Possibilities to control Ichthyophthirius multifiliis infestation with medicated feed in rainbow trout

In the present study, the treatment of ichthyophthiriasis with medicated feed was investigated in rainbow trout, Oncorhynchus mykiss, and chub, Leuciscus cephalus. The anti-parasitics toltrazuril and imidocarb; the antibiotics doxycycline, erythromycin and sulphadiazine and the anti-inflammatory acetylsalicylic acid were tested. In vitro experiment revealed that all tested anti-parasitics and antibiotics were effective in killing the isolated trophonts and theronts. Minimum doses for killing 100 % of the viable trophonts and for inhibiting the development of theronts were 3 mg/L for doxycycline, 30 mg/L for erythromycin, 2 mg/L for imidocarb dipropionate, 30 mg/L for sulphadiazine and 20 mg/L for toltrazuril. Acetylsalicylic acid (40 mg/kg fish/day), doxycycline (3 and 6 mg/kg/day), erythromycin (40 mg/kg/day), imidocarb dipropionate (5.0 mg/kg/day), sulphadiazine (40 mg/kg/day), toltrazuril (20 and 40 mg/kg/ day) and combinations of doxycycline and toltrazuril (3 +20 mg/kg/day, 6+40 mg/kg/day) were tested as medicated feed. When administered as medicated feed, only doxycycline, toltrazuril and combinations of doxycycline and toltrazuril reduced the fish mortality and infestation level. Best results were obtained by feeding a combination of 6 mg/kg/ day doxycycline and 40 mg/kg/day toltrazuril. In O. mykiss, this treatment reduced the mortality rate from 100 to 50±14 % after 10 days and the infestation level from grade 4 (>100 trophonts per skin mucus sample) to 3.5 (50-100 trophonts). In L. cephalus, the mortality rate was decreased from 100 to 39±5 % and the infestation level from grades 4 to 2 (ten to 50 trophonts) after 10 days.