Epizootic Epitheliotropic Disease of lake trout (Salvelinus namaycush): history and viral etiology

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Abstract
A previously undescribed virus was isolated by physical (non-cultural) means from juvenile lake trout (Salvelinus namaycush) undergoing epizootics of unknown etiology. The enveloped virus, which measured 220 to 235 nm and had a capsid diameter of 100 to 105 nm, was extracted from moribund fish by isopycnic centrifugation and visualized by transmission electron microscopy. Virus was not found in apparently healthy fish. Horizontal transmission was demonstrated by waterborne exposure of healthy fish to 450 nm filtrates and to purified virus obtained by non-cultural methods from moribund fish, 100% mortality occurring within 39 and 43 d post-exposure, respectively. No mortalities occurred in groups of fish exposed to 220 nm filtrates or to 450 nm filtrates prepared from healthy control fish. Investigations of natural epizootics at 3 different hatcheries revealed that fry were more affected by the disease than fingerlings. Mortalities in a population of 3.2 million fry exceeded 99.7%. The virus appeared to be the etiologic agent of the epizootics based on the following criteria: (1) it could be obtained only from infected fish; (2) the disease could be induced with purified virus and with filtrates from infected fish prepared with filters of appropriate porosity; (3) it produced clinical signs and lesions consistent with those observed during natural epizootics; and (4) the antibiotic oxytetracycline proved ineffective against it. The virus has been tentatively classified as a herpesvirus and named Epizootic Epitheliotropic Disease Virus, based on the propensity of the agent for epithelial cells, especially those in the epidermis.

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