Q: Are bison allowed outside Yellowstone National Park now?
A: Since 1985, few bison have been tolerated outside the park. Bison can be a threat to human safety and can damage fences, crops, landscaping, and other private property. Of more concern, however, is the threat of a disease called brucellosis.

Q: What is brucellosis?
A: Brucellosis is a disease caused by the bacterium Brucella abortus, which came to North America centuries ago in European cattle.

Q: How does it affect the animal? 
A: Brucellosis can cause pregnant cattle, elk, and bison to abort their first calves after becoming infected. They will likely bear healthy calves in subsequent pregnancies. No cure exists for it, but it is not fatal.

Q: Can humans contract brucellosis?
A: In humans, the disease is called undulant fever or Bang’s disease. Although rare in the United States, humans can contract brucellosis by consuming unpasteurized, infected milk products or by contacting infected birth tissue. It cannot be contracted by eating cooked meat from an infected animal. Milk pasteurization eliminated the greatest threat to humans; today the primary risk is to hunters, large-animal veterinarians, and others who might handle reproductive tissues of an infected animal. If infected, people can be treated effectively with antibiotics.

Q: Why is brucellosis considered a big threat?
A:  Although the threat to humans is greatly reduced, the U.S. Department of Agriculture (USDA), through the Animal Plant Health Inspection Service (APHIS), continues efforts to eradicate brucellosis because of its risk to livestock. Ranchers stand to lose thousands, perhaps millions, of dollars if their cattle herds become infected. If the infected animals are traded, sold, or otherwise transported from their original herds, they can spread the disease across the country. To prevent such spread, infected individual cattle are killed, and the entire herd may be slaughtered.

Q: How is brucellosis transmitted?
A: Brucellosis is transmitted primarily when uninfected, susceptible animals come into direct contact with infected birth material. Because of this, females are the animals of concern with brucellosis; if they have an active infection, they can transmit the disease through their reproductive organs and milk. Other animals can pick up the disease at birthing time when they consume afterbirth. However, male animals are unlikely to transmit the bacteria to other animals. If the infection becomes chronic in a herd over a period of years, many of the animals develop an immunity that enables them to clear the bacteria from their bodies.

Q: How long does the bacteria persist in a natural setting?
A: Field testing has proven that brucella remains in the environment less than 30 days. The colder and darker the environment, the longer the bacteria remain viable. As long as bison and cattle are separated by several weeks, little risk exists of transmitting the bacteria between them.

Q: What is the risk of bison transmitting brucellosis to cattle?
A: The National Academy of Sciences states the risk is close to zero. Bison have a very low probability of transmitting brucellosis to cattle under natural conditions, primarily because management strategies prevent bison from commingling with cattle.

Q: Can bison or cattle be vaccinated against brucellosis?
A: All cattle that use overlapping ranges with bison are vaccinated for brucellosis when they are calves. The cattle vaccine for brucellosis, RB 51, is safe for bison calves and yearlings. When winter and capture conditions allow, bison calves and yearlings are vaccinated if they have been captured, tested negative for brucellosis exposure, and are being released. The National Park Service is considering vaccinating bison in the field, using methods that do not require handling individual bison. Because scientists now know more about bison movement patterns, group dynamics, and habitat distribution, they better understand where and when field vaccination could succeed.

Q: Have bison always had brucellosis?
A: No. Brucellosis was discovered in Yellowstone bison in 1917. They probably contracted the disease from domestic cattle raised in the park to provide milk and meat for visitors.

Q: How many bison have the disease?
A: About 50 percent of the park’s bison test positive for exposure to the brucella organism. Testing positive for exposure (seropositive) means the animal has been exposed to the bacteria at some time in its life; it doesn’t mean the infection is active or contagious. Currently, the only way to confirm an active infection is to test the tissue of a dead animal. These tests reveal that less than half of seropositive female bison were shown to have the infection.

Q: Do bison react to brucellosis the same as cattle?
A: Current information shows both species exhibit very similar clinical signs of brucellosis infection and very similar methods for transmitting the disease to other individuals. However, a scientific review of published and unpublished data indicates bison differ from cattle in their response to vaccines and possibly to standard testing for the disease. Studies are being conducted on wild bison to better understand the bison-brucella relationship, and to study these other questions.

Q: Do other wild animals carry brucellosis?
A: Yes. Brucellosis is present to varying degrees in the elk that live in the greater Yellowstone area. Elk populations on native winter ranges have seropositive rates of 0.5 to nearly 20 percent. Elk that use feeding grounds in the greater Yellowstone area have seropositive rates of 17 to 35 percent. Other animals carry the disease but are not considered a disease threat to cattle.

Q: What is “brucellosis class-free” status?
A:States that eradicate brucellosis from cattle receive “brucellosis class-free” status and can export livestock without restrictions and costly disease testing. Brucellosis infections in two cattle herds during a 24-month time period would downgrade a state’s status to Class A, and adversely affect the finances of ranchers. When one cow in a cattle herd becomes infected with brucellosis, the herd is quarantined and may be slaughtered to eliminate the infection. Federal and state indemnity funds partially compensate the livestock producer for this loss.

Q: Do the states around Yellowstone have this status?
A: Idaho and Wyoming have regained their brucellosis class-free status after losing it a few years ago when cattle herds were found to be infected with brucellosis. Montana was downgraded to Class A status in 2008 after two cattle herds became infected (one in 2007 and a second in 2008). The state can petition to regain its class-free status after one year, and after a scientific review of the risk and extensive testing of cattle close to the infected herds are completed. The U.S. Department of Agriculture will review the case and determine when to re-classify Montana as brucellosis free.

Q: How did cattle in these states become infected?
A: Elk are the suspected vectors in all three states, but the evidence varies. Investigators have genetic evidence that wildlife transmitted the disease in the second infection. Bison are not in that area, so elk are the assumed transmitters. Where the elk came from, when and how the disease was transmitted, and other details are still being investigated. But the transmission occurred in the Greater Yellowstone Ecosystem, which leads some people to believe bison are ultimately responsible for spreading the disease to cattle.