Detection of CMTV-like ranavirus following a Rana temporaria mass mortality event in a northern Italian alpine lake

High-mountain lakes are vulnerable to climatic and anthropogenic stressors, and infectious disease may further exacerbate impacts on alpine communities, particularly during seasonal temperature peaks. In August 2024, a mass mortality event of common frogs (Rana temporaria) occurred in a high-altitude lake in the Cottian Alps (Piedmont, Italy). During a one-hour survey, 78 dead frogs were recorded; nine carcasses (all adult males) were sampled for diagnostic tests. Six showed ventral hyperaemia and three of them had ulcerative lesions of the digits. Eight out of nine vitreous humour samples were culture-positive, with isolates including Hafnia alvei, Acinetobacter guillouiae, Acinetobacter proteolyticus, and Serratia proteamaculans. PCR screening of skin and pooled target organs was negative for Batrachochytrium dendrobatidis and herpesvirus, but detected ranavirus in four out of nine frogs. Phylogenetic analysis of sequenced major capsid protein and DNA polymerase fragments grouped the virus within the CMTV-like clade, with high similarity to reference sequences. This represents the first geolocated detection of a CMTV-like ranavirus in free-ranging amphibians in Italy. Although the advanced state of decomposition precluded histopathological evaluation and causality cannot be conclusively established, the concordance between molecular detection and gross lesions consistent with ranaviral infection supports a plausible role of ranavirus in the observed die-off. Our findings highlight the need for targeted surveillance in Italy’s alpine amphibians, including environmental DNA sampling and screening of non-native fish. Given ecological simplification and short reproductive seasons at high altitude, longitudinal monitoring is advisable to assess persistence, seasonality and potential spillover across life stages and sympatric species.