Wildlife managers have speculated on why New York's moose populations have failed to grow beyond 600 to 800 individuals, while moose numbers in other states are much higher. Now, a new study published in June in Ecology Letters has uncovered one reason: deer in the Adirondacks are spreading deadly parasites to moose.
Two types of parasites that often use deer as hosts, but rarely lead to illness in them, are much more problematic in moose, where they can cause many symptoms and be fatal.
The finding points toward reducing the number of deer in the area as perhaps the best option to curb the parasites and help moose populations grow in the future. That's because controlling the parasites themselves may not be possible, the researchers said.
"We might think of moose as being the stronger competitor compared to deer, because they can reach higher in the canopy and they're better suited to snowy, cold conditions," said Jennifer Grauer, Ph.D. '24, a postdoctoral associate in the Department of Natural Resources and the Environment in the College of Agriculture and Life Sciences and the study's corresponding author. "But this research really flips that idea. We saw that deer are actually winning in this competitive interaction, not by fighting or outcompeting them for resources, but by spreading these parasites that moose are not as good at handling."
One parasite, called a brain or meningeal worm (Parelaphostrongylus tenuis), creates neurological damage in moose. Symptoms include paralysis, blindness and disorientation, all of which can lead to death. Deer are the natural hosts.
The other, called giant liver fluke (Fascioloides magna), is a flatworm that causes severe liver damage and increased mortality in moose. Natural hosts include deer, elk and caribou.
Both parasites have complicated life cycles. They reproduce in their natural hosts, are passed into the environment through feces and have stages in intermediate hosts: snails and slugs for brain worm and aquatic snails for giant liver fluke. The parasites spread when new hosts eat snails, slugs or infected wetland vegetation when they forage. While both parasites can infect moose, they typically fail to reproduce in moose as they do in deer.
Neither of these parasites infects humans.
In the study, the researchers set up remote cameras in a 1,500-square-mile study area in the northern Adirondacks and monitored them for two years, to understand how many deer and moose were there and their locations. They also sampled deer pellets in these areas and analyzed them to gauge the prevalence of each parasite.
The team then created a statistical model that produced maps of where parasites, deer and moose were most abundant and tested how moose were responding to both the deer and parasites. "We saw that moose weren't really responding to the densities of deer in an area, but dramatically shifted what areas they used depending on parasite abundances," Grauer said.
Due to habitat loss, moose were missing from New York state for over 100 years prior to the 1980s, when they naturally returned from neighboring states and Canada. "They have increased in numbers since then, but they haven't rebounded the way people had expected and are still at lower numbers and densities than we see in other northeastern states," Grauer said.
Grauer works in the lab of Angela Fuller, leader of the New York Cooperative Fish and Wildlife Research Unit, courtesy professor in the Department of Natural Resources and the Environment in CALS and a senior author of the study. Co-authors include Joshua P. Twining at Oregon State University; Jacqueline Frair and David Kramer at the SUNY College of Environmental Science and Forestry; and Manigandan Lejeune, associate professor of practice in the Department of Population Medicine and Diagnostic Sciences, and Krysten Schuler, associate research professor in the Department of Public and Ecosystem Health, both in the College of Veterinary Medicine at Cornell.
The work was funded by the New York State Department of Environmental Conservation and the U.S. Fish and Wildlife Service.
