What’s killing Montana moose?

This story was published in the Ravalli Republic and Char-Koosta News.

On a November morning, it’s snowy in the Big Hole Valley.

Bouncing along ranch roads in a Montana Fish Wildlife and Parks truck, biologist Nick DeCesare circles and crisscrosses the valley, listening for beeps.

FWP biologist Nick DeCesare uses an antenna to locate a moose with a radio collar.

FWP biologist Nick DeCesare uses an antenna to locate a moose with a radio collar.

Ten moose in the valley wear radio collars that emit signals and allow DeCesare to find them using a radio and an antenna.

Loud beeps mean a moose is near. DeCesare pulls over, climbs into the bed of the truck, and aims the antenna in all directions.

He points to a ravine, a dip in the evergreens, to the west of the truck. “Right in there somewhere,” he says sagely. He hardly needs the equipment. After tracking her for a year, DeCesare knows this moose, her haunts and habits.

Today he’s worried about the animal.

Nearby ranchers have reported seeing a blind collared moose. He’s guessed by the locations of the sightings which moose it is, and he now prepares to approach her, to see for himself.

DeCesare heads for the ravine, trudging through a stand of evergreens and into a willowy marsh. Whenever you find yourself standing in muck, he says, you know you’re close to a moose.

And it’s true. There’s movement in the brush. She’s only a few yards away. It takes a few moments to take her in. An imposingly beautiful animal, she’s haggard. She looks thin and shaggy, her ears are mangled, and both eyes are a robin’s-egg blue: She’s blind.

Back in the truck, a deflated DeCesare says he thinks this might be a case of a parasite called the arterial worm, or Elaeophora schneideri. By blocking blood flow to the head, it can cause blindness and damage to the extremities like the ears, which without blood simply freeze off in the winter.

It also causes brain damage, “to the point where [the moose] is essentially incapable of functioning,” DeCesare says. The worm is carried by mule deer, which it doesn’t infect, and is transmitted to moose via horseflies.

In the last year, it might have claimed a couple of DeCesare’s collared moose.

Across the country, moose populations are in dramatic decline—and nobody knows why. In Minnesota, data shows moose populations are down about 75 percent over the last 20 years.

In Montana, there’s little data on moose. What we do know, which comes from surveying moose hunters, is troubling. Since the mid-1990s, self-reported hunter success rates have plummeted.

Consequently, Montana Fish Wildlife and Parks has cut the number of moose-hunting permits by more than 50 percent; just since 2008, the number has dropped from 578 to 368, and FWP has proposed to make even fewer permits available in 2014.

DeCesare and Montana FWP are beginning a 10-year research project intended to figure out what’s behind the decline. In the last year, DeCesare has collared 34 adult female moose in three parts of Montana: the Big Hole, the Cabinet Mountains, and the Rocky Mountain Front.

Tracking moose will allow DeCesare and other biologists to discern if moose are suffering from low birth rates or mortality, or some combination of both.

So far, six of DeCesare’s collared moose have died. Though each animal’s cause of death hasn’t been identified, the arterial worm is a suspect in a few of the cases.

“We know that we have this parasite,” says Jennifer Ramsey, FWP’s wildlife veterinarian. “We know that it’s a problem for moose. And it can be found in these populations. But we can’t really say how prevalent it is.”

In Wyoming, at least, there does appear to be an increase in the prevalence of the arterial worm. So says Amy Williams, a graduate student in the University of Wyoming veterinary sciences program.

Williams researches the parasite and the horseflies that transmit it. She says studies on Wyoming moose populations in the 1970s showed zero incidence of arterial worm. Today, she says, “You would definitely find some.”

Williams’ research hopes to shed light on this change. She says an increased incidence of arterial worm could be caused by increases in horsefly populations, especially the specific species or life stages that carry the parasite, or by changes in the environment or in moose populations themselves that make them more susceptible to the parasite once bitten.

One possibility is a change in weather.

“If you look at horseflies, they really are dependent on the environment and on the weather,” Williams says. “They really need those warm, dry summer days. The warmer and dryer they are, the more active horseflies are. … So if it’s warmer … horseflies are going to do better. Is that actually causing a change in the arterial worm presence? I don’t think you can make that kind of connection yet, but it’s worth investigating and keeping in mind.”

Back in the truck, DeCesare echoes this uncertainty. He suspects that a multitude of different factors are working against Montana’s moose. Between increased predation, habitat changes, disease, parasites, and warming temperatures, things get complex.

“All those things could be having an effect and they could be interacting with each other, which makes it complicated to tease out,” he says. “But it seems like there’s evidence in other parts of the country that all of those things could be important.”

DeCesare puts the truck in drive. “Someday,” he says, alluding to the eventual findings of his research, “we’ll know a lot.”