"The challenge and concern comes from the fact that we don't understand why it is more virulent in some years and why in some years it has no effect at all," Sime said.
A minor outbreak hit Yellowstone's wolves in 1999, but there has not been a flare-up since.
Ed Bangs, wolf recovery coordinator for the U.S. Fish and Wildlife Service, says outbreaks take place when exposure to the virus is coupled with other factors. But scientists do not know what secondary circumstances trigger the outbreaks.
"It may be something like the weather or poor food supply that makes the pups weaker," Bangs said, "but we don't know."
Sime noted that some scientists believe sheer numbers may play a role in the outbreaks.
Most of the wolves trapped by biologists in Montana test positive for exposure to parvo, she said, but few succumb.
"Outside the park the wolves don't interact with each other that much," she said.
When their numbers are higher, animals tend to pass diseases more readily, Sime added.
"For a lot of other wildlife diseases, density matters," she said.
Little to Be Done
Scientists believe parvo is the likely cause of the pup deaths in part because it attacks pups and not adult wolves.
"The reason pups die and adults don't is [pups] are weaker," Bangs said. "The pups get weaned and lose the protective elements they get from the milk, and they are suddenly susceptible to disease."
If the disease killing Yellowstone's pups turns out to be parvo, there is little that can be done for the wolves, experts say.
Dogs can be vaccinated against the virus, but it is not feasible to trap and vaccinate all the wild wolves in Yellowstone, park officials say.
Most wolf experts and wildlife biologists agree that if parvo is the cause, Yellowstone's population should ultimately rebound on its own.
Wildlife biologist David Mech studies wolves in Minnesota, where an outbreak of parvo killed many wolf pups in the 1980s.
"Ultimately what it does is retard the rate of increase of the population, but it doesn't reduce the population [in the long run]," said Mech, who is also founder of the International Wolf Center in Ely, Minnesota (wallpaper photo: gray wolf at Ely).
In the meantime, Yellowstone officials said they will closely monitor the park's wolves.
"Our hope is that we will learn more about what caused the outbreak," Yellowstone's Stahler said.
"We also hope the situation doesn't arise again in the next few years."
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