By Ashley Booth
Wind sweeps down off the mountaintops and ears twitch, legs tremble, tails shake. The air is cold and with the wind comes the smell of snow. Not the snow of warmer climes where the temperatures hover at freezing and snow masks the ground for a few days. No. This is a grass-covering, path-blocking, death-to-all-who-remain kind of snow. This first snowfall means that it is time to move. From high in the peaks of the Greater Yellowstone Ecosystem (GYE), thousands on thousands of elk migrate to lower elevations where the snow is not as deep, where the food is easier to reach, where the wind is not as cold. For many of the migrating female elk, the valleys amongst the towering Rockies will also become a birthing ground as spring approaches. However, for the unlucky pregnant females that carry a disease called Brucellosis, these valleys are a place where fetuses are lost. It is then, when abortions occur and diseased tissue is left behind, that the great elk migration becomes a problem for the cattle that live and graze on these lands.
Brucellosis, caused by the bacteria Brucella abortus, is a disease that leads to late-term abortions in elk, bison, and cattle, all of which are present in the Yellowstone area in high numbers. Transmitted through contact with aborted fetuses, the danger of infection is especially prominent in spring when abortion rates are high and cattle are released onto the same lands where elk remain after winter feeding has ended.
In most of the United States, should a single cow in a herd of cattle become infected, the entire herd must be slaughtered to prevent the spread of the disease. However, because of the relatively high prevalence of Brucellosis in the GYE, affected herds within this area can instead be quarantined, tested, and released after achieving disease-free status. Brucellosis can also, albeit rarely, be transmitted to humans through contact with an aborted fetus.
To determine a way to mitigate this growing risk to humans and livestock, Dr. Jerod Merkle, a postdoc with the Wyoming Cooperative Fish and Wildlife Research Unit at the University of Wyoming, is using historical data on food availability, behavior, and herd movement to develop a software model that will be able to forecast elk migration patterns and Brucellosis transmission risk. By predicting when and where elk herds will migrate and overwinter, producers will be able to adjust management strategies to keep cattle herds separate from elk and reduce the possibility of infection.
In order to evaluate efforts to manage the prevalence of Brucellosis in elk and develop a model of the potential for interaction and disease transmission between cattle and elk populations, Merkle and his colleagues examined GPS data for 253 female elk over a 15 year period from 22 different winter feeding grounds. The more than seven million data points gathered from the migrating elk were then used to assess habitat usage, migratory pathways, and elk density. Prevalence of Brucellosis among local elk populations was also measured.
Using information on disease prevalence and elk density, Merkle’s colleagues from Wyoming Game and Fish and USGS assessed the effectiveness of disease management strategies used on elk populations within the GYE. While the most logical option for managing Brucellosis infection within elk herds is vaccination, cattle vaccines do not appear to be effective in preventing the disease. An alternative method is implementing a low-density feeding strategy. During the snowier months when vegetation is scarce, wildlife managers provide food for elk herds on winter feeding grounds. This practice keeps elk off the nearby, privately maintained pastures, but the high densities of elk found on these feeding grounds can lead to an increase in disease. However, according to the group’s research, creating a feeding pattern that allows elk to spread out reduces individual contact with aborted fetuses, thereby reducing Brucellosis transmission and prevalence within elk populations.
From movement data, Merkle concluded that elk migrating out of high elevations clearly demonstrate partiality when selecting a winter range. Preferred habitat in winter is close to a feeding ground, on a south-facing slope at higher elevations, and in an area with greener vegetation and less snow. However, all of these preferences can vary each year based on weather throughout the region. In drought years, elk are more spread out and migrate back to higher elevations earlier, lessening the risk of transmission from infected elk to cattle. In years with greater snowfall, elk are concentrated primarily on the winter feeding grounds close to cattle pastures, increasing the chance of interaction and disease transmission between elk and cattle herds
Over the past 20 years, prevalence of Brucellosis has been increasing in elk populations throughout the GYE. While the reason for the increase is, as yet, unclear, higher elk densities at feeding grounds have been associated with a higher prevalence of disease. A similar increase in infection among cattle populations has been linked to increased contact with migrating elk, primarily on winter feeding grounds. While implementing low-density feeding to reduce elk infection rates appears promising, the real challenge is managing the interactions between elk and cattle to minimize transmission of Brucellosis to cattle herds. To that end, Merkle is continuing to develop a model that will be able to predict where elk will be on the landscape at any given point in time. This model, conjoined with knowledge of the prevalence of Brucellosis in elk populations, will allow managers and producers to predict where elk will be on the landscape and alter where cattle will be turned out for summer grazing. If cattle herds can be kept separate from the elk until they return to their summer range at higher elevations, the risk of contracting Brucellosis will decrease.
As snow begins to melt in the valleys, grass peeks out from beneath its wintry blanket, and the food at the feeding grounds tapers off. Calves are born, temperatures rise, and the elk begin to move, chasing the snow up the hills and towards the sky from which it fell. Summer approaches and cattle reappear on the landscape where elk once were. As the grass grows and the weather warms, the danger of infection retreats with the elk until they reach their destination high in the mountains on the public lands where this journey started thousands of years ago.
Merkle presented this research at the 2016 annual meeting of the Society for Integrative and Comparative Biology in Portland, Oregon.