A Day in the Field: Of Mice and Hantivirus

Buffalo Ranch Summer Day
NPS Photo - N. Herbert

from Yellowstone Science 25(1)

by Sarah Haas

Yellowstone’s northern range, undoubtedly one of the prime wildlife viewing spots in North America, harbors impressive bison herds, wolf packs, and meandering bears. Visitors are likely to depart from the park’s northern lands with at least one, if not more, checks on their wildlife card.

Beneath the awe of the large herds and charismatic megafauna lives a quiet, hidden life. The world of the bottom of the food chain is not as glamorous or appreciated as some of Yellowstone’s other attractions, but is no less as important. The small mammal communities of the northern range include species such as the deer mouse, bushy-tailed woodrat, and several kinds of voles and shrews. For every bison or wolf, there are probably thousands of these small animals, running over the landscape and providing a whole slew of ecological services: soil enrichment, predator nutrition, enhancing vegetative diversity, etc.

Some of my days in the field have been devoted to the large, captivating wildlife species that attract millions of visitors each year. In August 2015, I devoted a couple of days to learning about the wildlife that don’t receive much press, unless it is bad press. Accompanied by Jessica Richards of the park’s Wildlife Health Program, we spent one afternoon setting up live traps around the historic Lamar Buffalo Ranch to gather information on the prevalence of hantavirus in small mammal communities in that part of the park.

Rodents are natural reservoirs of hantaviruses, which can cause hantavirus pulmonary syndrome in humans, a rare but potentially deadly respiratory disease. Deer mice (Peromyscus maniculatus) are common in the park and a primary host for transmitting hantavirus, if they are infected. Many rodents, but especially deer mice and woodrats, take advantage of human structures such as barns and old cabins to gain protection from predators and improved shelter opportunities. The Lamar Buffalo Ranch was an ideal location to trap and assess the rodents living in the area.

Jessica Richards at work
Sherman live traps are set against travel corridors of rodents around the barn at Lamar Buffalo Ranch. Jessica Richards weighs and checks the reproductive status of a deer mouse to assess the health and condition of each individual

NPS Photos - S. Haas

Using peanut butter and oats, along with some cotton to keep the rodents warm once inside the traps, we laid out a grid of 120 traps surrounding the main barn and corrals. The next morning we carefully gathered up the traps, noting which traps were empty and which held a resident. Over 75% of the traps caught a rodent…sometimes two in one trap! Processing that many individuals was a marathon, and once started we did not stop until all the traps were empty and every capture was released back into the wild.

Jess Richards was the conductor of the processing operation, running the program with the skill and speed that only comes with handling thousands of small mammals. Pat Strong, another volunteer, and I provided back-up logistics: data recording, equipment preparation, trap stacking and rearranging, and finally releasing the rodents in the surrounding sagebrush.

From start to finish, nearly 3 hours passed between the first capture and mouse #92. All were deer mice, with the exception of one montane vole. Each rodent was checked for age, sex, reproductive status, and weight. Ear tags were placed on each individual, unless already present from a previous capture operation; a small blood sample was collected; and the animal was released.

Why the need for all this data? Hantavirus is a zoonotic disease, meaning it can pass from animals to humans. Rodents carry and can transmit the virus without detrimental impacts to themselves. For a human, however, infection can be fatal. The primary way a human can be infected is through inhaling airborne dust particles contaminated with saliva, urine, and/or feces from infected rodents. Early signs of infection include flu-like symptoms (fatigue, fever, aches, abdominal pain, nausea, etc.). The first symptoms usually develop within 5 weeks of exposure, with additional symptoms such as coughing and shortness of breath appearing shortly after.

Hantavirus is an important disease to understand due to the high prevalence of the virus in rodents such as deer mice (approximately 10-15%, with some areas far exceeding that rate), a species that is often found near human habitation. Understanding the relationship between zoonotic diseases and human-wildlife interactions is a key component of the National Park Service Wildlife Health Program. Although hantavirus is a disease to be taken seriously, the answer is not to eliminate all rodents from the planet, but rather to learn to live with them wisely. This can be especially critical in national parks, where the mission is to preserve ecosystems, including components that could cause human harm. Wise cohabitation with rodents leads to putting up a healthy distance between our living spaces and theirs, including keeping households and buildings safe, clean, and well maintained to exclude rodent entry.

There has fortunately never been a documented case of humans contracting hantavirus at Yellowstone; how- ever, an outbreak in Yosemite National Park in 2012 that resulted in the deaths of three visitors was a reminder of the presence and power of this virus in our environment. Most visitors who come to Yellowstone may never encounter a rodent such as a deer mouse, or even their diurnal and more respected cousins the squirrels, and probably leave the park with no sense of missing out. Perhaps the human-wildlife dangers in Yellowstone most people envision (bear attacks, bison goring, elk charging, etc.) keep the mind focused on the megafauna that dominate the landscape. When visitors return home, they keep the images and memories of their experiences in play, retelling stories of the large herds and sharp-clawed creatures they observed. Meanwhile, the mice that make up the food chain dance around our houses and whisper stories of their own—how they stole our scraps and made cushions out of our leftovers. Learn about these creatures and what they bring to our environment, both desirable and unwanted characteristics alike.

To learn more about the National Park Service Wildlife Health Program visit: www.nature.nps.gov/biology/wildlifehealth/
Hantivirus protection

Sarah Haas is the Science Program Coordinator at the Yellowstone Center for Resources. Her favorite rodent is the Utah prairie dog, a species she grew to know and admire while working at Bryce Canyon National Park.

Last updated: February 5, 2018