Histopathology of mortality in cultured rainbow trout fry in some coldwater hatcheries and fish farms of Iran

Causes of the mortality of cultured rainbow trout fry in coldwater hatcheries and fish farms in Iran were investigated. Some 104 tissue specimens of liver, kidney, spleen, pancreas, intestine, and gill from 59 diseased fries from Mazandaran, Fars, Markazy and Kohgiloyely Boyerahmad provinces were collected for histopathological studies. Tissues were immediately fixed in 10% buffered formalin for at least 24 hours. The fixed tissues were processed in an automatic tissue processor using standard procedure. After processing, tissues were embedded into wax and 5 micron sections were prepared using a rotary microtome. The sections were stained using II & E staining method, and examined under compound microscope. The affected fries were dark in color, and showed exophthalmia, ascitis, erratic swimming, faecal casts in the anal area and lethargy. They also gathered near the outlet of the ponds. Microscopic examination of the tissues revealed histopathologic changes including congestion, inflammation of the basal membrane of secondary lamellae, hyperplasia, and fusion of secondary lamellae and clubbing in some cases in the gill. Congestion of blood vessels, degeneration of kidney cells, necrosis of hematopoitic tissue and tubules, increase of melanin pigments and infiltration of inflammatory cells were observed in kidney. In liver congestion of blood vessels, increase of fat in hepatocytes, congestion and dilation of sinusoids with increased monocytes, increase in melano-macrophage number, vacuolation of hepatocyctes and focal necrosis were seen. Bile duct neoplasia (cholangioma) was also present in some cases. Spleen showed congestion, hemosidrosis, increase in melano-macrophage centers and necrosis in some cases. The pancreas tissue showed congestion, degeneration and necrosis of ashlar cells and islets of Langerhans. Congestion of sub-mucusa layer, fusion of mucosa layer, necrosis and detaching of the mucoid columnar epithelium were observed in the intestine tissue. The clinical and histo-pathological examination results suggested that the causative agent of the fry disease is likely to be a virus with the signs similar to UN disease