An epizootic of an epitheliotrophic lymphoplastic lymphoma in coho salmon Oncorhynchus kisutch

Abstract

Juvenile coho salmon Oncorhynchus kisutch reared at a freshwater hatchery on Vancouver Island, Canada, exhibited high, chronic losses associated with an epitheliotropic lymphoblastic lymphoma of thymic origin. Cumulative mortality of 45% occurred over an 8 mo period in one year class. The disease was observed in the fish until release from the hatchery for seawater migration. Saltwater challenge tests indicated that the affected fish had impaired osmoregulation and thus poor seawater survival was expected. Histological examination of the moribund fish revealed a marked proliferation of neoplastic lymphoid cells in the thymus. The cells infiltrated the gills, the integument of the opercular cavity, and the skin around the nares and eyes. The neoplastic cells were oval with a moderate amount of finely granular eosinophilic cytoplasm, and had a centrally located nucleus. The cell population was morphologically heterogeneous and was composed of large blastoid cells and apparently more mature lymphoid cells with hyperchromic nuclei and inapparent nucleoli. Cells with cleft nuclei and multinucleated cells were observed. Ultrastructural examination of the neoplastic cells revealed abundant rough endoplasmic reticula with no evidence of desmosomes. No particles suggestive of viruses were detected in or around the neoplastic cells. The disease was judged to be a neoplasia, rather than a reactive lymphoblastosis, because the proliferative cells were morphologically immature, there was little other inflammatory involvement or necrosis, and the lesions were invasive and persistent. The fish continued to die until release from the hatchery.