Disease Progression in Lake Trout (Salvelinus namaycush) Experimentally Infected With Epizootic Epitheliotropic Disease Virus (Salmonid Herpesvirus-3).

Epizootic epitheliotropic disease virus (salmonid herpesvirus-3; EEDV) is responsible for the death of millions of hatchery-raised lake trout (Salvelinus namaycush) in the Laurentian Great Lakes Basin. However, little is known about its biology, pathology, tropism, and host interactions. In this study, the presence and disease progression of EEDV were evaluated following exposure of naïve juvenile lake trout to EEDV via bath immersion under controlled laboratory conditions (n = 84 infected; n = 44 control). Individual tissues (n = 10 per fish), collected over 6 weeks, were analyzed for viral load by quantitative polymerase chain reaction, gross and histopathologic changes, and virus cellular targets using in situ hybridization. Skin, fin, and ocular tissues were the earliest viral targets and yielded the highest viral loads throughout the course of infection. Early gross lesions included exophthalmia, ocular hemorrhage, fin congestion, and hyperemia of visceral blood vessels. Advanced disease was characterized by multifocal to coalescing erosions and ulcerations of the skin, and congestion of visceral organs. Microscopically, there was cellular degeneration and necrosis in the epidermis and spleen, and lymphohistiocytic perivasculitis of the dermis, omentum, and the epicardium. EEDV DNA was first detected by in situ hybridization in epithelial cells of the epidermis, with subsequent labeling in the epithelial lining of primary and secondary gill lamellae. During advanced disease, EEDV was detected in endothelial and dendritic cells as well as blood monocytes. This study characterized EEDV tissue tropism and associated pathologic features, to guide research aimed at understanding EEDV disease ecology and improving strategies for disease control.