By Will Ferguson, College of Arts & Sciences
PULLMAN, Wash. – Jesse Brunner did a double take as he surveyed a pond in southern Arizona’s San Rafael Valley. It was home to endangered tiger salamanders and, over the course of one week, every salamander Brunner could find was sick or dying.
“At the time I thought we were missing something,” said Brunner, assistant professor of biology. “There are all sorts of things that can kill off a population of amphibians, from introduced predators to a dried out pond. We eventually recognized the culprit behind this particular die-off was a ranavirus.”
Benign can turn deadly
Ranaviruses are a group of DNA-based pathogens that can infect and destroy virtually every tissue throughout the bodies of amphibians, fish and reptiles.
Similar to influenza, SARS and other human viral infections, when ranaviruses are introduced into new places or species, they often go unnoticed and sometimes cause moderate to massive mortality events.
Brunner is part of an international effort to find out why ranaviruses are deadly when introduced to certain animal species and habitats but are relatively benign in others.
Part of amphibian decline
“Concern about ranaviruses has been growing in recent years with the discovery of ranaviruses being spread in trade and with mounting evidence that certain strains are found all around the world, sometimes causing mass die-offs of amphibians and certain fish and reptiles,” he said. “We think that, like other pathogens, ranaviruses are contributing to the global decline in amphibians.”
For decades after they were discovered in kidney cell cultures of northern leopard frogs in the 1960s, ranaviruses were thought to have little impact on their host populations.
This view began to change when scientists around the world noticed a global pattern of mass die-offs, first in fishes in the mid-1980s and later in amphibians around the turn of the century. Today, six species of ranaviruses are known to infect at least 175 species of amphibians, fish and reptiles across 32 countries and six continents, including the inland Northwest.
Going global with research
Brunner is associate director of the Global Ranavirus Consortium, an international group of scientists investigating the emergence of ranaviruses as a pandemic. In May, the consortium published “Ranaviruses: Lethal Pathogens of Ectothermic Vertebrates,” the first book on ranaviruses, and Brunner coauthored two chapters.
His research explores the susceptibility of different species to ranaviruses, how they spread within and between populations, the virulence of various strains and how best to contain and stop their spread. Brunner began researching ranaviruses in the 1990s and has seen numerous examples of how destructive they can be.
Massive die-offs overnight
In North America, roughly 50 percent of amphibian deaths are attributed to viral infections, presumably ranaviruses. Death is often sudden and massive: hundreds or thousands of apparently normal tadpoles might be present one day and more than 90 percent dead within several days. In one recent report, some 200,000 tadpoles died within 24 hours.
In 2009, Caren Goldberg, assistant professor in the WSU School of the Environment, documented a ranavirus outbreak in tadpole populations along the Latah Trail between Moscow and Troy, Idaho.
Effects of ranaviruses on fish and reptiles in the wild are less known but they also can be highly lethal. Outbreaks in wild pallid sturgeon, a federally endangered species, and eastern box turtles have occurred in the United States. Additionally, outbreaks in farm-raised fish and reptiles in captivity are acute, often fatal.
Tackling tough questions
Because not every ranavirus strain is deadly, two questions arise: why are some strains worse than others, and how are they spreading?
One pattern that’s becoming clear is that ranaviruses found in captive settings (such as aquaculture facilities and bait shops) are more virulent than wild strains.
For instance, work by WSU biology professor Andrew Storfer showed that a ranavirus strain isolated from a bullfrog farm grew significantly faster and caused significantly higher mortality in bullfrogs and spotted frog tadpoles than a strain isolated from a wild bullfrog population in Washington.
“Our concern is the introduction of infected, farm-raised amphibians may introduce novel, highly virulent viral strains into areas where hosts are adapted to other ranavirus strains,” said Sarah Meiners, a doctoral student in Brunner’s lab.
Screening to avert disaster
Farmed and wild bullfrogs and other amphibians are traded globally in enormous quantities, making international trade an important route for the translocation and introduction of ranaviruses. More than 28 million amphibians were imported into the United States 2000-2005 with an 8.5 percent prevalence of ranavirus infection.
In an effort to stop introduction of virulent ranavirus strains into vulnerable populations, Brunner’s team at WSU is developing non-lethal methods to screen populations or shipments of animals.
“At this point we don’t have a cure for ranavirus,” he said. “Our hope is that developing an effective screening method will be a good start.”