Histopathology of viremia-associated ana-aki-byo in combination with Aeromonas hydrophila in color carp Cyprinus carpio in Japan.

A disease in which 'viremia-associated ana-aki-byo' is combined with an Aeromonas hydrophila infection currently occurs and is highly transmissible in color carp Cyprinus carpio in Japan. In the present study, to determine the interrelation between a corona-like virus and A. hydrophila, we conducted transmission trials by cohabiting naturally diseased carp with healthy carp with skin that had been slightly damaged artificially. Experimentally exposed fish successfully replicated the combination of a corona-like viral viremia and A. hydrophila infection. Diseased carp displayed scale-sac edema, ascites and exophthalmus adding to the formation of skin ulcers. In addition to pathological changes due to the corona-like virus infection, various changes due to the A. hydrophila infection occurred. Anasarcous skin lesions exhibited a separated epidermis, expanded scale-sacs, and an edematous dermis accompanied by hemorrhage and necrosis. The underlying lateral musculature was edematous and possessed markedly atrophied muscle fibers. Hepatocytes were either atrophied or swollen and had a granular appearance. Renal tubular cells showed vacuolar degeneration, cloudy swelling, necrosis and destruction. Hemosiderin deposition occurred within macrophages in the spleen and hematopoietic tissue, and within hepatocytes. Cardiac muscle fibers exhibited degeneration and necrosis accompanied by hemorrhage in the myocardium of heart. These changes appeared to be induced by bacterial toxins because bacterial cells did not directly invade these affected tissues.