Histopathology Associated with Two Viral Diseases of Larval and Juvenile Fishes: Epidermal Necrosis of the Japanese Flounder Paralichthys olivaceus and Epithelial Necrosis of Black Sea Bream Acanthopagrus schlegeli

Epidermal necrosis was detected in hatchery‐reared larvae and juveniles of the Japanese flounder in 1986–1988 in Japan. Because healthy fish could be experimentally infected by immersion in an ultrafiltrate from diseased fish, it was concluded that the disease was caused by a viral infection. Diseased fish had white cloudy fins and body surfaces, as well as occasional ascites. Skin lesions contained necrotic epidermal cells that had intranuclear inclusion granules and displayed vacuolar degeneration. These damaged cells retained their interdigitations and adhered to one another to form papillate cell aggregates in the epidermis. Necrotic cells examined by electron microscopy contained intranuclear and intracytoplasmic viral particles that displayed features of a herpesvirus. A different disease, epithelial necrosis, occurred among hatchery‐reared larval black sea bream in 1987. Fish became experimentally infected with the disease organism by immersion in an ultrafiltrate of naturally infected larvae. Observations by light microscopy revealed the separation of rounded, necrotic epithelial cells in the epidermis of the fins and body surface, oral mucosa, gills, intestine, and rectum. Electron microscopic examination of necrotic cells revealed intracytoplasmic, enveloped virions that were similar to paramyxoviruses.