Brucellosis: Crying Wolf?

Just north and outside of Yellowstone National Park, deep snow covers the ground and bitter March winds blow through the pines. Because of the adverse winter conditions in the Park, the bison have had to seek better grazing. Following the easily traveled snowmobile trails, they have wandered out of the harsher conditions of the late winter park.  Using their heads to plow away snow to uncover the buried grass, a small group of bison graze.   Only the crunching of the snow under their hooves, the occasional snort of exhaled breath, and their rhythmic breathing along with the occasional bird song and the moaning of the pines swaying in the wind can be heard.  Suddenly, a shot rings out!  A bison cow goes down!  Then another shot and another!  Two more cows are hit.  One stumbles and then drops.  The other runs back toward the park bleeding profusely.  The rest have already bolted for the safety of Yellowstone.  The wounded one makes it back to within the confines of the park, but has stopped, exhausted, drained.  She tries to take another step in the deep snow, but cannot.  She lies down and slowly dies.  No one comes for her.  A few days later, her scavenged carcass will be found.  Back at the killing grounds, the hunters are celebrating.  They have thwarted another incursion of the feared bison onto the bison’s natural grazing habitat.

            So why fear the bison outside of Yellowstone Park?  They do not attack humans or other animals unless they feel threatened. They do not devastate the land as they graze. So why such hostility toward them as illustrated above, which is a fictionalized account of what has happened and continues to happen.  Bison roaming outside of the Park into Montana are either hazed back into the park or killed.  Hazing involves rounding up the bison and herding them back into the boundaries of Yellowstone or into a quarantine pen, which inherently causes trauma for them.  But herding of bison is difficult.  Killing is easier.  But why slaughter an animal carrying the last known wild bison genome?

            The Montana ranchers claim bison threaten the cattle herds with the disease brucellosis.  Brucellosis, a bacterial infection (B. abortus), causes abortions and still-births in cattle, bison, elk, bear, deer, etc.  If a brucellosis outbreak would occur and spread through a cattle herd, financial devastation could result for the rancher.  As a side note, any losses, ironically, would be covered by government subsidies. 

            Upon closer examination of the circumstances and history of brucellosis in bison, the claim of the ranchers and the Montana Livestock industry strike one as a smoke screen.  One would think that science should be driving the claim, but politics and profits seem to be the drivers.  The USDA has threatened the state of Montana with the loss of Montana’s brucellosis-free status, which permits shipment of untested cattle across state lines.  The loss of such status means the ranchers would have to pay to have the cattle tested prior to shipment, which incurs additional costs. To ensure no risk of cattle infection the eradication of brucellosis has taken on a zero tolerance approach, giving absolute priority to achieving a zero risk level for cattle infection [1].  In addition, since almost no brucellosis can be found in American cattle herds, the USDA has expanded its efforts to eradicate the disease. The US Dept. of Agriculture has turned its attention to eradication of brucellosis in wild animals.  Thus, agencies whose funding depends on the USDA are recruited into the cause [2].  The eradication of brucellosis has become politicized with agencies increasing their power and funding by joining the crusade to rid the wild animal population of brucellosis.  The crusade, of course, includes the method of slaughter as well as vaccination.  However, the slaughter only seems to be restricted to bison; other wild life are not included. The slaughter of the Yellowstone bison arose, then, out of fear and political expediency, not science.

            Brucellosis probably arrived with infected cattle imported by Spanish settlers into Mexico.  Sometime prior to 1917 infected cattle were introduced into the Greater Yellowstone Area [3][4] with the disease spreading to the bison.  But the transmission of brucellosis from bison to cattle has never been established.  The actual transmission path would have to be through the ingestion of infected birthing materials or from an aborted fetus.  Thus, bison bulls, calves, yearlings and non-pregnant bison cows would not pose a threat. Furthermore, infected pregnant bison females will only pass infected material in her first pregnancy since after the first pregnancy, the uterus develops protection, preventing infected material being shed in subsequent births [5]. 

In order for brucellosis to be passed on to cattle cows, the cattle cows would have to lick or eat the discharged reproductive debris.  But bison abortions or still-births are rare.  If they do happen, the most likely time is winter when cattle are not present.  Cattle are unable to withstand bitter winters and are not grazed near Yellowstone until June.  By this time, the bison would have returned to the park [6].  But even should there be aborted or birth materials in June or later, the brucellosis bacteria cannot survive warm weather or direct exposure to sunlight.  Besides, predators and scavengers would all but guarantee fetuses or infected reproductive material would not persist beyond mid-May [7]. Thus, the possibility of cattle cows contacting infected material is remote.

Recent studies have shown that bison are not the primary reservoir of the brucellosis bacteria.  The bison were once considered to be the primary source because of high levels of B. abortus.  But examining the extent and transmission characteristics in other wildlife has shown elk to be the primary source of B. abortus.  Even though elk have a lower level of B. abortus, they are more numerous and widespread than bison.  Bison rarely move outside of conservation areas and are subject to rigorous management practices that limit migration, making comingling with cattle almost impossible.  In contrast, elk are allowed to freely range, and make long-distance migrations between summer and winter grounds, increasing the probability of contact with livestock.  Genetic studies have confirmed this, demonstrating elk to be the likely source of infections, not bison [8].  This begs the question: Why are elk, the primary source of brucellosis infections, allowed to roam free and bison are not?  The revenue generated from elk hunting may have something to do with it.  Revenue from elk hunting activities amounts to approximately $11 million annually for the state of Montana alone [9].

            Hazing and killing of bison that have roamed outside of YNP are not the only methods employed to placate the fears of the ranchers. With the state of Montana and the USDA insisting, bison are periodically rounded-up and tested.  About 45% of the bison test positive for long-term antibodies.  Using a positive test as proof of infection, the bison is slaughtered.  But another test is performed after the bison has been killed to determine if actual bacteria are present.  Comparing these two tests reveals a drastic difference.  Very few slaughtered bison have the actual bacteria.  The bison, then, are being slaughtered simply because they have developed immunity to brucellosis [10]. The control of brucellosis is a “no holds barred” approach when it comes to bison. Of course, the slaughter includes both bulls and cows. But why slaughter the bulls who cannot transmit the disease, and are essential for breeding? The logic to the slaughter remains elusive unless it is understood as arising from an unjustifiable fear.

            In response to the brucellosis eradication efforts in bison a number of viable solutions have been offered.  The Buffalo Field Campaign ( has offered several. These include:

  • Develop herd management plans that adjust cattle grazing dates which would eliminate transmission potential;
  • The Greater Yellowstone Area could be exempted from the OIE certification process by keeping cattle out of the area.
  • The state of Montana could develop risk management strategies for domestic cattle that allow free roaming bison [11].

            In light of the evidence–genetic studies, the migration and grazing habits of bison, the elk herds as a primary brucellosis reservoir, the transmission dynamics, and the testing results—and the offered solutions, why does the slaughter of the bison continue? Based on the above, it appears to be a result of the efforts of the livestock industry in Montana to maintain political control, using the fear and the paranoia over brucellosis as the means to maintain that control.

[1] Lott, Dale F. 2002. American Bison: A Natural History. University of California Press. Berkely, CA. 111

[2] Callenbach, Ernest. 1996. Bring Back the Buffalo: A Sustainable Future for America’s Great Plains. Berkeley. University of California Press. 281-283. These actions of the USDA continue to this day.  Confirmed by Darrell Giest, Buffalo Field Campaign, in an email to the author dated 15-April-2019.

[3] Callenbach. 134-139

[4] Kamath, Pauline L., Foster, Jeffery T., et. al. Genomics reveals historic and contemporary transmission dynamics of a bacterial disease among wildlife and livestock. Nature Communications. 11-May-2016.

[5] Yellowstone Bison and Brucellosis: Persistent Mythology.  Retrieved from (3/30/19).

[6] Callenbach. 280.

[7] Yellowstone Bison and Brucellosis: Persistent Mythology.

[8] Kamath.

[9] Willcox, Louisa. 8 March 2017. The Last Stand for Yellowstone’s Bison.  Retrieved from

[10] Yellowstone Bison and Brucellosis: Persistent Mythology.

[11] Yellowstone Bison and Brucellosis: Persistent Mythology.

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