A new research paper was published in June by researchers at Exeter University analysing the data collected by Froglife in collaboration with the Institute of Zoology as part of the Frog Mortality Project. The study, called Anthropogenic and Ecological Drivers of Amphibian Disease was published in PLOS ONE and is open access. You can read the full study here: http://dx.plos.org/10.1371/journal.pone.0127037
This new paper is important for several reasons. We know from previous studies (Teacher et al. 2010) based from data collected as part of the same Froglife project, that common frog populations, particularly in the SE of England, have been strongly impacted by ranavirus. Ranaviruses are group of pathogens introduced in Europe from abroad most likely via the exotic pet trade, with goldfish or bullfrog transports. The disease they cause in amphibians, sometimes called red-leg disease, typically causes ulcerations on the skin, loss of limbs and death and common frogs are often killed in large numbers by it, both in the pond and in the garden, making this a heart-breaking event for people across the UK as they have to collect the dead frog bodies and burry them. As a consequence populations can entirely crash or they take a very long time to recover. In the study by Teacher et al. in 2010 frog numbers had declined 81% from 1996 to 2008 at sites where ranavirosis was causing recurring mortality events.
The new study from 2015 looks at associations between the severity of the ranavirus incidence and the presence of other species in the pond or differences in pond management. Understanding what makes the disease more severe could obviously help us prevent the worse kind of impacts on amphibian populations and hopefully contain the disease.
Interestingly, the authors have found that the severity of ranavirosis increases in the presence of exotic fish, typically goldfish. The use of garden chemicals was also associated with increased severity of the disease, so reducing or eliminating applications of slug pellets or herbicides in the garden, something that Froglife has been promoting for a very long time, now has even more relevance. Although the exact mechanisms through which fish make the disease outbreaks worse are unclear it is possible that fish amplify viral levels of the pathogen in the pond water or that their presence cause stress hormone production that reduces immune function in frogs. We know for a long time that amphibians develop differently in the presence of fish, responding to chemical cues in the water but the fact that fish presence is linked with disease incidents in an important find.
What you can do to help
Manage your garden and your garden pond in a wildlife friendly way, limit or eliminate the use of chemicals, use plenty of native species of plants and do not introduce fish into your pond. Also, do not move frogspawn or tadpoles between ponds as that can help spread diseases very quickly. If you suspect a case of ranavirus in a frog that you have found, please report it through the Garden Wildlife Health project.