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Study Points to Related, Multi-Host Viral Pathogens in Spanish Amphibian Declines

NEW YORK (GenomeWeb) – A new study suggests related, but previously undescribed, Ranavirus species are wreaking havoc on a range of amphibian hosts in Spain.

As they reported online in Current Biology this week, researchers from the UK and Spain relied on a combination of PCR-based testing, targeted sequencing, and comparative genomics to track down two extensively virulent Ranaviruses that appear to have emerged in Spain's Picos de Europa National Park.

The team's analysis of samples collected within the past decade indicated that these viruses have contributed to the collapse of the common midwife toad, common toad, and alpine newt communities in that area. Another three amphibian species have shown population declines, apparently due to the viruses.

"The capacity of these viruses to infect multiple species means that there is the possibility that some host populations may be extirpated due to infection," the study's corresponding author Stephen Price said in a statement.

"Pathogens that can exploit more than one host simultaneously are able to persist even when one host drops to low numbers, and eventually zero, because there is another susceptible host available," added Price, who was affiliated with Queen Mary University of London and the Zoological Society of London when the study was done. He is currently based at the University College London's UCL Genetics Institute.

Price and his colleagues saw signs of a third related ranavirus in northern Spain that so far seems to be causing lower amphibian lethality due to its more restricted host species range.

In around 2005, members of the team started to see signs of amphibian loss at locations within Picos de Europa, which led to more routine testing of amphibian populations in the park, the researchers explained. Suspected ranavirus infections detected at the time seemed to be causing mass amphibian deaths, with affected animals suffering from open sores, systemic hemorrhaging, and limb tissue death.

The Ranavirus genus, part of the Iridoviridae family, is known for containing large, double-stranded DNA viruses that sometimes affect fish and reptile species, but are prone to particularly dangerous infections in amphibians.

A clade of so-called FV3-like ranaviruses have been linked to declines in common frog populations in the UK and Southeast Asia, for instance, though that virus appears to have a limited host range.

In contrast, authors of the new analysis cautioned that the newly detected ranaviruses seem to infect amphibians more indiscriminately, contributing to declines in all six documented amphibian species found in Picos de Europa National Park.

Over the past few years, the group has detected similar infections at a site new outside the park called Galicia, where two amphibian species and one reptile species seem to be succumbing to dangerous ranavirus infections.

In an effort to better understand the causative agent(s) behind the widespread amphibian die-offs in northern Spain, the researchers used PCR-based molecular testing to search for ranavirus-like sequences in samples collected from amphibians at 15 Picos de Europa park sites between 2005 and 2012.

After verifying the presence of ranaviruses in these samples and ruling out widespread infections by other potential culprits, the team used targeted sequencing to further characterize the viruses — amplifying and Sanger sequencing half a dozen ranavirus loci in samples from three amphibian species in the park and three amphibian or reptile hosts at the Galicia site.

By comparing these with whole-genome sequences from Ranavirus species sequenced in the past, the investigators determined that amphibians within the park are succumbing to ranaviruses from the common midwife toad virus, or CMTV, group.

Meanwhile, amphibians and reptiles from the Galicia site, more than 100 miles west of the park, appear to be infected by a very closely related virus known as Bosca's newt virus, or BNV, a member of a different branch within the CMTV-like ranavirus clade.

The analysis pointed to the presence of yet another ranavirus found only at a site called Ándara Lake. That ranavirus representative, dubbed Ándaran Alytes obstetricans virus (AAOV), belongs to the same FV3-like clade as the UK common frog-killing virus.

While it can infect and kill the common midwife toad, the researchers noted that the latter AAOV virus appears to be somewhat less limited in its infectious range, causing deaths in common midwife toads, but so far not leading to amphibian losses en masse.

None of the newly described viruses turned up in amphibians tested at unaffected sites within the Spanish park, the study's authors noted, though they cautioned that species diversity remains very low in locales already affected.

"One of the main threats to biodiversity is the emergence of infectious diseases that can impact dramatically on entire communities," Price said in a statement.

"We've identified a striking example of two viruses that are repeatedly overcoming the species barrier with catastrophic consequences," he continued. "This presents us with a great opportunity to understand more about the ecology and epidemiology of important multi-host pathogens."

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