Research sheds light on how brainworm is killing Minnesota's moose

A moose calf follows its mother
In this 2014 photo provided by the Minnesota Department of Natural Resources, a moose calf follows its mother in the snow near Grand Marais, Minn. Minnesota's moose population is estimated at about 3,300, down from more than 8,000 in 2006.
Pete Takash | Minnesota DNR file via AP

New research has revealed important clues about the role that brainworm has played in the rapid decline in the state’s moose population, and how exactly the parasite is infecting such a large number of the iconic animal in Minnesota’s north woods.

The moose population in the Arrowhead region in the northeastern corner of the state has plummeted over the past 15 years, from around 8,000 animals in 2009, to an estimated 3,300 moose this year, according to the Minnesota Department of Natural Resources’ latest survey.

There’s a complex interplay of factors involved in the shrinking moose population, from parasites and infections to climate change to predation of moose calves by wolves and bear.

But one of the most concerning culprits in the moose decline is brainworm, a parasite carried by white tailed deer. Research has found it’s responsible for up to 25 percent of adult moose deaths in Minnesota.

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The parasite damages the nervous systems of moose, ultimately leading to paralysis and death. It weakens moose, making them more vulnerable to wolves. And it can cause bizarre behavior.

Tiffany Wolf, a professor in the College of Veterinary Medicine at the University of Minnesota, said several years ago scientists were baffled when someone found a moose fitted with a GPS collar swimming in circles in Lake Superior.

“And when that moose was euthanized, and its brain and spinal cord examined by our pathologist here at the University of Minnesota, it was diagnosed with brainworm,” said Wolf.

Scientists, though, haven’t understood exactly how or where on the landscape the parasite is getting transmitted to moose, and what land or wildlife managers might be able to do to prevent those infections.

“That's where some of this new research will really help us out,” said Seth Moore, director of natural resources for the Grand Portage Band of Lake Superior Chippewa.

A screen grab of a video of a wild animal.
A video screen grab of a moose at the Voyageurs National Park.
Courtesy of Voyageurs Wolf Project

Scientists have long believed brainworm infected grazing moose when the animals ingested tiny gastropods, slugs and snails, that live on the forest floor.

White tailed deer, which are not harmed by brainworm, shed the parasite in their feces. Those larvae are in turn consumed by the slugs and snails, some of which are as tiny as a pinhead.

Then moose inadvertently consume the gastropods as they crawl up vegetation.

That was the theory. Now it’s been confirmed in a recently published study in the Journal of Wildlife Diseases. Moore, Wolf and other colleagues collected 258 fecal samples on and around Grand Portage tribal lands, and found that moose do indeed ingest gastropods.

Researchers then used DNA analysis to identify three species of the snails and slugs that moose ate, including one that’s a well-documented host for brainworm.

Moore believes that knowledge will help forest and wildlife managers strategically intervene to reduce the density of those gastropods in areas frequented by moose, and possibly, reduce the transmission of brainworm.

“We have some some data in the pipeline that's going to be published that shows that some of our forest harvest practices can reduce the populations of the gastropods that transmit brainworm for fairly long periods of time,” Moore said.

Setting prescribed fires, for example, kills the gastropods, Moore said. And it takes several years for them to repopulate.

“So by learning what species of gastropods are affecting moose, it will help us understand how we should manage the landscape to improve the habitat for moose,” said Moore.

Moose mystery

There’s another question that has vexed scientists. When they’ve collected gastropods from the forest and sampled them, they’ve found very, very low rates of brainworm infection — less than 0.1 percent.

“So there seems to be this disconnect where deer have had tons of brainworm. And we know that moose are getting it and dying,” said Bill Severud, a professor of wildlife management at South Dakota State University who has studied moose in Minnesota for the past decade.

“But we aren't finding the gastropod hosts that are infected to a level you would expect, seeing the numbers of moose that are being impacted.”

Scientists suspected there were specific places on the landscape where moose and deer overlapped, where brainworm was being transmitted to moose through the gastropods.

A moose is wearing a radio collar.
A moose is wearing a radio collar that helps researchers track the movement of individuals within a population to better understand species behavior.
Seth Moore | Courtesy of Michigan Technological University

And because the Grand Portage Band has been placing GPS research collars on both deer and moose since 2010, they were able to analyze data from those collars to identify focused locations that both species used at different points in the year.

When they hiked to those locations, they found what are known as “mineral licks.” They are springs and seeps with trace elements such as sodium and calcium that moose and deer need to survive.

Scientists placed trail cams at these sites, and documented both moose and deer frequenting them, sometimes at the same time, and slurping up mud and soil to pick up the minerals.

Researchers also captured video of deer defecating at the mineral licks, and moose subsequently eating the soil. Additionally they found these areas also have a high number of the gastropod species that carry brainworm. The results were published in the journal Food Webs.

“So we think it seems to reason that that's an area where parasite transmission might be high,” said Severeud. “This might be one of the missing pieces that might explain at least some of the reasons why moose continue to be infected by brainworm.”

More research is needed to confirm the connection. But if mineral licks are increasing the odds of transmission of parasites like brainworm, they could also potentially spread chronic wasting disease if it makes it to the region, said Moore.

“So this is the area where we have to think about how can we apply what we know, in a landscape and natural resources management fashion, to try to break that vector chain of brainworm getting into moose.”