Transmission electron micrograph of a cell infected with ranaviruses, which gather in the cytoplasm and in the assembly bodies next to the contorted nucleus.
The Ranaviruses, like the Megalocytiviruses, are an emerging group of closely related dsDNA viruses which cause systemic infections in a wide variety of wild and cultured fresh and saltwater fishes. As with Megalocytiviruses, Ranavirus outbreaks are therefore of considerable economic importance in aquaculture, as epizootics can result in moderate fish loss or mass mortality events of cultured fishes. Unlike Megalocytiviruses, however, Ranavirus infections in amphibians have been implicated as a contributing factor in the global decline of amphibian populations.[3][4] The impact of Ranaviruses on amphibian populations has been compared to the chytridfungusBatrachochytrium dendrobatidis, the causative agent of chytridiomycosis.[5][6][7] In the UK, the severity of disease outbreaks is thought to have increased due to climate change.[8]
Etymology
Rana is derived from the Latin for "frog",[9] reflecting the first isolation of a Ranavirus in 1960s from the Northern leopard frog (Lithobates pipiens).[10][11][12]
Ranaviruses are large icosahedral DNA viruses measuring approximately 150 nm in diameter with a large single linear dsDNA genome of roughly 105 kbp[26] which codes for around 100 gene products.[27] The main structural component of the proteincapsid is the major capsid protein (MCP).
Genus
Structure
Symmetry
Capsid
Genomic arrangement
Genomic segmentation
Ranavirus
Polyhedral
T=133 or 147
Linear
Monopartite
Replication
Ranaviral replication is well studied using Frog virus 3 (FV3).[25][26] Replication of FV3 occurs between 12 and 32 degrees Celsius.[27] Ranaviruses enter the host cell by receptor-mediated endocytosis.[28] Viral particles are uncoated and subsequently move into the cell nucleus, where viral DNA replication begins via a virally encoded DNA polymerase.[29] Viral DNA then abandons the cell nucleus and begins the second stage of DNA replication in the cytoplasm, ultimately forming DNA concatemers.[29] The viral DNA is then packaged via a headful mechanism into infectious virions.[25] The ranavirus genome, like other iridoviral genomes is circularly permuted and exhibits terminally redundant DNA.[29]
There is evidence that ranavirus infections target macrophages as a mechanism for gaining entry to cells.
[30]
Transmission of ranaviruses is thought to occur by multiple routes, including contaminated soil, direct contact, waterborne exposure, and ingestion of infected tissues during predation, necrophagy or cannibalism.[11][32]
Ranaviruses are relatively stable in aquatic environments, persisting several weeks or longer outside a host organism.[11]
Epizoology
Amphibian mass mortality events due to Ranavirus have been reported in Asia, Europe, North America, and South America.[11] Ranaviruses have been isolated from wild populations of amphibians in Australia, but have not been associated with mass mortality on that continent.[11][33][34]
Pathogenesis
Synthesis of viral proteins begins within hours of viral entry[27] with necrosis or apoptosis occurring as early as a few hours post infection.[26][35]
Seasonal disease dynamics
There are several hypotheses for seasonal outbreak patterns observed for Ranavirosis mortality events.[36] Ranaviruses grow in vitro between 8-30 °C, however for most isolates, warmer temperature result in faster viral replication.[36] A combination of this optimal growth temperature along with shifts in larval amphibian susceptibility result in seasonal outbreak events most often observed during warm summer months.[37] Amphibian mortality events are often observed as larval amphibians reach late Gosner stages approaching metamorphosis.[38] As larval amphibians reach metamorphic stages of development, their immune system is reorganized prior to the development of adult tissues.[39] During this time period, amphibians are stressed, and their immune systems are down regulated. This decrease in immune function and warmer environmental temperatures allows for greater viral replication and cellular damage to occur. Across 64 mortality events in the United States 54% were found to occur between June-August.[37]
Environmental persistence
The environmental persistence of Ranaviruses is not understood well, however in realistic environmental conditions the T90 value of an FV3-like virus is 1 day.[40] The duration of persistence is likely affected by temperature and microbial conditions. It is unlikely that ranaviruses persist in the environment outside of host species between outbreak events.
Researchers have explored several pathogen reservoirs for the virus which might explain how the virus can persist within an amphibian community. In some amphibian populations, sub-clinically infected individuals may serve as reservoirs for the pathogen.[6] These sub-clinically infected individuals are responsible for reintroduction of the virus to the larval population. With ranaviruses being capable of infected multiple taxa, and with there being differences in susceptibility between taxa, it is likely that sympatric fish and reptile species may serve as reservoirs for virus as well. Interclass transmission has been proven through the use of mesocosm studies.[41]
Gross pathology
Gross lesions associated with Ranavirus infection include erythema, generalized swelling, hemorrhage, limb swelling, and swollen and friable livers.[11]
^Teacher, A. G. F.; Cunningham, A. A.; Garner, T. W. J. (10 June 2010). "Assessing the long-term impact of Ranavirus infection in wild common frog populations: Impact of Ranavirus on wild frog populations". Animal Conservation. 13 (5): 514–522. doi:10.1111/j.1469-1795.2010.00373.x. S2CID85889833.
^ abBrunner, Jesse L.; Schock, Danna M.; Davidson, Elizabeth W.; Collins, James P. (2004). "Intraspecific Reservoirs: Complex Life History and the Persistence of a Lethal Ranavirus". Ecology. 85 (2): 560. Bibcode:2004Ecol...85..560B. doi:10.1890/02-0374.
^Pearman, Peter B.; Garner, Trenton W. J. (2005). "Susceptibility of Italian agile frog populations to an emerging strain of Ranavirus parallels population genetic diversity". Ecology Letters. 8 (4): 401. Bibcode:2005EcolL...8..401P. doi:10.1111/j.1461-0248.2005.00735.x.
^Price, Stephen J.; Leung, William T. M.; Owen, Christopher J.; Puschendorf, Robert; Sergeant, Chris; Cunningham, Andrew A.; Balloux, Francois; Garner, Trenton W. J.; Nichols, Richard A. (9 May 2019). "Effects of historic and projected climate change on the range and impacts of an emerging wildlife disease". Global Change Biology. 25 (8): 2648–60. Bibcode:2019GCBio..25.2648P. doi:10.1111/gcb.14651. hdl:10026.1/13802. ISSN1354-1013. PMID31074105. S2CID149444899.
^De Matos, A. P.; Caeiro, M. F.; Papp, T; Matos, B. A.; Correia, A. C.; Marschang, R. E. (2011). "New viruses from Lacerta monticola (Serra da Estrela, Portugal): Further evidence for a new group of nucleo-cytoplasmic large deoxyriboviruses (NCLDVs)". Microscopy and Microanalysis. 17 (1): 101–8. Bibcode:2011MiMic..17..101A. doi:10.1017/S143192761009433X. PMID21138619. S2CID21932480.
^Blahak S., Uhlenbrok C. "Ranavirus infections in European terrestrial tortoises in Germany". Proceedings of the 1st International Conference on Reptile and Amphibian Medicine; Munich, Germany. 4–7 March 2010; pp. 17–23
^Chen, Z. X.; Zheng, J. C.; Jiang, Y. L. (1999). "A new iridovirus isolated from soft-shelled turtle". Virus Research. 63 (1–2): 147–51. doi:10.1016/S0168-1702(99)00069-6. PMID10509727.
^Goodman, R.; Hargadon, K; Carter, E. (2018). "Detection of Ranavirus in Eastern Fence Lizards and Eastern Box Turtles in Central Virginia". Northeastern Naturalist. 25 (3): 391–8. doi:10.1656/045.025.0306. S2CID91510246.
^ abcChinchar VG, Essbauer S, He JG, Hyatt A, Miyazaki T, Seligy V, Williams T (2005). "Family Iridoviridae". In Fauquet CM, Mayo MA, Maniloff J, Desselburger U, Ball LA (eds.). Virus Taxonomy, Eighth report of the International Committee on Taxonomy of Viruses. Academic Press. pp. 145–162. ISBN978-0-12-249951-7. OCLC937237481.
^ abGreen, D E; Converse, K A; Schrader, A K (2002). "Epizootiology of sixty-four amphibian morbidity and mortality events in the USA, 1996–2001". Domestic Animal/Wildlife Interface: Issues for Disease Control, Conservation, Sustainable Food Production, and Emerging Diseases. 969 (1): 323–339. Bibcode:2002NYASA.969..323G. doi:10.1111/j.1749-6632.2002.tb04400.x. PMID12381613. S2CID33944909.
^Green, D E; Converse, K A (2005). "Diseases of frogs and toads". Wildlife Diseases: Landscape Epidemiology, Spatial Distribution, and Utilization of Remote Sensing Technology.: 89–117.
More information on Ranavirus and other pathogens impacting amphibian populations, including Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans can be found at the Southeast Partners in Amphibian and Reptile Conservation disease task team web-page. [1]
