These diseases and their pathogens, with the unsuspecting support of humans and our global activities, all have been involved in microbial invasions of sorts. The transportation and sale of live bait is the latest example of a seemingly innocent human activity that may be responsible for spreading such diseases with dramatic ecological consequences.
Evidence for this comes from the genetic analysis of an emerging virus that has been implicated as a cause of infectious disease in amphibian populations in the western U.S. The genetic study indicates that the virus may have been transmitted to remote locations through the vector of live bait - infected salamanders being distributed and introduced to uncontaminated environments by fishermen and interstate bait wholesalers.
A research article by a team (see end of release) headed by Arizona State University ecologist James Collins forthcoming in the Journal of Molecular Ecology finds that the wild incidences of a devastating iridovirus in Arizona and Colorado are all very similar genetically, indicating a recently emerged or introduced strain. The strain also closely matches viruses isolated from imported salamanders found in bait shops, indicating a possible source for the pathogen's introduction.
The finding further exposes a complex picture of how emerging or invasive diseases are involved in amphibian decline -- the ongoing global disappearance of a broad group of animals -- and is additional evidence for how human activities such as farming and trade in live animals or biological products are affecting ecologies globally.
"Amphibians are clearly vulnerable to environmental change, but the fact that they are being so broadly effected by emerging diseases is telling us that emerging diseases are having a major effect on the biosphere," said Collins.
The finding has come out of a $3 million National Science Foundation funded project to study "Emerging Wildlife Diseases: Threats to Amphibian Biodiversity." Under the grant, Collins and his team have been focusing on defining how important a role pathogens play among a host of possible causes in the decline of amphibian populations worldwide.
Early in the project, the team began to focus on two apparently "emerging" pathogens in particular - a chytrid fungus and an iridovirus - both unfamiliar organisms, though geographically widespread. The chytrid fungus in particular appears to be responsible for major population declines and even waves of mass extinction occurring in Central America, throughout the Rocky Mountain region in North America, and in Australia.
Collins's lab was particularly focused on the iridovirus, which was responsible for observed mass die-offs of tiger salamanders in isolated stock ponds in southern Arizona. The team was initially mystified about how the virus was being introduced to the ponds, which can be isolated from each other by miles.
"We were really interested in the ways in which it could be moving around, and whether or not it can jump between species," Collins said.
The salamander virus is related to another iridovirus strain that is endemic in frog populations. But preliminary analysis by the team has found that the salamander-killing virus is even closer to a virus found in sport fish (including rainbow trout), which in turn the researchers suspect resulted when the virus jumped from frogs to fish. The salamander virus however, is distinct from either of these and may be a recent development because of its lack of genetic diversity.
"The basic story, based on the molecular analysis, is that the virus has a very recent history in the west and it is a history that suggests in its patterns that the viruses have been moved around in less than traditional ways - probably in anthropogenic ways," said Collins. "Bait is one of the ways it is probably getting around.
"We found evidence for this specific virus in salamanders in bait stores in Phoenix and also evidence of it in axolotls (salamanders that remain in the larval gilled form) in the Indiana University colony that are shipped all over the world for biomedical research."
Whether the virus strain is lethal in its original Midwestern host populations is still not known, but it is lethal to tiger salamanders in the wild in the west.
"Bait stores collect their animals in the Midwest, for the most part - Nebraska, West Texas, eastern New Mexico. We find virus that is molecularly indistinguishable from the bait stores in the wild in Colorado. Presumably it was shipped in from somewhere, perhaps Nebraska, into Colorado as bait. And imported bait is a big, multi-million dollar industry."
Though the effect of the virus on tiger salamanders is only a small part of the overall issue of amphibian decline, Collins thinks the team's current findings have a larger significance and may provide a method for putting together other pieces of the puzzle.
"This tiger salamander system is really a model for looking at the emergence of these diseases," he said.
The paper, "Emergence of an Amphibian Disease Perhaps Due to Human-Enhanced Spread," is by J.K. Jancovich, E.W. Davidson, N. Parameswaran, J.P. Collins and B.L. Jacobs at Arizona State University and J. Mao of Tougaloo College, V.G. Chinchar of the University of Mississippi Medical Center, and A. Storfer of Washington State University. It is forthcoming in the Journal of Molecular Ecology.
Source: James Collins, 480-965-3571