Immunohistochemical demonstration of phocine herpesvirus-1 antigen in harbour seals (Phoca vitulina) of the North Sea

Phocine herpesvirus-1 (PhHV-1) can cause acute pneumonia and hepatitis with high morbidity and frequently fatal outcomes in young harbour seals (Phoca vitulina) and, occasionally, in grey seals (Halichoerus grypus). Here, we report the virological and pathomorphological findings of a PhHV-1 infection in 10 out of 90 investigated young harbour seals, collected between January 2020 and December 2021 from the Dutch North Sea coast, found either dead or suffering from severe respiratory distress resulting in euthanasia. Macroscopic findings included poor nutritional condition, poorly deflated and slightly consolidated lungs, as well as a friable and pale liver parenchyma. Moreover, six seals showed several co-morbidities such as septicaemia, pancreatitis and pneumothorax. Microscopic investigation revealed predominantly a non-suppurative peribronchitis and necrotizing bronchitis. Multifocal to coalescing necrotizing hepatitis and non-suppurative inflammation of the liver were the most striking lesions. Eosinophilic intranuclear inclusion bodies were seen in bronchial epithelial cells and hepatocytes in three and six cases, respectively. Using a monoclonal anti-PhHV-1 glycoprotein B antibody, viral antigen was detected in situ, specifically intra- and perilesional, in the lung and/or liver of eight seals. Electron microscopy of two selected cases demonstrated numerous intranuclear viral capsids and, within the cytoplasm, enveloped mature virions. A pan-Herpesviridae polymerase chain reaction using a degenerative primer set was used to confirm PhHV-1 infection. Genome fragments were detected in lung and liver samples. Collectively, these data indicate that PhHV-1 infection was the most likely cause of disease in these 10 animals, although several co-morbidities were also present. This indicates that monitoring of PhHV-1 infection status in harbour seals, especially juvenile animals, is an integral part of population health surveillance of susceptible phocid populations.