slow-worm

What we do

Disease: Ranavirus

In the late 1980s a number of unusual Common Frog mortalities were reported to Froglife in the southeast of England. Frogs were found to be suffering from a variety of symptoms, sometimes with secondary bacterial infections. It was found the frogs were suffering from a disease called ranavirus.

After a dramatic increase in cases in the southeast throughout the late 1980s, the Frog Mortality Project was set up to monitor and report on the extent of this disease in the UK.   The project is a joint partnership between the Institute of Zoology (IoZ) at the Zoological Society of London and Froglife. The project has received thousands of disease reports and has collected information on the deaths of over 85,000 frogs. This database – the largest of its kind in the world – is currently the subject of a PhD study. In 2007, Froglife began assisting the Rana Project, a European research project to monitor the spread of ranavirus working with the IoZ and the Centre for Environment, Fisheries and Aquaculture Science (CEFAS).

This work has evolved into the Garden Wildlife Health (GWH) project, a partnership between IoZ, the Royal Society for the Protection of Birds (RSPB) and the British Trust for Ornithology (BTO).  Find out more about the GWH project here.

What you can do

If you suspect a case of ranavirus in a frog that you have found, please report it through the Garden Wildlife Health project.

We have summarised what to look out for with this disease below.  More detailed information is available about ranavirus at the Garden Wildlife Health website.

Ranavirus in a nutshell:

Symptoms

Ranavirus can be tricky to diagnose without a post mortem, but animals may display one or more of the following symptoms:

  • Drowsiness (lethargy)
  • Abnormal wasting (emaciation)
  • Redness of the skin (erythema)
  • Skin ulcers or sores
  • Bleeding (systemic haemorrhaging), especially from mouth/anus
  • Breakdown of limbs (limb necrosis)
  • Eye problems

The most common symptom is simply to find a large number of dead frogs in a short space of time. The animals are often thin and lethargic before death.

Cures or treatment

Recent research on ranavirus seems to suggest that affected populations do recover in the years after the initial outbreak, as it’s likely some individuals will be resistant to the disease.

Research also suggests that local populations can make a good recovery, so it’s a case of waiting for the disease to pass through and hoping it leaves some survivors who are resistant. Despite years of research, there is no known cure or treatment for this disease, nor is one likely to be found in the near future.

Unfortunately there is nothing you can do to help any remaining frogs.

When you are disposing of the bodies it is important that you do so responsibly by either burning or burying them. Do not place them in the rubbish as this could help to spread the disease. Do not move your frogs, spawn or pond plants to other ponds as this could potentially spread the disease.

You may decide to clear out your pond after an infection but there are no guarantees this will help.

Some people may be tempted to try and put suffering frogs ‘out of their misery’ but this is not really advisable as it’s always possible they could be suffering from another, non-lethal, disease. There is also the possibility that the frogs could develop some kind of immunity to ranavirus. However, for advice on euthanasia please contact your local vet or wildlife hospital using Find a Vet to find your local surgery.

What will happen to the frog population?

Some populations recover without further mortalities, others suffer recurrent mortalities, whilst some were completely eliminated. At ponds where there were recurrent mortalities, the overall size of the frog populations decreases. In addition, analysis of the frogs’ immune systems revealed that frogs might be adapting to ranavirus infection with ‘resistant’ frogs becoming more widespread in infected ponds. Frogs may also adjust their mating behaviour by either choosing to mate with other healthy frogs or through an inability of sick frogs to effectively compete for mates.

Although tadpoles are often badly affected by ranaviral disease elsewhere in the world, and British tadpoles are susceptible to ranavirus infection in the lab, spawn and tadpoles from British ponds may be infected only rarely. Rather, infections here are among adult frogs and computer modelling has shown that disease could be maintained in individual ponds with infections occurring in adult common frogs alone. However, the virus is now known to infect toads and newts as well as at least one introduced species so the possibility that other species are involved in the persistence and spread of disease remains.

Where did this disease come from?

Isolation of the virus from a number of garden ponds enabled a comparison with other ranaviruses from around the world and – based on similarities between the viruses – a likely introduction to Britain from North America was suggested.

How this introduction occurred is still not known but suspicion lingers around the involvement of imported amphibian species (e.g. Bullfrogs) as well as goldfish (since some ranaviruses infect fish).

Originally the disease was found in southeast England but since then it has spread across the UK. It’s been recorded in Cornwall, Cardiff, Lancashire and Newcastle. There are often ‘hot-spots’ of disease – clusters of incidents reported in one area – but it’s unclear how it jumps from one location to another.

It is still largely unknown how the disease spreads between ponds/frog populations but we do know that it is more likely to spread in certain conditions. Ranavirus seems to be temperature and density dependant – this is why there is a peak during the summer months of July and August when air temperatures are high and when there are large numbers of frogs in and around ponds.

As it is not clear how the disease spreads we advise against introducing plants, spawn or animals from other ponds and also donating these from your own pond. Introducing fish should also be avoided – as well as being detrimental to your amphibian population they could be a potential source of disease.

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