Histopathological study of the flounder with anemia found in various places in Japanese coastal waters

Histopathological studies were carried out to investigate a previously unknown disease characterized by severe anemia in both wild and cultured Japanese flounder, Paralichthys olivaceus. Light microscopy revealed necrotic cells in the hematopoietic tissues of affected fish. No other tissues showed any notable pathological changes except for atrophy of the hepatopancreas. Virus-like particles of about 27 nm in diameter were observed with electron microscopy in cells in the hematopoietic tissue of the kidney and spleen, including granulocytes and red blood cells (RBCs) which exhibited abnormal features of cytoplasmic organelles. However, the relationship of these particles and the disease is uncertain. RBCs in the blood smears from affected fish were often deformed and staining of the cytoplasm was very weak. These histopathological and hematological characteristics were common among affected flounder from various places in Japan, suggesting that the anemic fish observed in this study were affected by the same disease.