Monitoring, Treating Brucellosis in Bison
This bison is part of a 13-head herd involved in a brucellosis vaccine study at
the National Animal Disease Center in Ames, Iowa.
A new chapter is being written in the history of brucellosisa disease
that costs U.S. beef and dairy farmers about $30 million a year.
In a multiagency effort, scientists from USDA's Animal and Plant Health
Inspection Service (APHIS) and Agricultural
Research Service (ARS) are monitoring the incidence of brucellosis among
bison living in Yellowstone National Park.
Brucellosis is caused by the bacterium Brucella abortus. In cattle,
infection with the organism induces abortions, decreases fertility, and reduces
milk production, says Steven C. Olsen. A veterinarian at the
National Animal Disease Center
(NADC) in Ames, Iowa, Olsen is leading the ARS part of the program to
study how brucellosis infects bison and how it may be transmitted to cattle.
No treatment or preventive drug has ever been developed for cattle
brucellosis. Since the early 1940s, vaccines based on B. abortus strain
19 have been the chief defense against this devastating diseaseone that
can be transmitted from animals to humans.
People can get brucellosis through handling infected carcasses at slaughter
or from infected cows during calving. It causes the disease called undulant
fever and produces severe flu-like symptoms that can last for monthsor
years, if left untreated. Consumption of unpasteurized milk and dairy products
can also cause undulant fever.
A bane of cattle producers since the 1840s, brucellosis has been nearly
eradicated in this country. Much of the success of the cooperative
federal-state brucellosis eradication program that began in 1934 can be
credited to the subsequent partnership of ARS and APHIS.
Currently, 37 states have been classified brucellosis-free. Free status
means that no cattle or domestic bison have been found infected in a state for
12 consecutive months. Thirteen states maintain a class A status, which
indicates a herd infection rate of less than 0.25 percent.
Cattle ranchers in a few states, however, fear losing brucellosis-free
status because the last major sources of the B. abortus organism are the
free-living elk and bison in the Greater Yellowstone Area. This includes
Yellowstone and Grand Teton National Parks and adjacent land.
The National Park Service has set a goal of eliminating brucellosis in bison
and elk in the Yellowstone area by 2010.
Last winter, nearly 1,300 bison left the park to forage in cattle-populated
areas of Montana, Wyoming, and Idaho. APHIS monitored the prevalence of
brucellosis among these animals and found that about 50 percent tested positive
for exposure to brucellosis, according to APHIS veterinarian Jack C. Rhyan in
For the last 2 years, ARS and APHIS researchers have repeatedly sampled a
small number of bison (both exposed and unexposed to B. abortus) by
using radio collars to track their movements in Yellowstone National Park.
Technician Aileen Duit checks a culture sample for the persistence of Brucella
abortus strain RB51 in the bloodstream of a vaccinated bison.
"Over the next 5 years, our goal is to better understand this disease
in bison so that a program can be developed to eradicate it," says Olsen.
"Texas A&M researchers have shown that bison can transmit the B.
abortus organism to cattle, but no one knows for sure how the organism
infects bison or if the potential for its transmission could be reduced if
bison were vaccinated."
Last year, ARS researchers demonstrated the effectiveness of a new vaccine
for calves using B. abortus strain RB51. This vaccine made by Colorado
Serum Company in Denver, Colorado, is now the official vaccine in many states,
having replaced the strain 19 vaccine. As an alternative to strain 19, the RB51
vaccine solved a major obstacle in the eradication program.
"Vaccines made with strain 19 can produce false signs of infection in
blood tests of vaccinated animals, making identification of truly infected
animals difficult," says Olsen.
He and the ARS research team of Mark G. Stevens, Mitchell V. Palmer, Shirley
M. Halling, Betsy J. Bricker, and Norman F. Cheville tested the vaccine in
"These researchers performed years of work behind the
scenesvaccinating calves, raising them to breeding age, waiting until
they were pregnant, and exposing them to the bacteria to see if the vaccine
prevented abortions," says Carole A. Bolin, who heads the Zoonotic
Diseases Research Unit at NADC.
The NADC researchers also tested the new vaccine in a few experimentally
infected bison at Ames.
"Our preliminary results are encouraging," says Olsen. "The
RB51-vaccinated bison showed an immune response comparable to what has been
achieved in cattle vaccinated with RB51.
"Another plus: None of the RB51-vaccinated bison shed the live
bacterium in the environment. This information is important, because we need to
know if other animal species could get the live organism and become
infected," he says. "Of particular concern are moose, because they
can die if infected with B. abortus."
Olsen found that the vaccine takes about 18 to 24 weeks to clear from the
bison's circulatory system. This is longer than the 13 weeks or so it takes to
clear in cattle. "We want the live vaccine to stay long enough to provide
immunity, then disappear," says Olsen.
This year the bison will be bred and any pregnant females will be infected
with the Brucella organism to see if the vaccine protects them.
RB51 was first identified in 1981 by Gerhardt Schurig, a microbiologist with
Virginia Polytechnic and State University at Blacksburg, Virginia.
More than 1,000 pregnant cows were inoculated with the live RB51 vaccine in
studies conducted in Alabama, Kansas, Georgia, Texas, and Florida. Of these,
only one animal aborted because of RB51. The information gained from this study
helped define the way abortion occurs in cattle with brucellosis and provided
the researchers with a better understanding of the disease. By
Olsen is in the USDA-ARS Bacterial Diseases of Livestock Research Unit,
Ames, IA 50010; phone (515) 663-7230