Brucellosis and Bison

Del Hensel
Mountain Man Bison Inc.

Reprinted from Bison World, volume 22, No. 3, July/August/September 1997 pp. 32-33


Brucellosis is a disease that occurs in bison and other livestock. It was common in the 1950’s but programs have nearly cleaned the disease out of the United States livestock population. Bison in the Yellowstone Park area continue to be a reservoir for the disease.

Brucellosis and Bison

Bison are rarely infected with brucellosis. The theory is that soon after the turn of the century when bison were rescued from near extinction and co-mingled with cattle, some bison acquired brucellosis from domestic cattle herds. Prior to the 1940s, brucellosis was prevalent in cattle and other domestic animals. After World War II the United States Department of Agriculture began an aggressive effort to eradicate brucellosis in all domestic animals. There were 124,000 cattle herds infected in 1957 and today there are 26. Millions of dollars have been spent and the USDA feels confident that brucellosis will be completely eradicated in domestic animals by December 31, 1998.

Brucellosis was eradicated from all the public bison herds (except Yellowstone) in the early 1980s and from all the private herds (except one) about that same time. The infected

private herd is in the process of eliminating the disease and expects to be brucellosis-free within the next few years. All bison shipped inter-state are tested for brucellosis prior to

being moved, (except in some brucellosis free states). There are well over 200,000 bison in the United States and only these two herds remain infected.

What is Brucellosis?

Brucellosis is a contagious disease, caused by various species of the genus Brucella that infect domestic animals, wildlife, and humans worldwide. Livestock are the primary hosts of Brucella but it can also affect dogs, horses, and other animals.

In ungulates, transmission typically occurs through ingestion. The incubation period varies widely depending on exposure dose, age, sex, etc. The organism typically localizes in the udder and/or the lymphatic system and reproductive tissue. Abortion is the characteristic sign of acute brucellosis. Other signs include retained placenta, infertility, reduced milk production, lameness, swollen joints, and swollen testicles. The organism is shed in aborted tissues, reproductive tissues, and discharges, especially just prior to, during, or soon after abortion or live birth. The organism also may be shed in milk for variable lengths of time. Some infected cattle, bison and elk intermittently shed the organism. There is no treatment or cure known for animals or humans infected with Brucella. Animals can overcome the clinical signs but may develop recurrent infections and thus they may be a source of exposure and possible infection for other animals.

In humans, brucellosis is known as undulant fever. Symptoms include recurring fever, muscle and joint aches, swollen testicles, headaches and nausea. Although insidious, undulant fever is rarely fatal. Humans may become infected with Brucella by contact with organisms through exposure to infected animals and their infectious tissues. Entry into the body is through ingestion, contact with mucus membranes (e.g., eyes), through an open wound or through intact skin. Those at highest risk for contracting the disease are livestock handlers, slaughter industry workers, veterinarians, people drinking unpasteurized milk and hunters field dressing animals. The risk is greatest when handling infected females during the last half of pregnancy. The risk can be substantially reduced if adequate precautions are taken-such as protective clothing, gloves, and washing thoroughly after exposure.

Undulant fever has become uncommon in humans (approx. 70 cases per year reported) but some feel that due to such low prevalence, the disease may be diagnosed improperly or go undetected. In Montana there have been two confirmed cases of hunters contracting undulant fever from elk.

There has now been a confirmed case of a horse contracting brucellosis from elk in the Jackson, Wyoming area. Brucellosis appears as fistulous withers in horses and may be more prevalent than expected. Infected horses must be euthanized.

Brucellosis and Yellowstone

According to the United States Department of Agriculture and many animal scientists, allowing brucellosis to proliferate in bison and elk in Yellowstone Park is definitely unacceptable. It is unhealthy to the animals afflicted, causing them great pain and suffering. It is dangerous to the humans, especially to hunters and researchers who come into contact with contaminated carcasses in the greater Yellowstone area. The greater Yellowstone area consists of much more than just Yellowstone Park. There is also the possibility of horses and other animals acquiring the disease.

Three years ago, the three states that surround Yellowstone, (Idaho, Montana and Wyoming) created a committee, "The Greater Yellowstone Interagency Brucellosis Committee" (GYIBC), consisting of all the affected state and federal agencies directly impacted by the bison and elk in the greater Yellowstone area. This committee has been meeting three to four times a year for three years and has made some significant decisions, the most important being to eliminate brucellosis in the greater Yellowstone area by 2010.

For the past several years the bison herd has been propagating at a tremendous rate. Prior to 1997, the population size numbered nearly 4,000 in an area that most feel can only support 2,000 or less. Many starving bison have been leaving the park to find food and have been killed by regulatory agencies to prevent the spread of brucellosis. Many more bison starve to death than are killed by humans. If the bison could not leave the park, many more would die in the park from starvation, because there is only so much vegetation, and supplemental feeding is not allowed in the park. The winter of 1996/1997 has been tremendously hard with early snows, then rain that created a frozen crust that animals could not break through. Estimates now place the herd at 1,200-1,500.

For the next several years Yellowstone will be back in a more normal situation regarding the bison population. With less population, fewer bison will want to leave the park and fewer will die of starvation. This will give the GYIBC some time to come up with a plan to eventually solve the brucellosis problem. With few bison being killed, the news media will go on to more exciting stories and we will temporarily forget all about the problem of the Yellowstone bison.

But if the bison population numbers are not controlled, the story will be back in the headlines again some day.

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