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

Abstract

Juvenlle coho salmon Oncorhynchus k~sutch reared at a freshwater hatchery on Vancouver Island, Canada, exhibited h ~g h , chron~c losses assodated with an ep~theliotropic lyrnphoblast~c lymphoma of thymic orlgln Cumulative mortality of 4SoA occurred over an 8 rno period in one year class. The d ~s e a s e xvas observed in the fish until release from the hatchery for seawater migration Saltwater challenge tests ind~cated that the affected fish had ~rnpaired osmoregulation and thus poor seawater surv~val was expected. Histolog~cal examlnat~on of the moribund flsh revealed a marked proliferation of neoplastic lymphoid cells In the thymus. The cells infiltrated the gills, the Integument of the opercular cavlty, and the s h n around the nares a n d eyes. The neoplastic cells were oval w ~t h a moderate amount of finely granular eoslnophllic 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 wlth hyperchrom~c n u c l e ~ and lnapparent nucleol~ Cells with cleft nuclei and multinucleated cells were observed. Ultrastructural exarninat~on of the neoplastic cells revealed abundant rough endoplasrnic reticula w ~t h no evldence of desrnosomes. No particles suggestive of viruses were detected in or around the neoplastic cells The dlsease was judged to b e a neoplasia, rather than a reactive lymphoblastosis, because the proliferative cells were morphologically immature, there was l~ttle other lnflamrnatory involvement or necrosis, and the lesions were invasive a n d persistent The fish continued to die until release from the hatchery.