PHARMACOLOGICAL TREATMENTS AGAINST ICHTHYOBODO-NECATOR (HENNEGUY, 1883) IN RAINBOW-TROUT, ONCORHYNCHUS-MYKISS (WALBAUM)

Various antiparasitic drugs belonging to several pharmacological group s were tested by bath administration for in vivo activity against a na tural infestation of rainbow trout, Oncorhynchus mykiss (Walbaum), by the flagellate protozoan Ichthyobodo necator. Fish were also monitored for signs of drug toxicity. Complete elimination of infestation in al l fish was achieved only by bithionol (25 mg l-1 for 3 h on two consec utive days). Ketoconazole, toltrazuril, amprolium, sulphaquinoxaline, quinacrine, N-metylglucamine, chloroquine, 1,3-di-6-quinolylurea, dimi nazene aceturate and paromomycin were not effective. Diminazene acetur ate (100 mg l-1, 3h) was the only drug which was clearly toxic. The co mparison of the response of symptomatic and asymptomatic infested fish to a formaldehyde bath indicates that poor health caused by I. necato r may greatly increase the susceptibility of fish to the toxic effects of a drug or chemical.