A virus that has slipped into several European countries is alarming herpetologists, as it ravages amphibians. A type of ranavirus (RV) is being blamed for gruesome deaths and declining populations of a wide range of species in the Picos de Europa National Park in northern Spain, according to research published online today in Current Biology. “This is the best example to date of RV being a serious threat to amphibian populations,” says Karen Lips of the University of Maryland, College Park, who was not involved in the research.
The virus adds to the woes of the world’s amphibians, which have been declining at a worrying rate. A major culprit, the chytrid fungus Batrachochytrium dendrobatidis has afflicted a wide range of species since it was discovered in 1998. In particular, it has apparently driven many species of frogs extinct in the tropics. The new RV, in contrast, seems to be a problem for temperate species.
Unusual amphibian deaths in the Spanish park were first noticed in 2005. With help from Jaime Bosch of the National Museum of Natural Sciences in Madrid, park biologists have kept close tabs on six common species of amphibians that live there. They’ve been seeing sick animals with necrotic tissue, open sores, and internal hemorrhages. Some vomit blood. “It’s not a pretty sight,” says Stephen Price, a molecular biologist at University College London.
In 2007, the pathogen was identified and named the common midwife toad virus (CMTV), a new kind of RV. RVs have surfaced on every continent except Africa and infect a broad range of hosts, including fish and reptiles, but it is uncommon for them to kill large numbers of individuals in more than one or two species.
That's exactly what they are doing in Picos de Europa National Park, the new study shows. Pulling together population trends across the park, the team found that the common midwife toad (Alytes obstetricans) has been most severely affected, with populations crashing. The common toad (Bufo bufo) and the alpine newt (Mesotriton alpestris) have taken a nosedive as well. Three other species, including salamanders, are susceptible, too.
When Price analyzed tissue samples from the park, he found that CMTV was the only pathogen present in the five ponds with declining populations. CMTV was absent from three other sites in the park with healthy animals. The park is relatively pristine, so other kinds of stress such as pollution are probably not to blame.
It’s not clear how the virus passed through the park gates. The various infected ponds are separated by many kilometers of rugged terrain, raising the prospect that humans rather than amphibians spread the virus within the park. “Muddy boots are about my best guess right now,” Price says.
There are other mysteries, too. Little is known about whether the virus is spreading or where it first came from. Closely related viruses have been found in China and in commercially exchanged amphibians, so trade is a possible route, which raises the odds of more introductions. "There’s a strong possibility we’re at the beginning of a broader emergence," Price says. CMTV was implicated in deaths of more than 1000 newts and frogs in the Netherlands in 2010, infected bullfrogs in Belgium, and more recent deaths in France.
Virologists are wondering what the next chapter will bring. "I am fascinated and worried about yet another challenge to amphibians and reptiles around the world," says Jesse Brunner, who studies RVs at Washington State University, Pullman.