Spread of Deadly Virus Tied to Forest Decline

Around 2004, large numbers of aspens in the West began dying off, and with no immediately identifiable cause, scientists dubbed the phenomenon “sudden aspen decline.” Ultimately, the die-back was pinned on a severe 2002 drought and heat wave that left aspen stands vulnerable to pests, cankers and fungi.

Now, a new study suggests that the decline of the West’s aspens is not just marring the landscape, but also helping to spread a strain of hantavirus fatal to humans.

The sin nombre virus — Spanish for “nameless virus” — is carried primarily by deer mice, whose numbers have surged in areas hit hard by the aspen die-backs, researchers from Fort Lewis College in Durango, Colo., reported at a scientific conference this month. Mice in areas of severe die-backs were three times as likely to carry the virus than those in less affected areas, the researchers found. The sin nombre virus was unknown to science before an early 1990s outbreak in the Four Corners region, where the borders of Utah, Colorado, Arizona and New Mexico meet. Humans who inhale virus particles, typically through dust mixed with mouse urine, droppings or saliva, can develop hantavirus pulmonary syndrome. The virus has a fatality rate of around 40 percent, but infections are still rare, with between 20 to 40 cases reported annually in the United States. Hantavirus infections remain concentrated in the Southwest, particularly in areas where sudden aspen decline is common, said Erin Lehmer, a biologist at Fort Lewis College.

Scientists reported last year that the aspen die-back in the West had slowed after a series of cooler, wetter years. But research also suggests that the aspens remain vulnerable to sudden shifts in climate and that further forest declines are highly likely in coming decades because of climate change. The spread of hantavirus among mice in the wake of the aspen die-offs should already be considered an “unintended consequence of climate change,” Dr. Lehmer said. She noted that other studies have shown an increase in human hantavirus infections in Germany during years of above-average warmth.

Dr. Lehmer’s own study is being prepared for publication.

“The bottom line is that climate change is tending to introduce diseases where they haven’t been before, because it’s changing the entire dynamics of plant and animal ecosytems,” she said.