Ranaviruses are members of the Iridoviridae, a group of large double stranded DNA viruses that affect cold-blooded vertebrates. While ranavirus infections in amphibians have been known since the 1960's, they were not associated with disease and death until the 1980's, when large-scale mortality and disease events were first seen.
Ranaviral disease, infection and mortality have now been reported on every continent (except Africa, and this is likely due to no one looking there yet). Infection by a ranavirus has now been made a "reportable disease" by the OIE (the CDC for animals).
The emergence of ranaviruses has been linked to population declines of the common frog (Rana temporaria) in the UK and some populations of amphibians in the US and Canada. They are suspected to play a role in many other population declines that have been seen in North America and Europe, but research to support this is needed.
Ranaviruses are transmitted between individuals in a variety of ways. Contact between individuals or contaminated water, through ingestion of infected materials (e.g. scavenging or cannibalism) or in some cases from an infected parent to the offspring. The complex nature of the life cycle of amphibians allows for many opportunities for ranavirus transmission and makes the likelihood of eradicating the virus very difficult, since amphibian ranaviruses are infections of entire communities and not just individuals.
Similar to the chytrid fungus, ranaviruses have been spread by the movement of infected animals. The trade in amphibians for pets, food and research is at best loosely regulated. These loose regulations have lead to the spread of diseases and the establishment of feral amphibian populations in Europe, North America and elsewhere.
In North America, ranaviruses have been spread by the use of amphibians as bait. The conditions that these animals are kept and transported in are over-crowded and for the most part unsanitary. These conditions are the perfect breeding ground for bacterial infections and the spread of diseases.
Infection with a ranavirus does not always cause disease and/or death, however, infected animals can still pass the virus to naïve animals. All types of amphibians can be affected by ranavirus infections: urodeles (salamanders and newts), anurans (frogs and toads) and likely caecilians as well.
In North America, most of the information that we have on ranavirus infections, disease and mortality events associated with ranavirus infections come from larvae and tadpoles, though we do know that adults can be infected with the ranavirus and suffer disease/mortality because of it. In some amphibian communities and populations the adults are the key in maintaining the infection between years.
Ranaviral diseases in larvae and tadpoles have very similar signs. Bellies tend to become red with hemorrhages and bloat slightly. The tails tend to become damaged especially at the edges and can also have hemorrhages in them. They become lethargic and lose their appetites, and in some cases spend much more time at the surface. This behavior combined with the loss of some of the predator avoidance responses makes them easy prey.
In the case of adult amphibians that have ranaviral disease, there are some clinical signs that the same for both anurans and urodeles: lethargy, emaciation, reddening of the lower abdomen and lower limbs (although this is a general stress response, so using this alone is not appropriate to diagnose disease), edema (especially of the abdomen), loss of digits (again, depending on where you are this may not be caused by a diseases sometimes frogs loose a finger or toe over the winter if it gets frozen). In salamanders and newts, the tail can also be affected and it can be partially lost (again not always caused by a disease) or it can have hemorrhages on it.
The internal signs of ranaviral disease include hemorrhages of the internal organs, specially the gastro-intestinal tract and reproductive organs. Some animals can appear quite healthy with large fat bodies (energy reserves) yet their internal organs are full of hemorrhages.
There are several ways that ranavirus infections can be identified. However, to establish a link between the presence of an infection and the presence of disease is a process that requires someone who is very familiar with amphibian diseases, such as a veterinary pathologist. To identify what ranavirus is present, a combination of PCR, cell culture and sequencing gives the best results.
This method is not specific for ranaviruses. Using a light microscope, all that can be seen are viral inclusion bodies in the cells. When electron microscopy is used, the shape of the virus can be seen. Therefore, further diagnostic methods are needed to identify the specific virus present.
From the tissues of infected animals, the virus is isolated through cell culture methods. For ranaviruses, usually fat head minnow or EPC cells are used because they are fish cells (ranaviruses also infect fish) and have been used for a long time to grow ranaviruses in the lab. Again here, further diagnostic methods are needed to identify the specific virus that is present. Tissues that are most commonly used to isolate ranavirus infection from an animal are the kidney, liver and spleen.
Negative isolation attempts do occur from infected amphibian tissues, the success of an isolation attempt depends on many factors, including how the sample was treated when it was collected (especially how many times it was frozen and thawed, the more times that this has happened, the less likely you are to get a good result) and the condition of the original sample: rotten tissues do not give the best results.
This method uses extracted DNA from tissues (usually the kidney, liver or spleen). The sample is then screened for the presence of viral DNA using one of several probes (usually for the gene that encodes the capsid protein of the virus, the major capsid protein (MCP)). This test is specific for ranaviruses, but the sensitivity of the test is dependant on many factors. A negative PCR result does not necessarily mean that a ranavirus is not present and care needs to be taken to minimize false negatives. To identify the specific strain of ranavirus that is present, sequencing is used.
This is a rather new technique to identity ranavirus infections in amphibians. The advantage that this method has is that it gives an idea of how much virus is present in a sample.
Unfortunately, there are no hard and fast rules to identify likely places for ranavirus infections. One pond can experience high levels of infection, disease and mortality where the one less than a mile away can be infection free. With increasing research about the prevalence of infectious diseases in amphibian populations, the area in which amphibians are affected by ranaviruses is also increasing.
It looks like most amphibian species are susceptible to ranavirus infections, however not all are equally likely to develop disease. Ranid frogs and Ambystomatid salamanders appear to be most affected by ranavirus infections and disease.
The length of time that a ranavirus has been present in an area is also an important factor in the development of disease. A good example of this is Oliver Pond in Ontario, Canada. When ranavirus outbreaks were first reported there, mass mortality of wood frog (Rana sylvatica) tadpoles (nearing an estimated 100%) were seen. However, over time, mortality decreased and in 2005 there were still infected wood frog tadpoles in the pond but no large-scale mortalities. Although this pond has several other species of amphibians, only the wood frog tadpoles were affected by the mass mortalities. So, even within the same pond the presence of a ranavirus can have very different effects on different species.
In the UK, common frogs (Rana temporaria) have been experiencing long term ranavirus-associated mortality events in some populations for nearly twenty years. It is only recently that investigations have linked the presence of ranaviruses in common frogs and this is because of the detailed long-term population data that was kept.
In most cases, there are not enough data to state with any certainty that ranavirus infections/disease/mortality have been the cause or have contributed to the declines because no one has looked. This is why full pathological investigations into mortality and disease events are important because we need all the information possible to correctly assess the situation.
Some amphibians can carry ranavirus infections and show no signs of disease. North American bullfrogs (Lithobates catesbeianus) and African clawed frogs (Xenopus laevis) are two such species (although ranavirus-associated mortalities have been documented in tadpoles of both) that are thought to have spread the ranavirus on large scales. Animals that show no signs of disease, but are infected with a ranavirus, can still pass the disease on to other animals. In some cases, these animals that are infected by show no signs of disease are the link that keeps the infection in the pond between years.
Amphibian ranaviruses are a serious problem in many amphibian populations and communities. Due to the fact that animals can be infected without showing signs of disease, moving animals between ponds (including eggs) should be avoided. Amphibians should not be used as bait for fishing because of the potential to spread the disease and also because the infection could spread to fish.
Assessing the long-term impact of Ranavirus infection in
wild common frog populations by Teacher et al. 2010
Ecology and Pathology of Ranaviruses by Gray et al. 2009
From chytrid to ranavirus: Another disease is devastating frog populations by John Platt, Scientific American, 10/14/2010
This page was graciously prepared by Dr. Amanda L. J. Duffus.
Dr. Duffus completed her PhD at the Institute of Zoology, Zoological Society of London and the School of Biological and Chemical Sciences, Queen Mary, University of London where she researched many aspects of the ecology and genetics of the ranavirus in the UK’s native amphibians. Prior to that, she completed her MSc at Trent University, Peterborough, ON, Canada where her research investigated ranavirus infections in amphibians at the community level. Her undergraduate work was done at Queen’s Univerisity, Kingston, ON where she completed a specialist degree (BScH, SSP-Biol) with distinction and was involved in aquatic ecology and plant evolutionary genetics research. For inquiries pertaining to amphibian ranaviruses she can be reached at: firstname.lastname@example.org