Nephropathia epidemica: mild variant of hemorrhagic fever with renal syndrome.

The diagnosis of nephropathia epidemica (NE) is primarily not histological, but because biopsy samples sometimes reach the pathologist, knowledge of histology may be of diagnostic value, in addition to clarifying the pathogenesis. The authors collected 80 biopsies from 65 patients taken after the onset of NE from various hospital files in Finland. Light microscopic, morphometric, electron microscopic, and immunohistochemical methods were applied in studying these samples. There was slight tubular dilatation and interstitial edema best evidenced during the first month after the onset of the disease. There was limited electron microscopic evidence of endothelial cell damage or reaction, also in the glomeruli. Occasional tubular necrotic cells or mitoses were found. About half of the cases were positive for IgG or IgM, and C3 and fibrin in the tubular basement membrane. Samples of the recovery phase showed interstitial fibrosis. Medullary interstitial hemorrhages were seen in 60% of samples of the first two weeks after onset. Interstitial inflammatory changes were diffuse in the early phases and often focal thereafter. During the first two weeks there was congestion and mild hypercellularity of the glomeruli. There was also slight prominence of mesangium, and evidence of endothelial cell damage or reaction. Osmiophilic glomerular deposits were scanty. Focal immunoglobulin (usually IgG) and complement deposits were present in the glomeruli, but less intense than in classical glomerulonephritis. Without medullary hemorrhages in the biopsy, the suggestive diagnosis could rest on interstitial edema, diffuse but sparse inflammatory infiltrate, dilatation of occasional tubules, changes suggestive of tubular cell death, and congestion and slight hypercellularity of the glomeruli.