Ranavirus: update from Froglife

Dr Amber Teacher, formerly of the Institute of Zoology (ZSL), has outlined some of the findings from her research into ranavirus, a frog disease that appears in many UK gardens in the summer months.

Amber’s results are thanks largely to the interest and involvement of our supporters in providing data from their back gardens, through our Frog Mortality Project, for over fifteen years…

“Many people will have submitted reports of frogs found dead in your gardens to Froglife. These records have been archived over the years, and have been used for research into a viral disease called Ranavirus.

This disease can cause skin ulceration which can get so bad that the frogs’ limbs degrade. It can also cause haemorrhaging of the internal organs, though this is not visible from the outside. Ranavirus often causes ‘mass mortality events’, where a great number of frogs are found dead over a short time-span.

Studies by Dr Andrew Cunningham, a veterinary researcher based at the Institute of Zoology, have shown that this disease is likely to have been introduced from North America.

Some surprising results have come of this study – it seems that Ranavirus can have a range of possible effects on frogs.

The disease appears to be able to be cleared by some frog populations, to affect other populations on a yearly basis, or to cause local frog populations to die-off. This range of responses implies that some frog populations are better able to cope with the infection than others.

The researchers looked more closely at the populations that are affected by recurrent disease, and found that these frogs seem to be doing something rather strange – healthy frogs seem to be mating with healthy frogs, and diseased frogs seem to be mating with other diseased frogs.

It is generally thought that common frogs mate fairly randomly, and are not choosey about who they mate with. It is likely that the frogs that have been infected are not strong enough to mate with fit, healthy frogs, and so are left to mate with other infected frogs.

The researchers also looked at the genetics of the immune system in these diseased frogs. In particular they studied a gene which is directly linked to the frogs’ ability to cope with viral infections, and they found that this gene is different in frog populations that are affected by disease, compared to those that have never been affected by disease. This is the first evidence that wild common frogs might be adapting to cope with infection, and may indicate that they are capable of becoming immune over time.

However, although these are promising results for the long-term future for this species, the disease remains a major welfare issue which affects a great number of frogs. Also, whilst the frogs may eventually be able to resist infection, the process of specialising their immune systems to deal with Ranavirus may leave them more susceptible to other diseases.

Without the help of Froglife's supporters, this work would not be possible, so we are extremely grateful for your continued interest and support.”

Such collaborative research between Froglife and the Institute of Zoology is set to continue; there is another PhD project currently underway which is looking at which other species can be affected by Ranavirus. New links are also being forged with Imperial College London, where there is expertise on modelling the spread of diseases.

Froglife would like to thank those that contributed to our Frog Disease Appeal in late 2008 – and to Natural England for additional financial support for our data-entry officer, Sophie Webster.

The Frog Mortality Project is a joint project between Froglife and the Institute of Zoology (ZSL).

For more information on the Frog Mortality Project click here.