To cope with amphibian die-offs caused by ranavirus, it is important to know the underlying ranavirus prevalence in a region. We studied the ranavirus prevalence in tadpoles of two native and one introduced anuran species inhabiting agricultural and surrounding areas at 49 locations across eight provinces of South Korea by applying qPCR. The local ranavirus prevalence and the individual infection rates at infected locations were 32.6% and 16.1%, respectively, for Dryophytes japonicus (Japanese tree frog); 25.6% and 26.1% for Pelophylax nigromaculatus (Black-spotted pond frog); and 30.5% and 50.0% for Lithobates catesbeianus (American bullfrog). The individual infection rate of L. catesbeianus was significantly greater than that of D. japonicus. The individual infection rate of P. nigromaculatus was related to the site-specific precipitation and air temperature. The individual infection rate gradually increased from Gosner development stage 39, and intermittent infection was confirmed in the early and middle developmental stages. Our results show that ranavirus is widespread among wild amphibians living in agricultural areas of South Korea, and mass die-offs by ranavirus could occur at any time.

Ranaviruses are a primary cause of amphibian extinctions. More consistent ranavirus-infection reports and genetic characterizations of identified viruses are urgently needed, particularly from Asian countries. The objectives of this study were to obtain the partial major capsid protein (MCP) gene sequences (506 bp) of the ranavirus responsible for infecting frogs in South Korea, as our previous research had confirmed using qPCR, and to evaluate their genetic relationships with other previously reported ranavirus sequences. Three different ranavirus MCP sequences were obtained from Pelophylax nigromaculatus and Lithobates catesbeianus. All six different types of MCP sequence from the ranavirus identified in South Korea to date belonged to the Frog virus 3 (FV3)-like virus group in the genus Ranavirus. To better understand the origin and spread of ranaviruses in South Korea, further infection reports and full genome analyses of the identified ranaviruses are needed.

The slender racer, Orientocoluber spinalis, is a monotypic species found in northeast Asia. We collected 67 O. spinalis samples from the Republic of Korea (hereafter, South Korea) and 7 from China and Mongolia and investigated their genetic diversity and population structure. In South Korea, O. spinalis populations were mainly found on Oeyeondo, Uido, and Udo islands and Woraksan Mountain and showed low genetic diversity in the analysis of concatenated mitochondrial sequences of the cytochrome b (Cytb) and NADH dehydrogenase subunit 4 (ND4) genes. Orientocoluber spinalis populations in South Korea showed low differentiation and likely diverged recently. Orientocoluber spinalis may have colonized the Korean Peninsula from China and Mongolia, but this route is not confirmed due to the lack of samples from the Democratic People’s Republic of Korea and middle eastern China. Considering its extreme rarity, low population density, and low genetic diversity, O. spinalis should be designated an endangered species in South Korea, as it is in Russia, Mongolia, and Kazakhstan.

Background: Several species of amphibians in agricultural areas are often infected with ranaviruses; however, the biological or ecological factors that cause this infection are not well understood. In this study, we investigated whether local tadpole density, Gosner developmental stage, and weather conditions affected ranavirus infection in Dryophytes japonicus tadpoles in rice paddies over three months. Results: During the study, eight samplings were undertaken between June 6 and August 21, 2022. No die-off of tadpoles occurred, but 20 of 110 tadpoles (18.8%) were found to be infected with ranavirus. The tadpole density at the sampling site and Gosner stage of the sampled tadpoles were not related to the daily ranavirus infection rate. The mean daily highest temperature during the two weeks prior to the sampling date and the mean daily lowest and highest temperatures during the week prior to the sampling date were negatively related to the daily infection rate. Conclusions: Our results suggest that low and extreme temperatures caused by flooding and draining of paddy fields or climate change in summer could be a significant risk factor for ranavirus infection in summer-breeding frogs in agricultural areas.