Ranavirus is a genus of viruses from the family Iridoviridae with a worldwide distribution. At least 91 amphibian species of 14 families from North and South America, Europe, Asia, Australia and Africa are affected. The ranaviruses are the second most common infectious cause of death in amphibians after the chytrid fungus Bd. According to ICTV, as of March 2021, there are seven listed virus types (ATV, CMTV, EHNV, ENARV, FV3, SCRV, SGIV). Three of them mainly affect amphibians (FV3, CMTV, ATV) and four fishes (EHNV, ENARV , SCRV, SGIV). Reptiles can also be affected. In addition to the virus types mentioned, there are numerous other candidates that should still be listed. Classifying viruses as strains or species is not always easy. In addition, the viruses can evolve quickly and take on new properties. A recombination of two or even more viruses to form a new virus is also possible. It is assumed that all viruses of the genus Ranavirus arose from a fish virus (Jancovich et al. 2010 [1]). Infections with ranaviruses often cause extensive damage to several organs, especially the liver of amphibians, fish and reptiles (Miller et al. 2015 [2]). Theoretically, the ranaviruses can also infect mammals, but this only works if the temperature is below 32°C (Gray et al 2015 [3]). Ranavirus epidemics in amphibians and fish typically begin in mid- to late summer, while outbreaks in reptiles occur irregularly. Of the three ranavirus species (FV3, CMTV and ATV) that are known to infect amphibians, FV3 or FV3-like viruses and CMTV or CMTV-like viruses have been detected in Europe. ATV is currently only known to be associated with infections of salamanders in the western United States.

Frog Virus 3 (FV3)

FV3 was the first ranavirus to be discovered. It was discovered in leopard frogs (Lithobates pipiens), which were studied for kidney tumors (Granoff et al. 1966 [4]). The tumors in the leopard frogs were not caused by FV3 but by a herpes virus (RaHV1). In addition to FV3, another herpes virus, RaHV2 (also called FV4), was also discovered in leopard frogs. We have cancer research to thank for the early discovery of these three viruses. FV3 outbreaks have occurred around the world, including North America, South America (Brazil, Uruguay, Venezuela), Europe (Croatia, Denmark, Germany, Netherlands, Spain, United Kingdom, Switzerland), Asia (China, Japan), and Australia.

Local declines of common frogs in the UK
In Europe, FV3 first became noticeable in 1985 with the local common frog die-off (Rana temporaria) in Great Britain, particularly in the south and east of England (Cunningham et al. 1996 [5]). Since then, mortality events in common frogs caused by FV3 have increased significantly in the UK (compare Figure 1). It is thought that FV3 arrived in the UK from North America in the 1980s. Long-term declines in the grass frog population of up to 80% were found at the outbreak sites in England (Teacher et al. 2010 [6]). In contrast to North America, where affected species such as the ice frog (Lithobates sylvaticus) and other species often die as tadpoles (Gray et al. 2009 [7]), dead common frogs (Rana temporaria) are observed mainly as adults during the summer months (Cunningham 2001 [8]). The common frogs show symptoms such as bleeding or ulcers (compare Figure 2). In the laboratory, tadpoles of common frogs and common toads were exposed to FV3 for 30 days. Very high doses of FV3 resulted in a high mortality rate in the common frog tadpoles, the tadpoles of the Common toads, on the other hand, were a lot more resistant (Duffus et al. 2014 [9]).

Figure 1: Ranavirus mortality events in the UK from 1992-2010
© Price et al. 2016

Figure 2: Common frog (Rana temporaria) affected by Ranavirus with skin ulcers and loss of toes.
© Zoological Society of London

The majority of healthy common frogs show little or no symptoms
FV3 infection causes death in tadpoles and stressed adults, but often causes only invisible subclinical infections in healthy adult common frogs and resolves within two weeks. It is likely that environmental stress leading to immunosuppression increases the pathogenicity of ranavirus infections.

Other species affected
In addition to grass frogs, mortality associated with FV3 was also observed in other European amphibians such as common toads (Bufo bufo), midwife toads (Alytes obstetricans), pond frogs (Pelophylax esculenta), sea frog (Pelophylax ridibunda), pond newt Lissotriton vulgaris) and mountain newt (Ichthyosaura alpestris). become (Miller et al. 2011 [10]). It can be assumed that other amphibian species in Europe may also be carriers of FV3. However, a comparable mass extinction to that of the grass frogs in Great Britain has not yet been observed in the other European species. Furthermore, the mechanism of FV3 is still unknown. It is not clear why many species are less affected or less affected. more resistant and asymptomatic carriers are.

Mass deaths of common toad tadpoles and metamorphs due to a combination of pathogens?
In the European part of Russia, dead tadpoles and metamorphos of common toads have been repeatedly observed in a pond in Moscow Oblast over the years. In 2011, an investigation revealed FV3 or a virus similar to FV3 and the chytrid fungus Bd (Reshetnikov et al. 2014 [11]). It is the first documentation of these two pathogens in Russia’s natural environment. However, it is assumed that other factors (e.g. toxic algal blooms, oxygen levels, etc.) or other pathogens are responsible for the fatal events. This unsolved case shows how complex the biology of pathogens can be.

List of sources

[1] J. K. Jancovich, M. Bremont, J. W. Touchman, B. L. Jacobs, 2010: Evidence for Multiple Recent Host Species Shifts among the Ranaviruses (Family Iridoviridae). J Virol. 2010 Mar; 84(6): 2636–2647 link

[2] D. Miller, A. Pessier, P. Hick, R. Whittington, 2015: Comparative pathology of ranaviruses and diagnostic techniques. In: Gray MJ, Chinchar VG, editors. Ranaviruses. Springer International Publishing; Cham: 2015. pp. 171–208 link

[3] M. J. Gray, G. V. Chinchar, 2015: Ranaviruses: lethal pathogens of ectothermic vertebrates. Cham: Springer International Publishing; 2015 link

[4] A. Granoff, P. E. Came, K. A. Rafferty, 1965: The isolation and properties of viruses from Rana pipiens: their possible relationship to the renal adenocarcinoma of the leopard frog. Ann N Y Acad, Sci 126:237—255 link

[5] A. A. Cunningham, T. E. S. Langton, P. M. Bennett, J. F. Lewin, S. E. N. Drury, R. E. Gough & S. K. MacGregor, 1996: Pathological and microbiological findings from incidents of unusual mortality of the common frog (Rana temporaria). Phil. Trans. Roy. Soc. B 351, 1539-1557 link

[6] A. G. F. Teacher, A. A. Cunningham, T. W. J. Garner, 2010: Assessing the long‐term impact of ranavirus
infection in wild common frog populations.
Anim Conserv 13:514–522 link

[7] M. J. Gray, D. L. Miller, J. T. Hoverman, 2009: Ecology and pathology of amphibian
Dis. Aquat. Org. 87, 243-266 link

[8] A. A. Cunningham, 2001: Investigations into mass mortalities of the common frog (Rana temporaria) in Britain: epidemiology and aetiology. Royal Veterinary College (University of London) (cited 9 May 2013) link

[9] A. L. J. Duffus, R. A. Nichols, T. W. J. Garner, 2014: Experimental evidence in support of single host maintenance of a multihost pathogen. Ecosphere 5(11):142 link

[10] D. Miller, M. Gray, A. Storfer, 2011: Ecopathology of ranaviruses infecting amphibians. Viruses. 2011 Nov;3(11):2351-73 link

[11] A. N. Reshetnikov, T. Chestnut, J. L. Brunner, K. Charles, E. E. Nebergall, D. H. Olson, 2014: Detection of the emerging amphibian pathogens Batrachochytrium dendrobatidis and ranavirus in Russia. Dis. Aquat. Org. 110, 235–240 link