What’s killing Idaho moose? In many cases, infestations of ticks and other parasites

BOISE – Nearly a year after launching a research effort to study why moose populations in Idaho have been declining, researchers with the Idaho Department of Fish and Game are starting to get some answers.

According to a Fish and Game news release, the agency partnered with researchers from the University of Idaho to attach radio tracking collars to 112 adult cow moose in early 2020 to study survival rates and causes of death.

Across the country, moose populations have been declining since the 1990s, and moose biologists have told the Idaho Statesman in the past that there are likely several contributing factors – from increases in parasites that can prove fatal for moose to prevalence of old growth forests that aren’t ideal moose habitat.

“We have seen the most severe declines in northern Idaho and southeast Idaho, but we don’t know exactly why they are declining,” Hollie Miyasaki, a moose biologist with Fish and Game, told the Statesman in July 2019.

Researchers have now collected several months of data from the moose radio tracking collars, and the early results are already offering clues about moose survival. About 89% of the collared moose survived through the fall, Fish and Game reported.

“It really was a pleasant surprise,” wildlife research manager Mark Hurley said in the news release. “We expected survival to be lower than that, given that moose populations are declining throughout the southern part of their range, including in Idaho.”

Tick, parasite infections prove major problems

Officials performed necropsies on the animals that died and found that more than half had died of parasites or disease. The majority of those deaths were due to emaciation from winter ticks, which are a species of external parasite that prefers to feed on species like moose, deer and elk. The ticks suck blood from the host animal, causing anemia and making it difficult for the host animal to maintain proper nutrition.

In 2019, Fish and Game moose biologist Kara Campbell told the Statesman more moose are facing massive infestations of winter ticks as climate change causes warmer seasons.

“High tick loads used to die off in the winter, but with warmer winters, we don’t see tick die-off that we used to see,” Campbell said. “So moose can accumulate ticks at high loads, up to 10,000 ticks per moose.”

In February, a Vermont Fish and Wildlife Facebook post about winter tick infestations in moose went viral. Officials had found one animal with an estimated 90,000 ticks feeding on her.

Though Idaho tick infestations aren’t that severe, necropsies on 26 noncollared moose turned up similar findings for Idaho Fish and Game as the necropsies on collared animals.

“One of the most interesting things we’ve found is all the different ways moose are dying,” wildlife research coordinator Shane Roberts said in the news release. “And there are a lot, many of which are driven by diseases and parasites. It can be several encompassing things, but many factors are associated with winter tick infestations, which seems to be the No. 1 issue so far.”

Fish and Game also noted that blood samples from all 112 moose in the study showed one-third had some kind of parasite, and 85% had anaplasmosis, a tick-borne bacterial infection. Analyses of blood samples from more than 600 other specimens, gathered by hunters who harvested moose, showed similar results.

“Those are some causes of mortality we shouldn’t be seeing in any species – tick-related deaths, or deaths related to external parasites,” Hurley said. “But we are seeing them, and they are really specific to moose.”

Other causes of death included infectious and inflammatory diseases, vehicle collisions, noninfectious diseases, neurologic diseases and predation, officials said.

Study turns to calf survival

Despite the prevalence of parasites in adult moose, the 89% survival rate in the study sample proved promising. Researchers plan to turn their attention from adult mortality rates to moose calves.

About 85% of the moose in the study were pregnant when they were collared. Of the pregnant moose, only about 70% had calves with them when researchers tracked the animals in the spring. Of the group seen with calves in the spring, 69% of the moose cows still had their calves with them in the fall.

“It is possible some pregnancies aren’t being carried to term, or there is a significant cause of early calf mortality, and we will continue to look at that,” Roberts said. “While there will always be some calf mortality before we’re able to observe them the first time after birth, there is a discrepancy worth looking into further.”

Researchers plan to put radio tracking collars on the calves to track their survival from six months to 1 year old, when they’re susceptible to predator attacks, as well as tick- and parasite-related fatalities. They’ll also focus on the parasites and diseases they’ve observed in the moose to try to determine what role they play in moose mortality.