Ranavirus genotypes in the Netherlands and their potential association with virulence in water frogs (Pelophylax spp.)

Ranaviruses are pathogenic viruses for poikilothermic vertebrates worldwide. The identification of a common midwife toad virus (CMTV) associated with massive die-offs in water frogs (Pelophylax spp.) in the Netherlands has increased awareness for emerging viruses in amphibians in the country. Complete genome sequencing of 13 ranavirus isolates collected from ten different sites in the period 2011–2016 revealed three CMTV groups present in distinct geographical areas in the Netherlands. Phylogenetic analysis showed that emerging viruses from the northern part of the Netherlands belonged to CMTV-NL group I. Group II and III viruses were derived from the animals located in the center-east and south of the country, and shared a more recent common ancestor to CMTV-amphibian associated ranaviruses reported in China, Italy, Denmark, and Switzerland. Field monitoring revealed differences in water frog host abundance at sites where distinct ranavirus groups occur; with ranavirus-associated deaths, host counts decreasing progressively, and few juveniles found in the north where CMTV-NL group I occurs but not in the south with CMTV-NL group III. Investigation of tandem repeats of coding genes gave no conclusive information about phylo-geographical clustering, while genetic analysis of the genomes revealed truncations in 17 genes across CMTV-NL groups II and III compared to group I. Further studies are needed to elucidate the contribution of these genes as well as environmental variables to explain the observed differences in host abundance.蛙病毒属病毒（Ranaviruses）是全球范围内侵染变温脊椎动物的致病性病原。荷兰地区曾发现与侧褶蛙属（Pelophylax spp.）两栖动物大规模死亡事件相关的普通助产蟾病毒（common midwife toad virus, CMTV），该发现提升了荷兰国内对两栖动物新兴病毒的关注度。2011至2016年间，研究人员从10个不同采样点收集了13株蛙病毒分离株，全基因组测序结果显示荷兰境内存在三类分布于不同地理区域的CMTV毒株群。系统发育分析（phylogenetic analysis）表明，荷兰北部的新兴蛙病毒归属于CMTV-NL I群；II群与III群病毒分离自荷兰中东部及南部的宿主动物，且与中国、意大利、丹麦及瑞士报道的两栖动物相关CMTV蛙病毒拥有更近的共同祖先。实地监测显示，不同蛙病毒群定植的采样点中，侧褶蛙宿主种群丰度存在显著差异：在CMTV-NL I群分布的北部区域，伴随蛙病毒感染引发的死亡事件，宿主种群数量持续下降且几乎未发现幼体，而在CMTV-NL III群分布的南部区域则未出现此类现象。对编码基因串联重复序列（tandem repeats）的分析未获得关于系统地理聚类的确定性结论，但基因组遗传分析显示，相较于I群，CMTV-NL II群与III群的基因组中共存在17个基因的截短变异。未来仍需开展进一步研究，以阐明这些基因及环境变量对宿主种群丰度差异的调控机制。