Parapoxvirus infection in harbor seals (Phoca vitulina) from the German North Sea

In the summer of 2000, proliferative lesions of the skin and oral mucosa were observed in 26 young harbor seals (Phoca vitulina) from a rehabilitation center in Schleswig-Holstein, Germany. Verrucose, roundish nodules, approximately 1-2 cm in diameter, were presented in the oral cavity, especially on the tongue. Some animals developed similarly sized spherical dermal elevations with ulceration on flippers, chest, neck, and perineum. Necropsy of one animal showed multifocal, verrucose nodules in the oral cavity and a mild tonsillitis. Histologically, the nodules were characterized by ballooning degeneration of the outer parts of the spiny layer and stratum granulosum, with large eosinophilic cytoplasmic inclusions and a perivascular to interstitial lymphohistiocytic infiltration accompanied by fibroblastic proliferation and neovascularization. Negative staining of mucosal tissue homogenates demonstrated parapoxvirus-like particles. The presence of parapoxvirus was confirmed by polymerase chain reaction, using primers specific for parapoxvirus of ungulates. By in situ hybridization, using a parapox-specific, digoxigenin-labeled DNA probe, abundant parapoxvirus DNA-positive epithelial cells were detected in the stratum granulosum and the outer parts of the spiny layer. There was no parapoxvirus-positive signal in the adjacent submucosa. Although DNA analysis revealed that the causative agent can clearly be distinct from terrestrial parapoxviruses, lesions resembled parapoxvirus infections in other terrestrial species, and the pattern of virus DNA distribution indicated a direct effect of the virus on keratinocytes. In contrast, changes in the corium may be considered an indirect response mediated by the virus or the immune system.