An epizootic of lymphoma in northern pike, Esox lucius L., from the Åland Islands of Finland

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Abstract
An epizootic of lymphoma in northern pike, Esox lucius L., occurring in the Åland Islands in Finnish waters of the Baltic Sea is described. Macroscopic inspection of 19 tumorous pike, 17 males and two females, as well as tissue observations by light and electron microscopy showed that this lymphoma is similar to those previously reported in esocid fishes from North America, Ireland and Sweden. Haematoxylin and eosin‐stained light microscopic sections of tumour tissue suggested its classification as a lymphoblastic or stem cell lymphoma. Tumour cells cultured during several months at 4°C showed morphological features of differentiation and resembled plasma cells. No conclusive evidence of the presence of viruses in the lesion was obtained.