Although transmission stu&es showed that an infect~ous agent was involved in an eplzootic mortality in hatchery-reared fingerling and yearling lake trout Salvelinus namaycush, routine diagnostic examinations repeatedly f a ~l e d to detect infectious or paras~tic agents. Histological examinatlon consistently showed epithehal hyperplasia inside the mouth, on the snout, and over the body. Electron microscopy revealed a putative virus associated with the hyperplaslc tissue; however, the virus has yet to be isolated in cell culture. Morphologically, the virus resembled a herpesvirus, but its taxonomic grouping has yet to b e firmly established. In laboratory studies, the infection could be transmitted by cohabitation with diseased fish, by exposure to water from tanks holding diseased fish, and by bath challenge with filtrates (0.45 pm and 0.22 pm) of hyperplastic epidermal tissue. As judged from laboratory challenges and hatchery observations, the disease d ~d not appear to affect brook trout Salvelinus fonhnalis, brown trout S a l n ~o trutta, rainbow trout Salnio gairclneri, or Atlantic salmon Salmo salar.
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McAllister et al. (1989) studied this question.