Microscopical lesions and antigen distribution in bovine fetal tissues and placentae following experimental infection with bovine herpesvirus-1 during pregnancy.

As part of a larger investigation, gross and histopathological examinations were carried out on six aborted and one non-viable calf born to heifers inoculated with bovine herpesvirus-1 (BHV-1) early in the third trimester of pregnancy. Antibody titres in sera collected from the dams confirmed seroconversion following inoculation. Samples of liver, lung, kidney, brain, heart, spleen, hepatic lymph node and placenta were subjected to histopathological examination. Immunohistochemistry for the detection of BHV-1 antigen was performed on liver and placenta from each calf, and on the full range of tissue from three of the six calves. Six dams aborted between 15 and 50 days post-inoculation (dpi) whilst one produced a live but non-viable calf at 51dpi. Consistent microscopical findings in tissues from the six aborted calves were multifocal coagulative necrosis in the liver and necrotic placentitis. The latter was characterized by villous necrosis, necrosis of vascular endothelium and infiltration of necrotic villi by mixed inflammatory cells. Other findings included multifocal necrosis in kidney, spleen and hepatic lymph node as well as haemorrhage in the lung and kidney. Immunohistochemistry confirmed the presence of BHV-1 antigen in association with these lesions and also revealed focal labelling of the endothelium of small blood vessels and surrounding glial processes in the brains of three calves. Virus isolation confirmed the presence of BHV-1 in the placentae from the six aborted calves and in pooled tissues of three of the fetuses. It is concluded that the pathogenesis of BHV-1 abortion involves infection of vascular endothelial cells in multiple tissues including placenta and brain. Furthermore, histopathological examination in suspected cases of BHV-1 abortion should include placenta as well as fetal viscera, and immunohistochemistry is a valuable tool for confirming a diagnosis of infection with the virus.