Effects of sea lice treatment on health status in cultured rainbow trout (Oncorhynchus mykiss)

The objective of the work undertaken was to study the effect of azamethiphos (Salmosan) treatment on blood biochemistry and tissue histopathology in aquaculture reared rainbow trout to determine its impact on the health status. Fish serum biochemical analysis showed significant increases (p<0.05) in total bilirubin and free haemoglobin concentrations recorded 24 hours after the treatment. Levels of serum iron were significantly increased 48 hours after the treatment. Markers of kidney damage showed increased levels in all time points after the treatment together with high levels of inorganic phosphorus and urea at 4 and 10 days after the treatment, respectively. Typical marker for liver condition included ALT, which showed increased activity from 48 h after treatment onwards, while globulin concentration was decreased 4 days after the treatment. The levels of copper and zinc were significantly lower on day 4 and day 10 after the treatment (p<0.05). The ammonia concentration was significantly lower only on day 4 (p<0.05) when compared to concentrations before the treatment. After histopathological evaluation gills showed highest damage to the secondary lamellas in form of blood congestion, telangiectasia and excessive bleeding 4 days after the treatment. The tissue damage of kidney 24 hours after delousing treatment showed increased deposits of hemosiderin in proximal and distal tubules, while after 10 days glomerular necrosis was evident. Liver damage was visible as after delousing treatment after 24 hours as mild necrosis of hepatocytes while severe dilatation of sinusoids and hemosiderin deposits were visible 4 days after the treatment. Splenic tissue showed excessive hemosiderin deposits and increased number of melano-macrophage centres (MMC) at 4 days after treatment when compared to fish from before treatment. These results indicate the possible impact of sea lice treatment on trout health through intravascular haemolysis caused by the chemical, and consequently the pathophysiologic process of haemoglobin metabolism and its products causing chronic injury from hemosiderin deposits. Blood biochemistry has a good potential to approach fish heath status and was correlated with organ/tissue damage. The kidneys were significantly affected and weakened after the treatment showing a more sensitive response than the other studied organs. We conclude that organ injury resulting from haemolysis and hemosiderin deposits should be considered as a cause of decreased organ function after sea lice treatments.