Abstract
Background:
A serious mortality epizootic affected the populations of common carp (Cyprinus carpio L.) in several fish farms that are located at Al–jadeda–Diyala province, Iraq, during the autumn 2022. The diseased fish were showing clinical signs associated with a viral cause.

Aim:
To conduct molecular detection using polymerase chain reaction (PCR) to examine the carp interstitial nephritis and gill necrosis virus (CNGV) outbreak, and characterize histopathological changes in major organs, as well as serum liver enzyme profile and kidney function profile.

Methods:
A total of 50 live common carp (Cyprinus carpio L.) with weight of 128 to 2,300 g that showed clinical signs such as necrosis on the gills, overproduction of mucus, or pallor in the skin were randomly sampled from five positive farms. Blood samples were obtained from the caudal vein for measurement of the level of biochemical liver enzymes (alanine transaminase and aspartate amino transferase) markers for kidney function (creatinine). At 15 d postimmunization, fish (n = 3) from each group were euthanized, and tissues (gill, liver, spleen, kidney, and intestine) were aseptically removed. Hematoxylin/eosin-stained histopathology tissues were processed by routine methods after fixation with 10% neutral buffered formaldehyde. Gill tissue was maintained in phosphate-buffered saline for DNA extraction and PCR amplification to a region of the conserved genome between CNGVs for molecular analysis.

Results:
The positive presence of CNGV in the fish samples was confirmed by PCR. Histopathological analysis showed extensive lesions in all the analyzed organs, such as severe gill necrosis, hepatitis, splenitis, extensive interstitial nephritis, and enteritis. They were associated with marked increases in serum liver enzyme activities and alterations of renal end-points.

Conclusion:
This is the first molecular confirmation of CNGV infection as the cause of an outbreak of disease in common carp in Iraq. The virus caused severe systemic lesions with characteristic histopathological alterations in major organs and remarkable hepatic and renal function impairment. These results bring CNGV as a serious new threat to carp farming in the region.

Key words: Carp interstitial nephritis and gill necrosis virus; Histopathology; Blood biochemistry; Common carp.