How a Tiny Brain Worm Is Killing Moose Across North America

On the first day of my visit to Yellowstone National Park, I got up at dawn and wandered over to the river flowing a few hundred feet from my campsite. Through the mist rising from the water I spotted a magnificent bull moose, standing motionless, its enormous antlers leaning back as it pointed its muzzle to the sky.

Entranced, I stood still, feeling blessed by this magical moment.

Now I read that these impressive animals are in serious trouble.

Moose are the largest of all the deer species. Males are immediately recognizable by their huge antlers, which can spread six feet from end to end. These animals are so tall that they prefer to browse higher grasses and shrubs because lowering their heads to ground level can be difficult.

We learned recently that half of the world’s wildlife has disappeared in the past 40 years, and it seems that moose are not exempt from this trend.

Moose Populations in Drastic Decline

Moose populations are in steep decline in North America, in their normal habitats from Montana, Wyoming and British Columbia, to Minnesota and New Hampshire.

The state of Minnesota, for example, had two separate moose populations 20 years ago. One of them has all but disappeared since the 1990s, dropping from 4,000 to less than 100. The other population, in northeastern Minnesota, is dropping 25 percent a year and is now fewer than 3,000, down from 8,000.

“Something’s changed,” says Nicholas DeCesare, a biologist with the Montana Department of Fish, Wildlife and Parks who is counting moose in Minnesota. “There’s fewer moose out there, and hunters are working harder to find them.”

Scientists are trying to figure out exactly what is causing the moose die-off.

Climate change is a definite suspect. Winters have grown substantially shorter across much of the moose’s range. Average winter temperatures in northern Minnesota have increased more than four degrees over the past 40 years. And that’s bad news for a couple of reasons.

Scientists think warmer winters and longer summers may be weakening the heat-sensitive moose and giving wolves more time to hunt them.

Parasites also have more time to infect them.

Then there’s the brain worm.

Historically, deep winter snow kept deer out of moose country, so the animals didn’t mix much. But a series of warm winters, as have happened recently in the Northeast and Midwest, can allow deer, and brain worms, to move north into the boreal forest.

This tiny parasite coexists happily with white-tailed deer, living in the connective tissue around the brain and spinal cord (or the meninges). But in areas with lots of deer, moose pick up the worm, too. And that’s a big problem for the moose. Some moose seem able to fight off the parasite, but others start walking in circles or just stand around until they become prey or die.

Scientists Working to Solve the Mystery

The National Wildlife Federation describes one study:

In early 2013, Minnesota biologists outfitted 110 adult moose with GPS collars and mortality sensors—and a year later added 36 more to replace those that already had died. They also put collars on 34 calves in May 2013. In a similar effort, New Hampshire and Maine biologist radio-collared 103 moose, half of them calves, in January 2014. The devices not only track locations, they also send messages to researchers when an animal may be dead, making it possible for the biologists to race in to collect carcasses or samples for analysis.

By March 2014, only nine of the 34 Minnesota radio-collared calves were still alive; the rest had mostly been killed by wolves, others by bears. More than a fifth of the adults also died.

So now it’s a race against time to understand exactly what’s causing this decline, and then figure out what to do about it.

The researchers admit they may not come up with answers before all the moose are gone from Minnesota, but hope that what they ultimately learn may save the moose populations in the rest of North America.

I just hope I can return to Yellowstone next year and catch a glimpse of another beautiful bull moose.

Photo Credit: Thinkstock


Jim Ven
Jim Ven3 years ago

thanks for the article.

Carole R.
Carole R3 years ago

Save the moose.

Jennifer H.
Jennifer H3 years ago

What is wrong with this statement? "“Something’s changed,” says Nicholas DeCesare, a biologist with the Montana Department of Fish, Wildlife and Parks who is counting moose in Minnesota. “There’s fewer moose out there, and hunters are working harder to find them.”

If there are fewer and the numbers are drastically delclining, why in the heck are they still being hunted? Ignorance on the part of DFW AGAIN! That is what is alarming here. They are still hunting and then trying to pass off the herd loss to the wolves to be able to justify their mass killing and ESA removal.

Wildlife C.
Wildlife C.3 years ago

A peer reviewed definitive study published in August by the world's leading wolf researcher David Mech attributes the large population decrease to wolves. It's not brain worm nor hunters nor climate change nor vampires nor space aliens. It's wolves. Unless a large and continuous effort is made to severely restrict (insert kill) wolf populations it's highly likely Minnesota might well lose it's iconic moose in much of the state.

Mandy H.
Mandy H3 years ago

Stop the hunters from killing them right now that would help to reduce the amount of damage we people have already done.

Angev GERIDONI4 years ago

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Thank you for sharing

Angev GERIDONI4 years ago

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mari s.
Mari S4 years ago

Let's protect the moose!

Better yet, let's ban hunting altogether!

Donnaa D.
donnaa D4 years ago


Glenda L.
Glenda L4 years ago

What about all the hunters.