What’s Killing Minnesota’s Moose? Studies Reveal Sites of Deadly Brainworm Transmission

Carried by deer and spread by snails and slugs, a lethal parasite is infecting the large ungulates, which have recently declined dramatically

Moose walking in the woods
Moose numbers have plummeted in Minnesota. NPS Photo / Kira Heeschen

In recent years, Minnesota’s moose population has plummeted. Now, scientists are gaining a better understanding of one of the main drivers of the large ungulates’ demise: a devastating parasite known as brainworm.

The worms, called Parelaphostrongylus tenuis, hitched a ride to Minnesota inside the brains of white-tailed deer, which serve as their main host but only rarely suffer ill effects from being infected. Moose, in contrast, have no natural defenses against the parasites. The roundworms bore tunnels throughout the mammals’ brains, and infected moose will exhibit bizarre behaviors, such as walking in circles, before slowly dying of starvation or hypothermia.

When infected deer defecate, they also pass brainworm larvae in their feces, which snails and slugs then consume. Scientists had long suspected that moose were accidentally eating those infected snails and slugs while munching on leaves, twigs and other plant matter. They also guessed there might be specific geographic locations where moose were becoming infected because they overlapped regularly with deer.

Now, a pair of new studies sheds more light on the process of brainworm transmission. One paper, published last month in the Journal of Wildlife Diseases, confirms moose are indeed eating snails and slugs. To reach this conclusion, scientists collected and analyzed 258 moose fecal samples. From the samples, they identified at least three species of gastropods the moose had ingested, including the quick gloss snail (Zonitoides arboreus), which is a “well-documented host” of brainworm, per the paper.

“There’s been a ton of research on the deer part of this [brainworm] cycle,” says study co-author Tyler Garwood, an ecologist at the University of Minnesota, to the Duluth News Tribune’s John Myers. “But understanding the other pieces of the cycle, the slugs and snails, is helping explain how this is impacting moose.”

The second study, published in the journal Food Webs this month, suggests moose primarily consume the infected snails and slugs at sites known as mineral licks, which deer also visit regularly.

Moose and deer need sodium and calcium to survive, so they converge on areas with springs and seeps that have traces of these minerals. Researchers figured this out by analyzing GPS data from collars on both species, then setting up trail cameras at the shared sites. The cameras showed deer defecating at the mineral licks, as well as moose eating the soil and nearby vegetation. Scientists also discovered snails and slugs that typically carry brainworm at these sites. Together, the findings suggest mineral licks are hot spots for brainworm transmission.

moose sniffs grass and a deer stands in the background in a black and white trail camera photo
A trail camera image shows a moose and deer together at a mineral lick. South Dakota State University

Both of the new studies could inform wildlife managers’ on-the-ground tactics for protecting moose from brainworm infection. For instance, they may set prescribed fires or do targeted timber cutting in strategic areas of the forest, which could kill off the snails and slugs that might harbor the parasites.

“Any sort of forest management treatment might decrease the gastropod numbers, and that might lead to a decrease in the number of infected gastropods on the landscape,” says study co-author William Severud, a natural resource management researcher at South Dakota State University, in a statement.

Brainworm has become a leading cause of death among Minnesota’s moose in recent years, responsible for an estimated 25 to 30 percent of mortality. The moose also face other challenges, including predation from a growing gray wolf population in the region, tick-related threats and rising temperatures brought on by human-caused climate change.

In fact, global warming has enabled white-tailed deer to expand their habitat, bringing them north into the range of the moose—and allowing them to spread brainworm to the cold-loving mammals, Vox’s Liz Scheltens wrote last year.

The total moose population in Minnesota has plummeted from 8,000 individuals in 2009 to around 3,700 in recent years, per the Minnesota Department of Natural Resources. But, at least for now, moose numbers appear to have stabilized.

Indigenous communities, like the Grand Portage Band of Lake Superior Chippewa, also known as the Ojibwe people, consider moose to be a “cultural keystone” species that is not only an important source of food, but also plays a significant role in spiritual practices, per Vox. More broadly, moose are an indicator species that can help signal the overall health of Minnesota’s boreal forest ecosystem.

For these and other reasons, conservationists and wildlife managers want to do everything they can to help save Minnesota’s moose. And that process can start with research.

“Our intent comes from the background of us wanting to help,” Edmund Isaac, a wildlife biologist with the Grand Portage Band of Lake Superior Chippewa, said to Northern News Now’s Dan Wolfe last year. “Moose are kind of losers on the landscape under the light of climate change.”

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