | Source |
|---|---|---|
| Route | Oral, in medicated feed | Peer-reviewed / Merck |
| Dose | 50 µg/kg fish body weight per day | Peer-reviewed |
| Duration | 7 days (consecutive) | Peer-reviewed |
| Target tissue level | About 50 to 60 ppb in muscle and skin | Peer-reviewed |
A one-week course provides prolonged protection against the attached life stages of sea lice, but the drug is then cleared from the fish over several weeks, a period that has important resistance implications.
Resistance and integrated pest management
Reduced sensitivity and resistance of sea lice to emamectin benzoate are widely documented: most major salmon-farming regions, including Norway, Scotland, Atlantic Canada and Chile, reported resistance within about 5 to 10 years of the product's introduction. A key driver is sub-lethal exposure: after treatment, newly arriving juvenile lice meet declining, sub-therapeutic drug levels while the fish clears the drug over roughly 8 to 10 weeks, selecting for resistant lice. For this reason emamectin benzoate is used within integrated pest management, rotating chemical classes and combining medicines with non-medicinal methods such as cleaner fish and thermal or mechanical delousing.
Environmental and regulatory note
Emamectin benzoate is toxic to non-target crustaceans and other aquatic invertebrates, so its use and discharge are environmentally regulated. It is registered for sea-lice control in feed in several salmon-farming countries (for example the United Kingdom, Norway, Chile and Ireland); approval, dose and any withdrawal requirements vary by country and apply to food fish, so consult a fish-health veterinarian and current local regulations before use.