Figure 4. Histopathologic lesions and viral RNA in European perch infected with Bern perch virus. A) Histopathologic lesions in gills (hematoxylin and eosin stain) showing epithelial hypertrophy and hyperplasia, multifocally leading to lamellar fusion (stars) and multifocal epithelial lifting due to edema (closed arrowheads). Scale bar indicates 25 μm. B) In situ hybridization detection of Bern perch virus RNA in gills (brown labeling): positive macrophages, more pronounced in proliferated areas, and endothelial cells. Inset: higher magnification showing positive macrophages (open arrowheads) and endothelial cells (arrows with open heads). Scale bar indicates 50 μm.