ABSTRACT
Ranavirus disease emerged as a cause of annually recurring epidemic mortality of common frogs (Rana temporaria) in Britain in the late-1980s. Affected frogs present with a peracute disease characterized by systemic haemorrhage, or with a chronic disease characterized by skin ulceration, but no internal gross lesions. Common toads (Bufo bufo) have also been found with haemorrhagic ranavirus disease. In order to investigate possible differences in the pathogenesis of ranavirus infection for each main disease syndrome, we studied a range of tissues from both naturally and experimentally infected frogs using anti-ranavirus immunohistochemistry. Ranavirus was located in a variety of cells, including fibrocytes, epithelial cells, lymphocytes, hepatocytes and melano-macrophages, but fewer tissues were infected in frogs with the skin ulcerative syndrome than in frogs with systemic haemorrhagic disease. Specifically, and in contrast to frogs with haemorrhagic syndrome, there was no labelling for viral antigen in the splenic lymphocytes, pancreas or gastrointestinal epithelium in frogs with ulcerative syndrome. Intracytoplasmic virus inclusions were seen in the liver, kidney, pancreas and stomach of frogs with systemic haemorrhagic disease, but not in frogs with the ulcerative syndrome. Immunohistochemical labelling of selected tissues from an affected toad demonstrated ranavirus antigen in the skin and viscera. This technique demonstrates that, in comparison to ranavirus ulcerative syndrome, the haemorrhagic form of ranavirus disease is associated with virus infection of a wider range of internal organs and identifies the infection of certain tissues, such as the spleen, which might be important in the pathogenesis of the haemorrhagic disease.