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[Infectious pancreatic necrosis virus: acquired susceptibility to trout serum neutralizing factor after repeated passage in cell culture].

It has been shown previously that in normal sera from IPN free Rainbow Trout a "6 S molecule" neutralized Sp type IPN Viruses routinely used in European fish diseases laboratories. These viruses were unable to reproduce the disease under experimental conditions. After a survey of trout farms, a Sp type pathogenic virus was isolated (isolate 31/75). After one passage on RTG2 cells this virus was fed to 7 weeks old fry and led to a 70% mortality. In contrast, a routinely passed and cloned virus (27/70) gave no noticeable mortality. This freshly isolated virus produced only small plaques on RTG2 cells (27/70 produced small and large plaques) and was not neutralized at all by the "natural" 6 S neutralizing molecule from serum. After repeated RTG2 cell culture passages of 31/75 virus, large plaques appeared at the 7th passage, and a large plaque was cloned. The virus thus cloned (8th passage) was fully neutralized by 1/200 normal trout serum. This acquired susceptibility can account for the loss of pathogenicity of cell cultured IPN viruses, and could be a "marker" of non pathogenic IPN viruses which have been recorded in trout farms.