Physiological, biochemical and morphological studies of Baltic salmon yolk-sac fry with an experimental thiamine deficiency: relations to the M74 syndrome

Abstract

Sea-run Baltic salmon (Salmo salar) populations are suffering from the M74 syndrome, a reproduction disorder affecting both broodfish and their progeny. The syndrome is usually manifested during the middle part of the yolk-sac fry stage and has been shown to be associated with a thiamine (vitamin B(1)) deficiency. Development of the disease is reversible by thiamine treatments of broodfish or progeny. This study aimed at investigating the ability of the thiamine antagonist pyrithiamine, administered by microinjections 3 days after hatch, to cause M74-like signs i.e. typical clinical symptoms, high mortality rates and histopathological changes. Furthermore, the effects of pyrithiamine on hepatic activities of the thiamine-dependent enzyme transketolase (TK), the glucose-6-phosphate dehydrogenase (G6PDH) and the cytochrome P4501A (CYP1A) were evaluated. Six family groups with differing thiamine status were sampled on three occasions during the yolk-sac fry stage. All pyrithiamine exposed groups, with the exception of the one with the highest thiamine concentration, showed M74-like symptoms and suffered from high mortality. Enzyme activities were not different in pyrithiamine groups as compared with controls. However, the TK-activities were strongly associated with the thiamine concentrations. The G6PDH-activity demonstrated small variations with the highest activities in the M74-groups. The [TK]/[G6PDH]-ratios were considerably lower in the M74-groups than in the healthy controls, indicating an imbalance between the oxidative and the non-oxidative part of the pentose-phosphate shunt due to a deficit in thiamine. The pyrithiamine-injections induced several M74-like symptoms including incoordination, lethargy, whitened liver and yolk-sac precipitates. They also caused high mortality rates, in addition to lowered glycogen levels and increased prevalence of necrotic brain cells. Moreover, the study demonstrates that the TK, G6PDH and CYP1A-activities are associated with the thiamine content.