Organophosphates are acetylcholinesterase inhibitors and cause excitatory paralysis leading to death of sea lice when given as a bath treatment. Dichlorvos was used for many years in Europe and later replaced by azamethiphos, the active ingredient in Salmosan, which is safer for operators to handle. Azamethiphos is water-soluble and broken down relatively quickly in the environment. Resistance to organophosphates began to develop in Norway in the mid 1990s, apparently due to acetylcholinesterases being altered due to mutation. Use has declined considerably with the introduction of SLICE, emamectin benzoate.