Decline of Western U.S. trees swells population of rodents that carry sin nombre virusBy Susan MiliusWeb edition : Friday, January 7th, 2011
DANGEROUSLY CUTE Deer mice carrying a deadly virus are becoming more common in the West as a sudden and mysterious loss of aspen trees has effects rippling throughout the ecosystem, new research suggests. David Cappaert/Michigan State University, Bugwood.org
SALT LAKE CITY — Recent diebacks of aspen trees in the U.S. West may end up increasing the risk posed by a lethal human pathogen, a new study suggests.
A tree-killing syndrome called sudden aspen decline that has wiped out swaths of trees across the West in the past decade has also changed the kinds, numbers and interactions of creatures living around the trees, researchers have found — including some carriers of human disease. Deer mice at hard-hit sites in 2009 were almost three times as likely to carry sin nombre virus — which can be fatal to humans — compared with mice in less-ravaged aspen stands, Erin Lehmer of Fort Lewis College in Durango, Colo., and her colleagues reported January 4 at the annual meeting of the Society for Integrative and Comparative Biology.
The deer mouse Peromyscus maniculatus looks ironically cute in pictures at meeting presentations, but the Centers for Disease Control and Prevention ranks it as the main rodent reservoir for sin nombre virus. Infected deer mice don’t show many symptoms, but people inhaling virus wafting from mouse urine or saliva can get quite sick with hantavirus pulmonary syndrome.
Unknown to medicine until 1993, hantavirus pulmonary syndrome starts with muscle aches, chills, fever and stomach upset. Later, fluid fills the lungs; more than a third of victims have died. In 2010, the CDC logged 560 cases in 32 states stretching from California to Maine, but mostly in the West.
“Both plant diseases and animal diseases are rapidly emerging globally, and we should be looking for ways that the two might interact,” said Richard Ostfeld of the Cary Institute of Ecosystem Studies in Millbrook, N.Y., who studies Lyme disease transmission.
What caused sudden aspen decline seems to be more complex than a single pathogen, Lehmer noted. Severe drought from the late 1990s into the 2000s stressed aspens and then may have allowed cankers, fungi and other maladies to deliver death blows.
Lehmer and her colleagues compared aspen stands with minimal, moderate and high damage in the San Juan Mountains of Colorado. In places that still had most of their aspens, researchers found more species of small mammals than in the devastated plots. And in the healthier aspen stands, the most abundant small mammal was the montane vole, which doesn’t make a good host for the virus.
In study sites that had lost at least two-thirds of their aspens, the researchers found fewer species of small mammals. The most abundant of those species was the deer mouse, which isn’t as choosy about its habitat as the vole is. Lehmer speculated that infection might have risen among deer mice as their growing dominance in the landscape let them encounter each other more frequently and get into more mouse fights. Sin nombre spreads readily among rodents through bites.
Results from the aspen study so far seem to parallel the Lyme disease story, Ostfeld says. He and his colleagues have found that as people have fragmented habitat for wild animals, species that make poor hosts for the Lyme pathogen and its tick vector have dwindled in number. In these less diverse landscapes, however, the white-footed mice that carry Lyme disease thrive and readily pass around infections. So what’s bad for wild habitat ends up being bad for human health.