Remote Vaccination of Bison

Evaluation of Whether to Implement Remote Vaccination of Yellowstone Bison

Follow the park planning process for this issue.

National Park Service’s Decision

A Final Environmental Impact Statement was prepared to address a 2000 commitment by the National Park Service (NPS) to evaluate whether to vaccinate free-ranging bison inside Yellowstone National Park for the non-native disease brucellosis (caused by the bacteria Brucella abortus) using a rifle-delivered bullet with a vaccine payload.

The NPS has concluded that the implementation of remote vaccination at this time would not substantially suppress brucellosis in Yellowstone bison and could have unintended adverse effects to the bison population and reduce wildlife viewing opportunities due to:

  • Our limited understanding of bison immune responses to brucellosis suppression actions such as vaccination;
  • The absence of an easily distributed and highly effective vaccine (e.g., 10 to 15% reduction in infection; short duration of immune protection; cannot vaccinate females in second half of pregnancy);
  • Limitations of current diagnostic and vaccine delivery technologies (e.g., inconsistent vaccine hydrogel formulation; short rifle range; no rapid diagnostics for live animals);
  • Effects of bison nutrition, condition, and pregnancy/lactation during winter that lessen protective immune responses from vaccination;
  • Potential adverse consequences (e.g., injuries; changes in behavior) to wildlife and visitor experience from intrusive brucellosis suppression activities (e.g., capture; remote vaccination); and
  • Chronic infection in elk which are widely distributed and would almost certainly re-infect bison if brucellosis prevalence in bison was significantly reduced from current levels.

Therefore, the NPS will continue current management under the 2000 Interagency Bison Management Plans, as adjusted. This management plan will continue to conserve a large, wild, and genetically diverse population of bison and preserve important natural aspects and behaviors of this historic and iconic population by minimizing human intervention and avoiding unintended consequences that could result from remote vaccination. This management plan also includes an adaptive management process to answer uncertainties and make improvements, while protecting and preserving the historic, cultural, and natural resources of Yellowstone National Park.

This rationale was supported in February 2013 by an independent panel of vaccination and wildlife experts whose consensus conclusions included the “best available data do not support that remote vaccination of bison with the currently available vaccines will be an effective tool for suppressing brucellosis in wild bison to a level that changes the IBMP management strategies.” For the foreseeable future, the eradication of brucellosis is not feasible given current technology and chronic infection in elk, which are widely distributed throughout the greater Yellowstone area.

Background of the Project

After intensively managing bison numbers for 60 years through husbandry and culling, the Superintendent of Yellowstone National Park instituted a moratorium on removing ungulates (hoofed animals) within the park in 1969 and allowed their numbers to fluctuate in response to weather, predators, and resource limitations. Bison numbers increased rapidly under this policy and the population distribution expanded to include lower elevation winter ranges near the park boundary by the early 1980’s. Within the decade, large winter migrations out of the park and into the state of Montana began to occur routinely during more severe winters. Attempts to deter these movements or bait bison back into the park failed and about 3,100 bison were removed from the population during 1984 through 2000. These migrations and removals led to a series of conflicts among federal and state agencies, environmental groups, and livestock producers regarding issues of bison conservation and containment of the disease brucellosis, which some of the bison carried. As a result, the federal government and State of Montana agreed to a court-mediated Interagency Bison Management Plan in 2000 that established guidelines for managing the risk of brucellosis transmission from bison to cattle by implementing hazing, testing for brucellosis exposure, shipments of bison to domestic slaughter facilities, hunting outside Yellowstone National Park, vaccination, and other actions near the park boundary to limit population abundance and distribution (eg. dispersal in to the state of Montana).

The NPS agreed in the 2000 Record of Decision for the Interagency Bison Management Plan to evaluate a remote-delivery vaccination program for bison inside Yellowstone National Park. Remote delivery is distinguished from hand (syringe) delivery that currently occurs in capture pens near the park boundary because it would not involve the capture and handling of bison. The most feasible strategy for remote delivery of vaccine at this time is using an air-powered rifle to deliver an absorbable bullet with a vaccine payload that is freeze dried or photo-polymerized. The goal of vaccination is to deliver a low risk, effective vaccine to calf and female bison inside the park to (1) decrease the probability of individual bison shedding Brucella abortus, (2) lower the brucellosis infection rate of Yellowstone bison, and (3) reduce the risk of brucellosis transmission to cattle outside the park. The migration of bison across the park boundary onto essential winter ranges in Montana would be preserved to facilitate their long-term conservation.

A Draft Environmental Impact Statement on the Remote Vaccination Program to Reduce the Prevalence of Brucellosis in Yellowstone Bison was released for public review on 28 May 2010. Alternative A described the current hand vaccination program under the Interagency Bison Management Plan that has been implemented at capture facilities near the boundary of Yellowstone National Park. Alternative B described a combination of the hand vaccination program at capture facilities and a remote-delivery vaccination strategy that would focus exclusively on young, non-pregnant bison. Alternative C described a combination of the hand vaccination program at capture facilities and remote-delivery vaccination of young, non-pregnant bison and adult females within the park prior to mid-gestation. For each alternative, the NPS analyzed potential environmental impacts divided into the following categories: Yellowstone bison population; other wildlife; threatened, endangered and sensitive species; ethnographic resources; human health and safety; visitor use and experience; and park operations.

Importance of Yellowstone Bison

The plains bison, also commonly known as buffalo, once numbered in the millions and ranged across much of North America, from arid grasslands in northern Mexico, through the Great Plains and Rocky Mountains into southern Canada, and eastward (south of the Great Lakes) to the western boundary of the Appalachian Mountains. Perhaps no other wildlife species is as symbolic of the American experience since bison were an inherent part of the spirit and culture of many native peoples and central to national expansion and development. Only a few hundred bison survived commercial hunting and slaughter during the mid- to late-1800s, with the newly established (1872) Yellowstone National Park providing refuge to the only relict wild and free-ranging herd (less than 25 animals). This predicament led to one of the first movements to save an endangered species and develop a national conservation ethic by a few visionary individuals, American Indian tribes, the American Bison Society, the Bronx Zoo, and federal and state governments. Bison numbers rapidly began to increase after protection from hunting and poaching, husbandry, and relocation, and today, there are more than 400,000 plains bison in North America.

Despite this success, several scientists recently concluded that plains bison are ecologically extinct across North America because less than 4 percent (%) are in herds managed primarily for conservation and less than 2% have no evidence of genes from inter-breeding with cattle. Instead, most bison are raised for meat production, mixed with cattle genes, protected from predators, confined in fenced pastures, and their mating structures are inhibited by low ratios of adult males in order to maximize offspring production. As a result, bison no longer influence the landscape as they once did by roaming across large areas while enhancing nutrient cycling, competing with other ungulates (hoofed animals), creating wallows (i.e., depressions in soil) when they roll on their backs to give themselves dust baths, and serving as a major converter of grass to animal matter.

Bison are massive animals that compete directly with humans and livestock for use of the landscape. Their preferred habitats include nutrient-rich valley bottoms where agriculture and development occupy most of the land, while public lands are more likely to be situated in mountainous areas above these valleys. Given existing habitat loss and the constraints modern society has placed on the distribution of wild bison, it is unlikely that many additional populations will be established and allowed to range across the landscape. Thus, the few remaining wild and free-ranging bison populations in North America are very important.

Yellowstone bison comprise the largest (2,400 to 5,000) wild population of plains bison and are one of only a few populations to continuously occupy portions of their current distribution. They are managed as wildlife in multiple large herds that move across an extensive landscape (more than 150,000 hectares or 372,000 acres) they share with a full suite of native ungulates and predators, while being exposed to natural selection factors such as competition for food and mates, predation, and survival under substantial environmental variability. As a result, these bison likely have important adaptive capabilities compared to most bison populations that are managed like livestock with forced seasonal movements among fenced pastures, few predators, and selective culling for age and sex classifications that facilitate easier management (e.g., fewer adult bulls). These bison also provide meat for predators, scavengers, and decomposers; contribute to nutrient recycling that enhances plant production and diversity; and allow visitors to observe this keystone species and symbol of the American frontier.


Brucellosis is a contagious disease caused by the bacteria Brucella abortus that was introduced into wild bison and elk in the greater Yellowstone area by domestic cattle in the early 1900s. This non-native disease can induce abortions or the birth of non-viable calves in livestock and wildlife. When livestock are infected, there is economic loss to producers from slaughtering infected animals, increased brucellosis testing requirements, and possibly, decreased marketability of their cattle. Brucellosis has been eradicated in cattle herds across most of the United States, with the exception of occasional outbreaks in the greater Yellowstone ecosystem where bison and elk persist as one of the last reservoirs of infection. Approximately 40 to 60% of Yellowstone bison have been exposed to Brucella abortus bacteria.

Vaccination of bison and cattle was included in the IBMP as the primary method to decrease brucellosis transmission, and over time, its prevalence in Yellowstone bison. Cattle operations in the Gardiner and Hebgen basins of Montana vaccinate all calves and most adults with vaccine strain RB51, which is the best available vaccine. Strain RB51 vaccine reduces the probability that abortions and further transmissions occur when vaccinated cattle subsequently become exposed to, and infected with, Brucella bacteria. However, most vaccinated cattle will become infected after exposure to infectious amounts of Brucella deposited by infected bison or elk. Thus, vaccinating cattle with strain RB51 will not eliminate the potential for brucellosis infection from wildlife.

Similarly, strain RB51 vaccine is not effective at preventing bison from becoming infected following exposure to sufficient doses of Brucella bacteria. In experiments, less than 15% of vaccinated bison exhibited protection against infection following exposure to Brucella bacteria. However, strain RB51 vaccine is moderately effective (50-60%) at reducing abortions and further transmissions by bison, including to cattle. Therefore, the primary reason for vaccinating bison would be to reduce the shedding of Brucella bacteria, and thereby the potential for further transmission, after individuals become infected.

Efforts to reduce the prevalence of brucellosis in bison using vaccination would be most effective through a region-wide effort that consistently and reliably delivers vaccine to most bison each year over decades. The most effective way to vaccinate bison is with a syringe because bison receive the intended dose in the correct site (just under the skin). Also, vaccinated animals can be marked to facilitate monitoring of protective immune responses and reproductive events. Optimally, vaccine delivery should occur in autumn, at least 12 to 16 weeks before potential exposure to Brucella bacteria in late February or March, to develop a protective immune response. However, bison mating behavior, migration patterns, and hunting seasons make it difficult to vaccinate sufficient females at this optimal time period each year to have a lasting effect on brucellosis suppression. During the summer bison congregate for the rut (breeding season) and relatively large groups are sustained through autumn. Breeding behavior and larger groups make bison more difficult to approach and vaccinate. Also, even in winters with severe snow pack, less than 50% of the bison in the population migrate to the boundary, where capture facilities are located. Most migrants tend to move to the boundary during late winter when pregnant females are late in gestation, their body condition is still declining and their stored energy reserves are being used to survive and produce the offspring. Thus, they should not be vaccinated at this time because they are least likely to produce adequate immune response to fend against any future exposure to the bacteria.

Approaches that target pre-reproductive females for vaccination, while removing reproductively active, likely infectious females could potentially be effective at reducing brucellosis transmission by focusing on methods to reduce shedding of the bacteria. However, to selectively treat (e.g., vaccination, culling) 50 to 100 pre-reproductive females and 50 to 100 likely infectious females in a given year, would require the capture and testing of more than 650 bison—which is more bison than often migrate to either the northern or western boundaries. More bison would need to be captured each year to meet minimum targets for effectiveness if the prevalence of brucellosis was suppressed, resulting in a limited decline in population seroprevalence. Vaccinated bison would need to be held in the capture pen for 21-days during hunting seasons due to concerns about consumption before the vaccine has been cleared from the animal’s system. In addition, vaccination of most females is not possible after hunting ceases in March/April as previously noted. Thus, vaccinating bison only at capture facilities is unlikely to achieve considerable, sustained decreases in brucellosis transmission or prevalence.

Delivering vaccine remotely using biobullets, darts, or bait is possible. However, oral and aerosol delivery mechanisms are not feasible at this time due to the uncertainty regarding their effectiveness to deliver a consistent recommended dosage to the intended portion of the bison population, while avoiding delivery to unintended bison and other species. In addition, the use of darts containing live Brucella abortus vaccine was considered but determined not feasible because of the liability to visitors and other wildlife from lost darts left about the landscape. The effective range of biobullet or dart delivery via air rifle is approximately 30 to 40 meters, which is ineffective for reaching bison inside the perimeter of a relatively large group. Also, it is uncertain whether each treated animal receives the intended dose, and there is no way to ever know because animals are not marked and subsequently tested for immune reaction. Furthermore, there are recurrent issues with biobullet vaccine formulation and encapsulation, projectiles fracturing or being too soft to penetrate the skin, and poor immunologic proliferation. As a result, it is difficult to estimate the portion of the population that is effectively vaccinated. In addition, capture and handling and remote vaccination are likely unpleasant experiences for bison. Therefore, they may begin to avoid humans and, as a result, it will probably become more and more difficult to vaccinate a large portion of the population.

The duration of vaccine-induced immune protection appears to be relatively short rather than life-long. Thus, booster vaccination of bison would likely be necessary. Furthermore, the extent of protective immune responses stimulated by vaccination may be reduced when vaccines are delivered to undernourished bison during winter. Similar to other ungulates in this northern mountain environment, bison are chronically undernourished by late winter due to the limited availability of relatively low quality forage (grasses and grass-like plants), most of which is senescent (i.e., old or dead) and covered by snow. This seasonally poor body condition and nutrition increases the vulnerability of bison to attack or reemergence of infections and coincides with increasing reproductive demands during late pregnancy that curb the resources bison can allocate to immune defense. Energy and protein reserves are too limited to simultaneously invest in both efforts, and as a result, immune responses are often suppressed during pregnancy. As a result, the vaccination of wild bison during winter may be less effective against brucellosis than suggested by the results of captive, experimental studies due to chronic under-nutrition and the nutritional demands of pregnancy and lactation that suppress immune responses.

For the reasons outlined above, there are numerous uncertainties and issues with vaccination that make it unlikely to achieve desired results and could have unintended adverse effects to the bison population and visitor experience. Key uncertainties include: 1) how effective will the vaccine be following delivery (duration and extent of immune response); 2) how many bison need to be vaccinated each year and is this effort sustainable; 3) can sufficient bison be captured for vaccine delivery via syringe or will remote delivery (i.e., without capture of the animal; biobullet, dart) be necessary; and 4) how will bison behavior change in response to vaccine delivery following capture or remote delivery? Therefore, a vaccination program for bison would be controversial, expensive, intrusive, and logistically challenging, with no guarantee of successfully reducing brucellosis prevalence near zero. Moreover, an effective vaccination program requires that all possible routes of re-infection be evaluated, treated, or effectively separated from the vaccinated population. In the past decade, brucellosis prevalence in some elk populations in the greater Yellowstone area has increased up to 20%, independent of Yellowstone bison, and all detected transmissions of brucellosis to cattle have been traced to elk. Thus, the potential for elk to maintain the disease and re-infect susceptible bison cannot be ignored if brucellosis prevalence in bison is significantly reduced from current levels.

Even if brucellosis prevalence could be reduced by 50% (i.e., to about 30%), which would be quite difficult to achieve given current technology and conditions, such a change would not have any significant effect on bison management practices or the risk of brucellosis transmission from bison to cattle, which is already extremely low compared to elk. Bison would still need to be managed to maintain separation with cattle and mitigate human safety and property issues. Testing requirements for livestock producers within the greater Yellowstone area would not change because elk would remain a far greater threat of brucellosis transmission to livestock than bison. There have been zero incidences of Yellowstone bison transmitting brucellosis to cattle, while at least 23 cattle and domestic bison herds have been infected with brucellosis by elk since 2002. The states in the greater Yellowstone area have not lost their class-free status in recent years despite multiple brucellosis outbreaks in cattle and domestic bison herds due to transmission by elk. Thus, there is no reason these states should lose their class-free status if there is one or more outbreaks due to transmission from wild bison.

Moreover, the implementation of aggressive, intrusive actions to suppress brucellosis in bison, while not taking similar actions to address increasing prevalence in elk across the greater Yellowstone area, is difficult to justify given the high costs and values that many residents, visitors, and tribal interests have towards bison. A reduction in brucellosis prevalence in Yellowstone bison will have little to no effect on the risk of brucellosis transmission to cattle if the prevalence of brucellosis in elk is stable or increasing throughout the greater Yellowstone area and substantial, region-wide actions are not taken to prevent comingling of elk and cattle during the elk abortion and calving season (which overlaps with cattle occupancy on private lands and with cattle turn-on dates throughout the area).


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