Emerging epidemic diseases of frogs in Britain are dependent on the source of ranavirus agent and the route of exposure

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Abstract
A series of transmission studies was conducted to investigate the aetiology, or aetiologies, of emerging fatal epidemic disease syndromes affecting the common frog (Rana temporaria) in Britain. The syndromes, characterized by skin ulceration or systemic haemorrhages, were induced upon exposure to lesion homogenates or cultured ranavirus. The re-isolation of ranavirus from experimentally affected frogs fulfilled Koch's postulates. Aeromonas hydrophila, previously associated with similar lesions, was not significant to disease development. Unexpectedly, disease outcomes were influenced by both the source of agent and the route of exposure, indicating that different ranaviruses with different tissue tropisms and pathogeneses (possibly similar to quasi-species in RNA virus populations) are circulating in the British common frog population. Our findings confirm that ranavirus disease has emerged as an important cause of amphibian mortality in Britain.