Dietary nicarbazin reduces prevalence and severity of Kudoa thyrsites (Myxosporea: Multivalvulida) in Atlantic salmon Salmo salar post-smolts

This experiment investigated a range of dietary nicarbazin concentrations for efficacy against the myxosporean parasite Kudoa thyrsites and for toxicity in seawater-reared Atlantic salmon post-smolts. Nicarbazin was incorporated into diets at 0 (control), 2.5, 5, 10 or 25 g kg− 1 such that each dose was replicated among three tanks and delivered intermittently over 2155 degree-days. Compared with controls, the prevalence and severity of K. thyrsites, determined histologically, were significantly reduced in fish fed diets containing nicarbazin. Nicarbazin residues in skeletal muscle, liver and skin were proportional to dietary concentration between 0 and 10 g kg− 1 and were associated with a red discolouration of the skin. Cumulative mortality ranged from 0% to 12.4% among groups and was significantly elevated in the 10 and 25 g kg− 1 groups, relative to control. Weight and condition factor were reduced because of reduced feeding responses to the medicated diets. Feeding responses and appearance of the skin returned to normal and mortality ceased upon resumption of non-medicated diets. In conclusion, dietary nicarbazin was efficacious against K. thyrsites in Atlantic salmon, when included in diets at concentration of at least 2.5 g kg− 1. Further research will optimise nicarbazin treatment strategies and establish tissue residues following cessation of treatment.

► Dietary nicarbazin reduced levels of Kudoa thyrsites in Atlantic salmon smolts.
► Nicarbazin residues in muscle and liver were proportional to concentration in diet.
► Nicarbazin caused mortality, reduced appetite and discoloration in some fish.
► Adverse effects ceased following cessation of dietary treatment.