Yellowstone moose are more vulnerable to tick-borne disease because it’s warmer

Warmer summers in the United States have led to a higher prevalence of ticks on the moose herd, making them more vulnerable to bacterial diseases such as Rocky Mountain spotted fever, according to a recent study published in the Journal of Wildlife Management.

The study — conducted by Montana State University biologists and the National Institute of Occupational Safety and Health — collected blood samples and examined ticks from 1,418 moose living near Yellowstone National Park and 250 in surrounding areas of northeastern Montana. It found that 55 percent of the moose had at least one species of tick during the warmer summer. In the earlier study, researchers found 44 percent of the animals had at least one tick.

The prevalence of Rocky Mountain spotted fever in moose calves, or pups, is about 15 times greater in the warmer summer compared with that of cooler summer, according to the study.

The tick that spreads Rocky Mountain spotted fever, the tick Aeromonas hydrophila, is known to spread the disease by spitting up infectious flakes before biting the affected animal. Since the rash-causing flu-like symptoms of the disease are usually present before the arthropod vector ever lays its eggs, the eventual attack could take days or weeks.

“These numbers indicate that several species of tick can carry a potential tick-borne infection,” lead author David Dungruang told the Spokesman-Review. “That means a person or animal could become infected and not know it.”

The study also found that tick abundance, in addition to being lower in the colder summer, was linked to the older age of moose in the lower half of the population — 11 to 13 years old.

Moose are a key target for tick-borne disease since they live close to people and other animals.

“The challenge is trying to take an animal that’s protected in winter and put it in a place it’s really hard to move,” study co-author Emalee Currier told The Washington Post. She is a registered nurse at the Montana Public Health Department.

The “crowded and thin” moose population in these areas is a particular cause for concern.

“Moose are wild animals with an important role in the ecosystem,” Currier said. “As the moose’s demographic has declined, their ability to effectively regulate the population and the health of the ecosystem is diminished.”

The prevailing theory is that they were cut off from the carcasses of wolves, which would pick up ticks during their winter food source — moose. The scientists also noted that Yellowstone National Park is one of the highest-elevation areas with very little snow and hosts a particularly good winter survival rate for moose.

But with several species of ticks presenting diseases, including Rocky Mountain spotted fever, the risk remains high.

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