Swine hepatitis E virus (HEV) antigen was detected immunohistochemically in formalin-fixed, paraffinembedded hepatic tissue from 30 naturally infected pigs. Thirty pigs from 30 different herds were selected on the basis of positive results for reverse transcription-polymerase chain reaction. Positive cells typically exhibited a red reaction product in the cytoplasm without any observable background staining. Swine HEV antigen was consistently detected in liver from all 30 pigs tested. A strong immunohistochemical signal was seen within a variable number of hepatocytes in multifocal lobules. The signal involved the majority of hepatocytes diffusely or was confined to foci of
liver cells. Positive immunohistochemical signals were also detected in small and large intestine, lymph node, tonsil, spleen, and kidney. The immunohistochemistry technique developed in this study proved useful for the detection of swine HEV in formalin-fixed, paraffin-embedded tissues
taken from naturally infected pigs and may be a valuable tool in studying the pathogenesis of swine HEV infection.