The disposition of ivermectin in Atlantic salmon (Salmo salar).

The disposition of tritium-labelled ivermectin (22,23 di-3H-avermectin, B1a) in Atlantic salmon was studied by whole-body autoradiography, liquid scintillation counting, and thin-layer chromatography. Ivermectin was slowly absorbed, the highest concentrations being found in lipid-containing organs. High concentrations were also found in the central nervous system, indicating that the blood-brain barrier in salmon is poorly developed compared to mammals. The excretion of the drug was very slow. The total amount of radioactivity in blood, muscle, liver and kidney diminished by only 35% from day 4 to day 28 after administration. Excretion was mainly by the biliary route, and enterohepatic circulation of the drug was apparent. The drug was mainly excreted in the unchanged form. Distribution to the central nervous system, and the prolonged excretion period, makes the drug unsuitable for the control of parasitic infestations in salmon.