Pathological and electron microscopic studies on cold water disease among cultured rainbow trout (Oncorhynchus Mykiss Walbaum)

The present study aims to investigate the etiology and pathogenesis of cold water disease among cultured rainbow trout (Oncorhynchus Mykiss Walbaum) reared at low water temperature on a private farm, and propose preventive and control measures to prevent the recurrence of the disease and its spread to the neighboring farms or to other countries. The disease is caused by Flavobacterium psychrophilum and is characterized clinically by high mortality rates (25%) with necrotizing skin and fin lesions. Septicemia with muscular and gill involvement is observed in severely affected cases. The histopathological and ultrastructure alterations in the infected trout explain the pathogencity of this microorganism in many organs other than skin and fins either directly by these bacteria or by its extracellular products, or indirectly via the hypoxia and ion imbalance that results from skin and gill damage. Although some antibiotics, such as trimethoprime, chloramin T and oxytetracycline, are effective on the causative agent, improvement in water quality, vaccination, increasing fish resistance and strict hygiene as well as quarantine measures would all help.