Parvovirus is killing wolf pups in Minnesota

About half of the wolf pups born in Minnesota each year are killed off by a highly contagious disease called canine parvovirus, according to new research published by a team of Minnesota researchers in a national journal.

The disease has stunted the growth of the state's gray wolf population at a time when wolves are increasing rapidly in number and expanding their range in Wisconsin, Michigan and western states.

"That's not happening in Minnesota, because there aren't quite enough of these wolves to do more than just maintain the population," said David Mech, senior research scientist at the U.S. Geological Survey and lead author of the study.

The state's gray wolf population is about 3,000 and increasing by about 4 percent a year, compared with increases of 16 to 58 percent in other states, Mech said.

Canine parvovirus is not new to Minnesota, but this is the first time its effect on wolves here has been studied in depth. The study concluded that between 40 and 60 percent of wolf pups die from the disease.

The virus was discovered in the late 1970s. It affects dogs, wolves and coyotes by attacking the gastrointestinal tract and causes loss of appetite, fever, vomiting and severe diarrhea. Animals become listless and dehydrated, and can die within a few days.

Mech said wolf pups younger than 12 weeks old are particularly vulnerable. "It almost always attacks fast-growing tissue, which is why it attacks young animals," he said. "If adults get exposed, they don't come down with real clinical symptoms most of the time."

For wolf pups, the disease is often fatal. If their mother was exposed to the disease, the antibodies are passed to newborn wolf pups, but their immunity wears off after 8 to 12 weeks. After that, the pups may be infected by other members of the pack.

If the mother was not exposed to the disease, the pups are safe for about 3 weeks while they remain in the den, usually in late April and early May. After they emerge to join the pack, they can be infected.

The virus spreads through feces that wolves and other canines smell, lick or ingest. It can be passed between dogs, wolves, coyotes, and possibly foxes.

Mech used data from an annual winter census of wolves in a study area east of Ely, Minn., estimates of pup survival across a broader area of the state, and the presence of parvovirus antibodies in several hundred wolves that were captured and tested. The data were drawn from research from the 1970s to 2004 and were published in the Journal of Wildlife Diseases.

Wolf expert not convinced

Dan Stark, wolf specialist for the Minnesota Department of Natural Resources, said the study has some merit, but the issue of wolf pup mortality needs further investigation.

For all that scientists have learned about wolf biology and behavior, he said, very little is known about the survival of newborn wolf pups. "It's one of those things that's really difficult to get a grasp on because you'd have to go into the dens and you'd have to handle the pups," he said.

It's true that Minnesota's wolf population grew steadily until the late 1990s and has stabilized over most of the past decade, said Stark, but disease may not play the largest role in keeping the population in check. Wolf pups also die of starvation and attacks by black bears and raptors, he said. An increase in roads and human interference also probably limit wolves from expanding into agricultural areas to the west and south, Stark said.

Mech agreed that wolf pups die from other causes, but said that disease plays a much greater role than previously realized.

"It's easy to see this wolf population as being in balance with its environment, but what's keeping it that way is parvovirus," he said. "Whereas in other areas, they are expanding."

One solution that protects dogs from the virus -- receiving vaccinations in a series of three shots -- is not practical for wolf pups in the wild, Mech said. Captive adult wolves suffering from the disease have recovered after being given intravenous electrolytes and other fluids, he said.

On alert on Isle Royale

Canine parvovirus is familiar to researchers on Isle Royale National Park, where scientists have studied the predator-prey relationship between wolves and moose for 50 years.

John Vucetich, a researcher in that project and assistant professor of wildlife at Michigan Technological University, said that someone broke park rules in 1980 and brought an infected dog to the island. Within two years the wolf population plummeted from 50 to 14 animals, he said. The surviving wolves developed an immunity and the parvovirus disappeared for years, said Vucetich, but it was discovered again in 2007 in two of six wolves tested.

So far the disease has not affected the birth or death rates of the 23 wolves on the island, said Vucetich, but he and colleague Rolf Peterson are watching. "We're definitely paying attention," he said.

Tom Meersman • 612-673-7388