Ranavirus

Globally, amphibians are currently facing massive declines and extinctions as a results of human activity(Carey et al. 1999; Daszak et al. 1999; Campbell Grant et al. 2020). While there are a number of factors involved, including the classics such as climate change and habitat loss, amphibians have also suffered losses as a result of emerging infectious diseases (Daszak et al. 1999, 2001; Greer and Collins 2008). An emerging infectious disease is defined as “a disease caused by a pathogen that is currently increasing in geographical range, is infecting an increased diversity of hosts, and/or has recently evolved”(Daszak 2000; Duffus et al. 2008). Here, we focus on an EID among amphibians: ranavirus. While ranaviruses have not received nearly as much attention as other amphibian disease (see my discussion of the amphibian chytrid fungus), it is in fact an EID that has been the cause of mass mortality events among amphibians in North America, Europe, South America as well as fish populations in North America, Europe, Asia and Australia(Cunningham et al. 1996; Bollinger et al. 1999; Gray and Chinchar 2015a). Here, I provide a brief outline of ranaviruses by describing their taxonomy, pathology and symptoms, as well as steps that could be taken to prevent or control the spread of ranaviruses.

The family iridoviridae is divided in to 4 genera: Iridovirus, Chloriridovirus, Lymphocycstivirus, and Ranavirus(Bollinger et al. 1999). Iridoviruses and Chloriridovirus only infect invertebrates, while Lymphocyctiviruses only infect fish. Ranaviruses, on the other hand, are known to infect amphibians, fish, and reptiles(Gray and Chinchar 2015a). Within the genus Ranavirus there are currently 6 known species(Gray and Chinchar 2015b). Interestingly, the fact that ranaviruses can infect such a wide variety of hosts has lead researchers to believe that some species could be acting as reservoir species(Duffus et al. 2008; Brunner et al. 2015). Here, a reservoir is defined as “a species that can harbor a pathogen by transmitting it among themselves and others, and is relatively unaffected by the disease”(Brannelly et al. 2018). As iridoviridae is a family of viruses, it is worth noting that all viruses within iridoviridae are large enveloped viruses which contain a linear double-stranded DNA genome (as opposed to RNA viruses)(Bollinger et al. 1999).

Like most viruses, ranavirus works by invading host cells and using host components to replicate. This involves passing through the host cell membrane, using proteins in the cell nucleus to synthesize DNA, and then synthesizing DNA and proteins in the cell’s cytoplasm. This is an incredibly simplified description of what goes on so if you happen to be a cell biologist reading this… sorry. For a more detailed description of how Ranaviruses invade and replicate within cells you can read Jancovich et al. (2015).

Now that we’ve outlined the taxonomy and biology of Ranaviruses, what about the ecology? What kind of world does it live in? Much about the ecology of Ranaviruses is still unclear. That’s why people like me are studying it! There are a lot of factors that could impact the risk of ranavirus within an ecosystem. For example, ranavirus tends to do better in more permanent ponds, as it can’t survive outside of water for longer than 2 weeks (Gray et al. 2009). Additionally…

Ranaviruses can infect amphibians either via direct contact between an infected individual and an uninfected individual, or the virus is able to survive in water and infect amphibians if they happen to swim in infected water. Ranaviruses infect their host by skin to skin contact, mostly between tadpoles (Brunner et al. 2015). Once infection occurs, a variety of symptoms can appear. The most well known and easily observed symptom is the reddening and swelling of the hind legs (Cunningham et al. 1996). Other symptoms can include loose feces, bloody stool, and skin ulcers (Bollinger et al. 1999).

Works cited

Bollinger, T. K., J. Mao, D. Schock, R. M. Brigham, and V. G. Chinchar. 1999. PATHOLOGY, ISOLATION, AND PRELIMINARY MOLECULAR CHARACTERIZATION OF A NOVEL IRIDOVIRUS FROM TIGER SALAMANDERS IN SASKATCHEWAN. Journal of Wildlife Diseases 35:413–429.

Brannelly, L. A., R. J. Webb, D. A. Hunter, N. Clemann, K. Howard, L. F. Skerratt, L. Berger, et al. 2018. Non-declining amphibians can be important reservoir hosts for amphibian chytrid fungus. Animal Conservation 21:91–101.

Brunner, J. L., A. Storfer, M. J. Gray, and J. T. Hoverman. 2015. Ranavirus Ecology and Evolution: from Epidemiology to Extinction. Pages 71–104 inRanaviruses: :Lethal Pathogens of Ectothermic Vertebrates.

Campbell Grant, E. H., D. A. W. Miller, and E. Muths. 2020. A Synthesis of Evidence of Drivers of Amphibian Declines. Herpetologica 76:101.

Carey, C., N. Cohen, and L. Rollins-Smith. 1999. Amphibian declines: An immunological perspective. Developmental and Comparative Immunology 23:459–472.

Cunningham, A. A., T. E. S. Langton, P. M. Bennett, J. F. Lewin, S. E. N. Drury, R. E. Gough, and S. K. Macgregor. 1996. Pathological and microbiological findings from incidents of unusual mortality of the common frog ( Rana temporaria ). Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences 351:1539–1557.

Daszak, P. 2000. Emerging Infectious Diseases of Wildlife– Threats to Biodiversity and Human Health. Science 287:443–449.

Daszak, P., L. Berger, A. A. Cunningham, A. D. Hyatt, D. E. Green, and R. Speare. 1999. Emerging infectious diseases and amphibian population declines. Emerging Infectious Diseases 5:735–748.

Daszak, P., A. A. Cunningham, and A. D. Hyatt. 2001. Anthropogenic environmental change and the emergence of infectious diseases in wildlife. Acta Tropica 78:103–116.

Duffus, A. L. J., B. D. Pauli, K. Wozney, C. R. Brunetti, and M. Berrill. 2008. Frog virus 3-like infections in aquatic amphibian communities. Journal of Wildlife Diseases 44:109–120.

Gray, M. J., and V. G. Chinchar. 2015a. Introduction: History and Future of Ranaviruses. Pages 1–6 in M. J. Gray and V. G. Chinchar, eds. Ranaviruses: Lethal Pathogens of Ectothermic Vertebrates.

———, eds. 2015b. Ranaviruses: Lethal Pathogens of Ectothermic Vertebrates. Springer International Publishing, Cham.

Gray, M. J., D. L. Miller, and J. T. Hoverman. 2009. Ecology and pathology of amphibian ranaviruses. Diseases of Aquatic Organisms 87:243–266.

Greer, A. L., and J. P. Collins. 2008. Habitat fragmentation as a result of biotic and abiotic factors controls pathogen transmission throughout a host population. Journal of Animal Ecology 77:364–369.

Jancovich, J. K., Q. Qin, Q.-Y. Zhang, and V. G. Chinchar. 2015. Ranavirus Replication: Molecular, Cellular, and Immunological Events. Pages 105–140 in M. J. Gray and V. G. Chinchar, eds. Ranaviruses: Lethal Pathogens of Ectothermic Vertebrates. Springer International Publishing, Cham.