Physiological and morphological effects of microinjection of oxythiamine and PCBs in embryos of Baltic salmon (Salmo salar): A comparison with the M74 syndrome

Since 1974, sea-run Baltic salmon populations have been afflicted by an ear ly life-stage mortality known as the M74 syndrome. The syndrome has been sh own to be associated with a thiamine (vitamin B-1) deficiency that causes n eurological disturbances associated with necrotic brain cells. Treatment wi th thiamine may counteract development of M74. In this study, eyed eggs of sea-run Baltic salmon were given the thiamine antagonist oxythiamine and th e commercial PCB-blend Clophen A50 by means of microinjection into the yolk sac. The aim was to study the effects of an experimentally induced thiamin e deficiency and how it affected the biotransformation system CYP1A using t he 7-ethoxyresorufin-O-deethylase (EROD) assay. After hatching, we attempte d to reverse the deficiency in half of each exposure group by immersion in a thiamine solution and investigated its effect on survival and EROD-activi ty. Yolk-sac fry from groups of eggs that were injected with oxythiamine, e ither with or without Clophen A50, demonstrated a loss of coordination, let hargy, exophthalmia, and whitened liver followed by complete mortality (100 %). Based on this and the time to death, between 124-193 posthatch degree-d ays (d degrees C), the effects of oxythiamine were comparable to those of M 74-development, however, dissimilarities were also noted. Thiamine treatmen t of oxythiamine injected groups delayed mortalities that were reduced to b etween 64.8 and 91.8%. A dose and time-dependent induction of EROD-activity recorded for Clophen A50 groups was strongly suppressed in oxythiamine gro ups. Histopathological examination of oxythiamine groups at 103 and 182 d d egrees C revealed reduced levels of hepatic glycogen, degenerating hepatocy tes and a higher prevalence of necrotic brain cells, all of which are patho logical features found in salmon yolk-sac fry affected by M74. Groups injec ted with Clophen A50 demonstrated no histopathological changes.