The Northern hemisphere is currently in the midst of spring, the season I have associated with frogspawn and tadpoles since I was a very young child. These days, I still experience child-like excitement when spotting amphibians in the wild, but there are ongoing challenges which must be overcome if amphibians are to thrive.
There are several factors threatening the existence of amphibians including pollution, loss of habitat, international trade in amphibian species, climate change as well as infectious diseases. Ranaviruses are some of the infectious pathogens contributing to dramatic loss of biodiversity of amphibian species.
Ranaviruses are emerging pathogens which have caused significant losses to amphibian species worldwide. Ranavirus is of such a concern that the World Organisation for Animal Health (OIE) have placed it on a list of notifiable pathogens. (Schloegel et al., 2010) This article will concentrate on the situation in the UK.
The UK boasts only two native frog species (common frog and pool frog), two native toad species (common toad and natterjack toad), and three newt species (Palmate, Smooth and Great Crested). Threats to any UK amphibian could be devastating for native wildlife and pond ecosystems. Currently ranavirus has caused the most devastating effects among the UK’s common frog populations (rana temporaria), and has been linked to several mass mortality events in this species.
Ranavirus behaves slightly differently in the UK than in some of the other geographic locations in which it has been studied. Ranavirus usually affects adult frogs in the UK, while in the rest of the world the majority of the effects are observed in the larval forms (tadpoles).
In infected individuals, the symptoms are varied. The virus frequently causes ulceration, either of the skin or internally. This may cause obvious wounds on the skin, or even missing digits, which can become secondarily infected by other pathogens. Cutaneous erythema (redness) may be visible, with swollen limbs, unusual swimming patterns and lethargy also observed. On post-mortem, haemorrhages are often discovered within the internal organs. The clinical signs are therefore non-specific and the overall appearance of infected frogs may be augmented by concurrent pathogens or pathologies of other origin.
Ranavirus first was observed in the UK in the1990s after several mass die off events in England. (Cunningham et al 1996) Continued study of those outbreaks and tissues archived at that time has revealed two ranavirus lineages detected in UK amphibians. These are termed CMTV-like (common midwife toad virus-like) and FV3-like (frog virus 3-like). The genetic data has shown that it is likely that FV3 in particular has been introduced to the UK at several times and different locations, although the exact routes and methods are unknown. (Price et al., 2017)
More work is required to fully elucidate the epidemiology of the viruses to clarify the host range and indicate any as yet unidentified species which may be involved in virus transmission. Ranaviruses are not species specific and have been detected in a variety of different species worldwide including fish, reptiles and various amphibian species. In the UK, ranavirus has also been detected in common toads and newts.
Although it may be possible to find an isolated affected frog, the primary indication of Ranavirus infection is mass die-offs in a population of frogs. Numerous individuals may be affected and perish within a short period of time. In the UK infections and mass die-off events were first reported in the late summer months, in South-East England. Since then positive virus samples have been identified in the West and North of England as well as in Wales and Scotland.
Three main outcomes for a population follow from infection. Firstly a transient pattern of infection may be observed, where after a period of pathology and mortality, there are no subsequent deaths. Secondly, catastrophic die-offs have been observed in some habitats where ranavirus kills all the frogs in that location. Finally, a persistent infection pattern has been noted. An initial infection period causes morbidity and mortalities in a population then among the remaining frogs, further mortalities are observed in subsequent years. These outcomes are without exception catastrophic for frog populations, which may not recover. (Teacher et al., 2010)
There is some evidence however that the frogs may mount adaptive immune responses to ranavirus. Behavioural adaptations have also been observed, where it was shown to be more likely that uninfected frogs would mate with each other. (Price et al., 2015; Teacher et al., 2009) So although much more needs to be known about ranaviruses to protect UK amphibians, and potentially other species too, there does appear to be indications that wild frogs are adapting to the presence of ranaviruses.
Everyone can play a role in preventing the spread of infectious amphibian diseases. If you spot healthy, normal amphibians sightings can be reported using the dragon finder app, which is part of the frog life group, which is focussed on conservation and education. Your sightings can inform future conservation efforts and research. Diseased amphibians or mass die off events should also be reported, this time via the Garden Wildlife Heath project. Finally, remember not to move any amphibians or spawn between ponds, to prevent spread of infection.
Cunningham AA, Langton TES, Bennett PM, Lewin JF, Drury SEN, Gough RE, et al. (1996) Pathological and microbiological findings from incidents of unusual mortality of the common frog (Rana temporaria). Philos Trans R Soc Lond Ser B-Biol Sci. 351: 1539–1557. pmid:8962441
Price, S. J., Garner, T. W. J., Balloux, F., Ruis, C., Paszkiewicz, K. H., Moore, K., & Griffiths, A. G. F. (2015). A de novo Assembly of the Common Frog (Rana temporaria) Transcriptome and Comparison of Transcription Following Exposure to Ranavirus and Batrachochytrium dendrobatidis. PLoS ONE, 10(6), e0130500. http://doi.org/10.1371/journal.pone.0130500
Price SJ, Wadia A, Wright ON, Leung WTM, Cunningham AA, Lawson B (2017) Screening of a long-term sample set reveals two Ranavirus lineages in British herpetofauna. PLoS ONE 12(9): e0184768. https://doi.org/10.1371/journal.pone.0184768
Schloegel LM, Daszak P, Cunningham AA, Speare R, Hill B. (2010) Two amphibian diseases, chytridiomycosis and ranaviral disease, are now globally notifiable to the World Organization for Animal Health (OIE): an assessment. Dis Aquat Organ. 92: 101–108. pmid:21268971
Teacher, A. G. F., Garner, T. W. J., & Nichols, R. A. (2009). Evidence for Directional Selection at a Novel Major Histocompatibility Class I Marker in Wild Common Frogs (Rana temporaria) Exposed to a Viral Pathogen (Ranavirus). PLoS ONE, 4(2), e4616. http://doi.org/10.1371/journal.pone.0004616
Teacher, A.G.F., Cunningham, A.A. and Garner, T.W.J. (2010). Assessing the long-term impact of Ranavirus infection in wild common frog populations. Animal Conservation 13, 514-522.