Although ranaviruses have been identified as the causative agent of mortality events for several ectotherms across the globe, our understanding of the host and geographical ranges of ranaviruses is hindered by a lack of virus characterization. The FV3 designation is based almost exclusively on the major capsid protein (MCP) gene. However, most ranaviruses show greater than 90 per cent sequence identity within their MCPs. Although diagnostic tests based on the MCP gene are useful for detecting ranaviruses in a sample, partial sequences of the MCP gene alone are insufficient for differentiating among ranavirus types. Therefore, designations such as ‘FV3-like’ are often used but should be interpreted cautiously, particularly in the absence of any sequence data (James Jancovich, California State University).
(a) Differential impacts on species
Studies involving well-characterized isolates demonstrate that similar FV3-like strains can cause mortality events in hosts as genetically diverse and geographically separated as the European common frog (Rana temporaria) in the UK, wood frog (Lithobates sylvaticus) near the Arctic Circle in Canada, various species chelonians in Europe, Australia and USA, and even the pallid sturgeon (Scaphirhynchus albus) in MO, USA. (Amanda Duffus, Gordon College; Danna Schock, Keyano College; Rachel Marschang, Höhenheim University; Ellen Ariel, James Cook University; Thomas Waltzek, University of Florida). Another distinct type of ranavirus, Ambystoma tigrinum virus (ATV), has thus far been found only in tiger salamanders (Ambystoma mavortium) in western North America. In addition, phylogenetic evidence shows that several testudine reptile ranaviruses are most closely related to amphibian ranaviruses, suggestive of repeated interclass host shifts (Rachel Marschang, University of Höhenheim), and there is phylogenomic evidence that ancestral ranaviruses infected fish hosts and subsequently jumped into amphibian and reptilian hosts (James Jancovich, California State University). Such urgency for a better characterization of ranavirus isolates, their geographical distributions in the wild, and their host ranges is underscored by the fact that ranaviruses can be translocated over large distances as a result of international commerce of their vertebrate hosts (Angela Picco, US Fish and Wildlife Service). The ramifications of possible ranavirus translocations as a result of aquaculture were mirrored in reports ranging from lethal ranaviruses in goldfish (Carassius auratus) in Thailand (Somkiat Kanchanakhan, Aquatic Animal Health Research Institute) and American bullfrogs in Japan (Yumi Une, Azabu University) and Brazil (Rolando Mazzoni, Universidade Federal de Goiás).
Experimental studies provide evidence that cross-class transmission is of critical importance in understanding spread. Within each class, hosts can be differentially impacted by infection. Among and within species, infection may range from no observable lesions or behavioural changes to severe lesions and rapid death (David Green, US Geological Survey; Anna Balseiro, Servicio Regional de Investigación y Desarrollo Agroalimentario (SERIDA); Debra Miller, University of Tennessee). In fact, Jason Hoverman (University of Colorado) tested the relative susceptibility of 19 North American amphibian species to two ranavirus isolates and suggested that more tolerant host species might serve as reservoirs; the most susceptible species he tested were the wood frog and gopher frog (Lithobates capito). Thus, species composition of communities may be a key factor in the dynamics of a disease outbreak.
(b) Environmental dependency of disease dynamics
Because ranaviruses infect ectothermic vertebrates, the surrounding environment should alter ranavirus–host interactions. David Lesbarrères (Laurentian University) described the role of potential abiotic and biotic mechanisms, such as temperature, larval developmental stages, density and competition for resources on the prevalence and virulence of the virus. Additionally, natural stressors (e.g. exposure to a predator) may interact with anthropogenic stressors (e.g. pesticides) to increase susceptibility to ranavirus (Jake Kerby, University of South Dakota). Finally, Andrew Storfer (Washington State University) demonstrated that anthropogenic movement of tiger salamanders have disrupted the natural co-evolutionary dynamics between this host species and endemic ATV strains leading to the evolution of viral virulence and disease emergence. Thus, to better understand the epidemiology of ranaviral diseases and forecast disease outbreaks, more investigation of host–pathogen genotypic interactions and their environmental dependence is required.
(c) Transmission and persistence
In many ways, ranaviruses are well suited to cause mass die-offs and even local extinctions. In addition to often being highly virulent, they are transmitted through several routes: water, brief direct contact and ingestion. Jesse Brunner (Washington State University) demonstrated that transmission among wood frog tadpoles is not density dependent but rather frequency dependent, presumably because tadpoles tend to aggregate. Pathogens transmitted in a frequency-dependent manner can, in theory, cause host extinctions, as Matt Gray (University of Tennessee) noted. However, this likely depends on site characteristics and the composition of the aquatic community. By contrast, transmission in chelonians is much less well understood as Matthew Allender (University of Illinois) explained how red-eared sliders were fairly resistant to ranavirus, and only became infected by intramuscular injection. Allender hypothesized that transmission between chelonians might require a vector, which would be a first for a ranavirus.
Ranavirus persistence in the environment remains poorly understood. It is increasingly clear that ranaviruses can infect a suite of hosts, and so reservoirs species are probably common, and infections do, on occasion, develop into a sublethal, persistent and apparently infectious form. It is worth noting that all of these means of persisting could also contribute to the spread of ranaviruses across the landscape and between distant locations. Moreover, reservoir species, environmental persistence and frequency-dependent transmission all facilitate disease-induced extinctions (Matt Gray, University of Tennessee).