Stig Mellergaard

Treatment of trichodiniasis in eel Anguilla / anguilla reared in recirculation systems in Denmark: alternatives to formaldehyde

Treatment of trichodiniasis in recirculation eel farms using formalin (37%) has been found insufficient. We have screened 30 different chemical substances for efficacy against natural infections with trichodinids (Trichodina jadranica) on skin and gills of eels. The following substances showed high parasiticidal effect: acriflavin (25 ppm), bithionol (0.1 ppm), chloramine T (50 ppm), Detarox AP® (45 ppm), malachite green (1 ppm), raw garlic (200 ppm), potassium permanganate (20 ppm) and Virkon PF® vet. (20 ppm). Preliminary screening revealed that the anthelmintic, bithionol, and the decomposable disinfectants Detarox AP and Virkon PF® vet. were potential therapeutics and these were tested on infected eel stocks in recirculation production units. The following treatments showed a high parasiticidal effect: bithionol at 0.1 ppm in a recirculation system including the biofilters; Detarox AP® (45 ppm) in recirculation systems with the biofilters temporarily isolated for 1 h and Virkon PF® vet. (40 ppm; 25 ppm at start supplemented with 15 ppm 1 h later) in rearing tanks temporarily isolated from recirculation systems for 3 h.

Association between trichodiniasis in eel ( Anguilla anguilla ) and water quality in recirculation systems

Abstract Association between various water quality parameters and the infection level of Trichodina jadranica in cultured eels were examined in nine recirculation farms in Denmark. Production units suffering from trichodiniasis ( n =4) had relatively high contents of organic dry matter (13–65 mg l −1 ), low concentrations of nitrate (150–500 mg NO 3 l −1 ) and high pH (6.2–7.4) in the process water compared to farms without Trichodina problems ( n =5), which had a relatively low load of organic dry matter (9–16 mg l −1 ), high nitrate concentration (250–900 mg NO 3 l −1 ) and a low pH (5.4–6.9). More detailed studies of the effect of water parameters on the infection level were conducted in two production units suffering from frequent outbreaks of trichodiniasis. In one production unit, the high and highly varying content of organic dry matter (21–83 mg l −1 ) was significantly and positively correlated ( r =0.37, p −1 )), no correlation was seen neither between a high and highly varying concentration of nitrate (635–1104 mg NO 3 l −1 ) and intensity of infection nor between a low and highly varying pH (5.45–6.39) and intensity of infection (2–300 trichodinids per fish) during a 17-day period. It is suggested that the infection pressure from trichodiniasis in farms with a relatively high load of organic dry matter (>15–20 mg l −1 ) may be reduced by reducing the content of organic dry matter in the process water.