Histopathological survey on pectoral fin rot in cultivated rainbow trout (Oncorhynchus mykiss) in Urmia, Iran.

Objective: The cultivated fish fin rot is a most important problem in aquaculture. The histopathological study is a way to understanding the importance of this lesion. The objective of this study was histopathological survey pectoral fin rot in cultivated rainbow trout. Method & Materials: In this study 1000 rainbow trout fish from 10 fish farms of Urmia, Iran were surveyed and affected fish with fin rot were collected randomly. Totally 50 fish were taken and transferred to pathology lab of Veterinary Faculty of Urmia University. Affected pectoral fins were fixated in 10% phosphate-buffered formalin and then sections were analyzed under light microscope. Results & Conclusion: The results showed in out of the affected cases with pectoral fin rot, 58% and 83% epiderm was hyperplastic and spongiosis, respectively. The latter one was characterized by aqueous swelling of basal layer cells, upper layer and also intracellular edema. The cellular and intracellular edema range was from slight to severe and formation of vesicle. This resulted information of gaps among cells, then cells were isolated and the layers of epiderm were taken apart. The distal cells underwent necrosis and exfoliations were resulted. In all above mentioned conditions inflammation and infiltration of cells were observed to some extents and in some cases these infiltrations were so severe that were extended to all layers of epiderm. In some cases the infiltration was less severe and limited to the regions under the epiderm. The cellular reaction ranged from an acute suppurative swelling to granulomatous swelling. Analysis of spongiosis and the mechanism involved in exfoliation of upper layer cells of epiderm and a high incidence rate of 83% indicated that process of fin rot was initiated following cellular and intracellular edema that gave rise to dissociation of cells and exfoliation of cells layers