Strong sea lice make for skinny salmon

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Kate Casey

The cover story in this month’s issue of Visión Acuícola portrays yet another unpleasant picture of Caligus sp., the nasty copeopod that has taken center stage as the adversary in fish health. Veterinarians throughout the industry agree that the two principal caligus species in the country’s salmon farming regions have developed a resistance to the once effective treatment, emamectin benzoate. The trade name SLICETM, developed by Schering-Plough Animal Health, has been remarkably effective for the treatment of the species Lepeophtheirus salmonis found in the northern hemisphere, but in Chile, in less than a decade of use the product is already losing its potency in treating the Caligus sp. As with the incorrect or overuse of antibiotics, the same goes for emamectin benzoate – after years of “misuse” so to speak, the parasite has developed a resistance to the drug. The term misuse is in reference to the fact that the application of the drug in Chile has not been the same as in other parts of the world. In the late 90’s, the original sales team of SLICETM in Chile proposed a treatment program to reduce the population levels of sea lice throughout specific geographic regions, similar to treatment planning seen in Norway. Unfortunately, the challenge of convincing all neighboring salmon producers to purchase the new drug, and make a joint effort of using it in a coordinated and cooperative manner, was nearly impossible. The paradoxical mentality of reducing of costs at all costs now serves as a harsh lesson. According to the owners of Centrovet, a fish health service provider, “From information provided by the salmon producers, we estimate that their annual cost in medications for the treatment of Caligus borders US$7.8 million.” Besides emamectin benzoate, most salmon producers try to combat their Caligus infestations by a using a rotation of alternative treatments such as hydrogen peroxide baths and injecting emamectin benzoate instead of adding it to the feed.