Iraqi fish breeding industry suffered a severe fatal outbreak in September and October 2018, where the high mortality rate was observed in Iraqi fish farms especially that depended on cage system breeding. The current study was conducted to investigate and diagnose the phenomena of high mortality in September and October 2018. Twenty-five affected common carp fishes were collected from five fish farms use cages system (five fishes each) on Euphrates River banks. Affected fish samples were analyzed via gross. Histopathology, PCR for viral detection, bacteriology and mycology techniques. The high mortality rate was observed in the all affected farms, which reach to 80% of farms fishes. Grossly, hemorrhage patches in skins of affected fishes were observed. Congestion and hemorrhage were observed in liver, kidneys, spleen and muscle. Also, erosion of gills filament with presence of white to gray patches in affected gills were observed. Histopathology results showed necrosis lesion in gills, liver and kidneys cells with presence of hemorrhage. The qPCR results for virus detection revealed that all tested samples were positive for Koi Herpes virus (KHV). The bacteriology results showed presence of five gram negative bacteria including three species of Aeromonas, Raoultella ornithinolytica and Shewanella putrefaciens. The mycology results showed the presence of Aspergillus fumigatus and Mucor spp. In conclusion, according to results the high mortality outbreak in fish that occurred in September and October 2018, Iraq was due to Koi Herpes virus infection with presence of secondary bacterial or fungal infection.


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