University researchers investigate Minnesota moose mortality

Scientists hope to find a way to reduce moose deaths caused by a parasitic worm transmitted by white-tailed deer.

Allison Cramer

In response to the declining Minnesota moose population, University researchers are studying possible factors contributing to the increase in mortality.

In 2016, about 4,000 moose lived in Minnesota — a 50 percent decrease from 2006. Researchers pinpointed a brain parasite as a major cause of moose death and hope to identify a way to save the state’s moose.

Up to 30 percent of natural mortality in recently studied adult moose can be attributed to a parasitic worm that naturally resides in the tissues around the brains of white-tailed deer, said Tiffany Wolf, a veterinarian and assistant professor in the University’s Department of Veterinary Population Medicine with nearly 10 years of moose research experience.

The deer pass the larvae of the parasite in their feces, and from there they are picked up by a snail or a slug, where they develop and become infectious.

The parasite isn’t dangerous for deer, but when moose are exposed to it during grazing, the parasites migrate to locations in the central nervous system other than the brain covering — where they cause damage and deadly disease.

Scientists have known about the parasite for years, but rising moose mortality has raised awareness among the public, said James Forester, an assistant professor in the fisheries, wildlife and conservation biology department.

“It’s not a surprise that animals are dying of P. tenuis,” Forester said. “The real question is, is there any kind of management intervention that we can do to try to increase the moose population?”

Changes in the population ranges of white-tailed deer have likely contributed to this problem, Wolf said. Today, white-tailed deer range further north than was normal in the past, meaning they share habitat with moose.

“The real problem now is … good habitat for moose is also good habitat for deer. The real question is why are there more deer?” Forester said. “It’s because there’s less snow pack in the wintertime, so populations are expanding.”

Moose are facing a “triple whammy” right now, said Forester, who studies the relationship between animals and changing landscapes. As deer populations increase, both the probability of brainworm infection and the number of wolves that might prey on sick moose increase.

“Moose are really getting exposed to these two pressures simultaneously, and they’re also being exposed to much higher temperatures, which is going to cause their immune systems to probably be compromised because they’re just hotter, and it might change their foraging patterns too,” Forester said.

Ronald Moen, an associate professor in the Department of Biology on the Duluth campus, is one of many researchers searching for solutions to the parasite problem.

Moen supervises several graduate students focusing on moose-related research. One of these students is studying snail distribution, and another is using tracking collars on deer and moose to analyze habitat overlap.

One proposed solution is reducing the density of the deer population in moose range, but it’s unpopular with local hunters, Wolf said.

“We’re thinking more about how the parasite is transmitted between deer and these gastropods and then on to moose, and if there’s a place along that chain of transmission that we can break,” Wolf said. “If there are options for habitat management to do that, then that’s what we’re hoping to find.”

Moen said one of the unsolved mysteries surrounding parasite transmission is that less than 1 percent of the snails carry the brainworm, so a moose or deer may have to eat as many as 1,000 snails to contract the parasite. Researchers aren’t sure how or why moose would consume this many snails while grazing on brush.

Forester said the only current possibilities for intervention are altering the moose habitat in some way or changing the density of the deer.

“There aren’t that many levers that we can pull,” Forester said.