Histopathological effects of waterborne copper nanoparticles on the gills of rainbow trout (Oncorhynchus mykiss) fingerlings before and after a recovery period

Present study was to elucidate the histological damage of copper nanoparticles on gills of rainbow trout before and after a recovery period. To that end, total 135 fingerlings with an average body weight of 30 g were randomly allocated into nine polyethylene tanks (with 90 l volume). Fingerlings were exposed to three concentrations of copper nanoparticles, namely, 0, 25 and 50 ppb for 21 days. The recovery period was also lasted for another 21 days post nanoparticle exposure with no longer nanoparticle addition to culture media to assess the capacity of fish to recover the gill histoarchitecture. The gill samples were taken at the end of each period and stained with H&E method. Results from the first stage indicated that 25 and 50 25 μg/l Cu-NPs resulted in filament shrinkage and epithelial hyperplasia. Also, telangiectasis and blood congestion on tips of gill filaments of 25 μg/l Cu-NPs exposed group were observed. Severe blood congestion and increased inter-lamellar space along with moderate local epithelial degeneration of secondary lamellas were noticeable in 50 μg/l Cu-NPs exposed fish. Tissue damage was observable even after a 21-day recovery period. However, the severity of pathological alterations was lower. In conclusion, it is conceivable that copper nanoparticles can cause noticeable damage to gills which in some cases could be noticed even after post exposure recovery period.