Abstract
Disorders of the gill during the phase of farming trout salmon (Oncorhynchus mykiss) become a significant problem. The term "complex gill disease" (CGD) refers to a variety of clinical gill disease manifestations that typically occur over the course of four seasons in salmon farms. The observed gross lesions and histological lesions are the end result of infection to pathogenic organisms. Fish pathological and normal states can be determined using histopathological examination. This study estimated the health status of farmed trout salmon using histopathological examination as a diagnostic tool by using the routine stain hematoxylin and eosin, and using special stain Alcian blue stain and periodic acid fast stains samples of gills of 200 trout (Oncorhynchus mykiss) from fish farms in Duhok were collected and fixed. All fish have lesions on the gills. Studies using light microscopy of the treated gills revealed significant histological changes which includes hemorrhage and congestion of the blood vessels of the primary and secondary lamella, facultative degeneration of epithelial cells, mononuclear inflammatory cells infiltration, necrosis epithelial cells of the secondary lamella, adhesions of primary lamella, hypertrophy of chloride cells and mucus cells, facultative degeneration of pillar and hyperplasia of chloride cells.