INFECTIOUS PANCREATIC NECROSIS OF SALMONI DS - ASSESSMENT OF METHODS AIMING AT CUTTING OFF EGG-ASSOCIATED TRANSMISSION

Rainbow trout ova were fertilized by milt treated with iodine followin g different schedules, in order to cut off infectious pancreatic necro sis (IPN) transmission. The efficacy of iodine was assessed in paralle l by virus neutralization during a 1 min. exposure. The schedule recom mended for the commercialized iodine solution (ova drained, covered wi th iodine solution and milt added immediately) was used first. When th e pH ranged from 5 to 8 and for iodine concentrations of 15, 25, and 3 5 mg/l, the fecundation rate was below 10 % and rose only at pH 9 (69 % for 25 mg/l iodine). In other hand, the efficacy against the virus d ecreased rapidly for pH values above 7, and efficacy could never match with safety at any pH. The same schedule used with 10 individual milt s showed striking differences in individual susceptibilities (fecundat ion rates from 1.6 to 37 % for 25 mg/l iodine at pH 8) which did not c orrelate with the quality of the milt as estimated by the concentratio n of the 42 kDa membrane protein in seminal fluid. Preincubation of mi lt from a few seconds to 1 minute in different concentrations of iodin e resulted in null fecundation rates, except when milt was in large ex cess. In that case, iodine lost all efficacy against the virus. Two ex perimental transmission experiments were done by incubating sperms wit h concentrated virus. A careful elimination of dead and abnormal eggs followed by an external disinfection (50 mg/l iodine, 15 min.) before hatching did not allow to cut off transmission. This indicates that th e virus can be transmitted without lethally infecting the embryo.