APHIS Factsheet

Veterinary Services February 2007

Questions and 

Answers About 

Brucellosis

Q. What is brucellosis? 

A. It is a contagious, costly disease of animals that 

also affects humans. Although brucellosis can infect 

other animals, its main threat is to cattle, bison, and 

swine. The disease is also known as contagious 

abortion or Bang’s disease. In humans, it’s known 

as undulant fever because of the severe intermittent 

fever accompanying human infection or Malta fever 

because it was first recognized as a human disease 

on the island of Malta. 

Q. How serious is brucellosis? 

A. Considering the damage done by the infection in 

animals—decreased milk production, weight loss in 

animals, loss of young, infertility, and lameness—it is 

a very serious disease of livestock. The rapidity with 

which it spreads and the fact that it is transmissible to 

humans makes it all the more serious. 

Q. What disease agents cause brucellosis?

A. The disease is caused by a group of bacteria 

known scientifically as the genus Brucella. Three species of Brucella cause the most concern: B. abortus, 

principally affecting cattle, bison, and cervids; B. suis, 

principally affecting swine and reindeer but also cattle 

and bison; and B. melitensis, principally affecting 

goats, but which is not present in the United States. In 

free-roaming bison and elk in the Greater Yellowstone 

area, the bacterium of concern is B. abortus. In cattle 

and bison, the disease primarily localizes in the reproductive organs and/or the udder. Bacteria are shed 

in milk or via the aborted fetus, afterbirth, or other 

reproductive tract discharges. 

Q. What are the signs of brucellosis?

A. There is no effective way to detect infected animals 

by their appearance. The most obvious signs in 

pregnant animals are abortion or birth of weak calves. 

Milk production may be reduced from changes in 

the normal lactation period caused by abortions and 

delayed conceptions. Not all infected cows abort, but 

those that do usually abort between the fifth and 

seventh month of pregnancy. Infected cows usually 

abort once, but a percentage will abort during

additional pregnancies, and calves born from later 

pregnancies may be weak and unhealthy. Even 

though their calves may appear healthy, infected cows 

continue to harbor and discharge infectious organisms 

and should be regarded as potentially serious sources 

of the disease. Other signs of brucellosis include 

an apparent lowering of fertility with poor conception 

rates, retained afterbirths with resulting uterine 

infections, and (occasionally) enlarged, arthritic joints. 

Q. How is brucellosis spread?

A. Brucellosis is commonly transmitted to susceptible animals by direct contact with infected animals 

or through an environment that has been contaminated with discharges from infected animals. Aborted 

fetuses, placental membranes or fluids, and other 

vaginal discharges present after an infected animal 

has aborted or calved are all highly contaminated with 

infectious Brucella organisms. Cows may lick those 

materials or the genital area of other cows or ingest 

food or water contaminated with the diseasecausing organisms. Despite occasional exceptions, 

the general rule is that brucellosis is carried from one 

herd to another by an infected or exposed animal. 

This mode of transmission occurs when a herd owner 

buys replacement cattle or bison that are infected or 

have been exposed to infection prior to purchase. 

The disease may also be spread when wild animals 

or animals from an affected herd mingle with 

brucellosis-free herds. 

Q. What is being done to fight brucellosis? 

A. Before 1934, control of brucellosis was limited 

mainly to individual herds. Today, there is a Cooperative State–Federal Brucellosis Eradication Program 

to eliminate the disease from the country. Like other 

animal disease eradication efforts, success of the 

program depends on the support and participation of 

livestock producers. The program’s Uniform Methods 

and Rules set forth the minimum standards for States 

to achieve eradication. States are designated 

brucellosis free when none of their cattle or bison are 

found to be infected for 12 consecutive months under 

an active surveillance program. For current brucellosis State classifications, see the updates posted at 

<www.aphis.usda.gov>. 

Q. What about free-ranging bison herds? 

A. The presence of brucellosis in free-ranging bison 

and elk in Yellowstone National Park and Grand Teton 

National Park threatens the brucellosis status of the 
surrounding States and the health of their livestock 

herds, which are free of the disease. Wyoming lost 

free status in 2004 and Idaho in 2005 due to comingling of free-roaming elk with local cattle herds. 

Wyoming regained its free status in September of 

2006. Reintroduction of the disease into a brucellosis-free State could have a serious economic 

impact on domestic livestock markets and potentially 

threaten export markets. The U.S. Department of 

Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) is working cooperatively with 

other State and Federal agencies to eliminate brucellosis from the infected free-roaming bison and elk 

populations in the Greater Yellowstone area. 

Q. How do epidemiologists help fight brucellosis? 

A. Epidemiologists are specially trained investigators 

who study the occurrence of disease in populations 

of animals in order to determine how the disease is 

spread and how best to control and eradicate the 

disease. Epidemiologists are concerned with disease 

in a group or population of animals and evaluate 

circumstances connected with the occurrence of disease. These veterinarians help eliminate brucellosis 

by identifying factors essential to its control, eradication, and prevention. 

Q. How costly is brucellosis to the livestock 

industry?

A. The livestock and dairy industries and the American consumer have realized great financial savings 

from the success of the Cooperative State–Federal 

Brucellosis Eradication Program. Annual losses from 

lowered milk production, aborted calves and pigs, 

and reduced breeding efficiency have decreased 

from more than $400 million in 1952 to less than $1 

million today. Studies have shown that, if brucellosis 

eradication program efforts were stopped, the costs 

of producing beef and milk would increase by an estimated $80 million annually in less than 10 years. 

Q. How effective is the Brucellosis Eradication 

Program? 

A. At the beginning of the program, brucellosis was 

widespread throughout U.S. livestock, but eradication efforts have had dramatic results. In 1956, 

124,000 affected herds were found by testing in the 

United States. By 1992, this number had dropped 

to 700 herds, and as of October 31, 2006, no known 

affected herds remained in the entire United States. 

A total of two affected herds were found and depopulated in fiscal year 2005. APHIS anticipates the 

Cooperative State–Federal Program to achieve the 

goal of nationwide eradication of brucellosis from 

domestic cattle and bison within the next year. 

Q. What is the basic approach to eradication?

A. Historically, the basic approach has always been 

to vaccinate calves, test cattle for infection, and send 

infected animals to slaughter. Identification of market 

animals for tracing, surveillance to find infected animals, investigation of affected herds, and vaccination 

of replacement calves in high-risk areas are important 

features of the current program. 

Q. How is infection found in cattle?

A. Two primary surveillance procedures are used to 

locate infection without having to test each animal in 

every herd. Milk from dairy herds is checked two to 

four times a year by testing a small sample obtained 

from creameries or farm milk tanks for evidence of 

brucellosis. Bison herds and cattle herds that do not 

produce milk for sale are routinely tested for brucellosis 

by blood-testing animals sold from these herds at livestock markets or at slaughter. In addition, some States 

require adult cattle and bison to be subjected to blood 

tests for brucellosis upon change of ownership even if 

sold directly from one farm to another. If brucellosis is 

detected, remaining animals from the herd of origin are 

tested.

Q. What happens when evidence of disease is 

found by surveillance testing? 

A. Once an infected herd is located, the infection is 

contained by quarantining all infected and exposed 

cattle and bison and limiting their movement to slaughter only, until the disease can be eliminated from the 

herd. Depopulation of brucellosis affected herds is 

recommended. Diagnostic tests are used to find all 

infected cattle and bison. Federal and State animal 

health officials test neighboring herds and other herds 

that may have received animals from the infected 

herd. In addition, officials test the source herds for any 

animals sold into the infected herd. In addition, officials test the source herds for any animals sold into the 

infected herd. All possible leads to additional infection 

are traced. 

Q. How does the brucellosis milk surveillance test 

(BMST) work? 

A. The BMST procedure (also known as the brucellosis 

ring test (BRT)) makes it possible to do surveillance on 

dairy herds quickly and economically. Milk or cream 

from each milking cow in the herd is pooled, and a 

sample is taken for testing. A suspension of stained, 

killed Brucella organisms is added to a small quantity 

of milk. If the milk from one or more infected animals is 

present in the sample, a bluish ring forms at the cream 

line as the cream rises. 

Q. How does market cattle identification (MCI) 

work? 
A. Numbered tags, called backtags, are placed on the 

shoulders of adult breeding animals being marketed 

from beef, dairy, and bison herds. Blood samples 

are collected from the animals at livestock markets 

or slaughtering plants and tested for brucellosis. If a 

sample reacts to a diagnostic test, it is traced by the 

backtag number to the herd of origin. The herd owner 

is contacted by a State or Federal animal health official to arrange for testing of his or her herd. Once the 

animals have been gathered, all of the eligible animals 

in the herd are tested at no cost to the owner. 

Q. Which animals are eligible for MCI testing? 

A. At slaughter, all cattle and bison 2 years of age or 

older are tested, except steers and spayed heifers. 

At market, all beef cattle and bison over 24 months 

of age and all dairy cattle over 20 months of age are 

tested except steers and spayed heifers. Pregnant or 

postparturient heifers are also eligible for testing 

regardless of their age. Herd tests must include all 

cattle and bison over 6 months of age except steers 

and spayed heifers.

Q. Why is identification of market cattle important? 

A. The key to the MCI program is proper identification 

of all animals so they can be traced to their herds of 

origin. Most livestock markets identify cattle and bison 

with numbered USDA-approved backtags. Backtags, 

as well as eartags and other identification devices, are 

collected and sent to the diagnostic laboratory along 

with the matching blood samples to aid in identifying 

ownership of test-positive animals. 

Q. What are the advantages of MCI? 

A. MCI provides a means of determining the brucellosis status of animals marketed from a large area 

and eliminates the need to round up cattle and bison 

in all herds for routine testing. MCI, along with other 

preliminary testing procedures, is effective in locating 

infection so control measures can be taken to contain 

the disease and eliminate it. 

Q. Does the brucellosis market cattle identification program satisfy the identification component of the National Animal Identification System 

(NAIS)?

A. No. A program for a single, specific disease such 

as brucellosis would not satisfy the identification 

component of NAIS. The goal of NAIS is to provide 

an infrastructure that will help producers and animal 

health officials respond more quickly and effectively 

to any significant animal disease event in the United 

States. Since NAIS is designed to support all animal 

disease programs, we cannot rely on disease-specific 

programs alone to achieve the goals of a national 

identification program. For example, the brucellosis 

market cattle identification program is limited to adult 

breeding animals being marketed from beef, dairy, and 

bison herds. If we were to rely on this program for 

national animal identification purposes, it would represent only a fraction of animals in the United States that 

are susceptible or vulnerable to any number of diseases of concern. 

Q. What is the brucellosis card test? 

A. It is a rapid, sensitive, and reliable procedure for 

diagnosing brucellosis infection. It employs disposable 

materials contained in compact kits. Brucella antigen 

is added to the blood serum on a white card. Results 

of the test are read 4 minutes after the blood serum 

and antigen are mixed. 

Q. Are there any other tests for brucellosis? 

A. There are a number of supplemental tests based on 

various characteristics of antibodies found in the blood 

and milk of infected animals. These tests are especially useful in identifying infected animals in problem 

herds and herds in which chronic brucellosis infection 

exists and from which infection is difficult to eliminate. 

Another diagnostic method involves culturing Brucella

organisms from infected tissues, milk, or other body 

fluids, from aborted calves or fetal fluids and 

membranes. 

Q. What animals are eligible for testing? 

A. With certain exceptions, herd tests must include all 

cattle and bison over 6 months of age except steers 

and spayed heifers.

Q. What is the incubation period of brucellosis? 

A. An incubation period is the interval of time between 

exposure to an infectious dose of organism and the first 

appearance of disease signs. The incubation period of 

brucellosis in cattle, bison, and other animals is quite 

variable, ranging from about 2 weeks to 1 year and 

even longer in certain instances. When abortion is the 

first sign observed, the minimum incubation period is 

about 30 days. Some animals abort before developing a positive reaction to the diagnostic test. Other 

infected animals may never abort. Generally, infected 

animals that do not abort develop a positive reaction to 

the diagnostic test within 30 to 60 days after infection, 

although some may not develop a positive reaction for 

several months to over a year. 

Q. Can brucellosis in animals be cured? 

A. No. Repeated attempts to develop a cure for 

brucellosis in animals have failed. Occasionally, 

animals may recover after a period of time. More 

commonly, however, only the clinical signs disappear 

and the animals remain diseased. Such animals are 

dangerous sources of infection for other animals with 

which they associate. 
Q. Can brucellosis be prevented? 

A. The disease may be avoided through good sanitation and management practices, as well as vaccination. Replacement animals should be tested when 

purchased and retested after a 30- to 60-day isolation 

period, during which they are kept separate from the 

remainder of the herd. These practices will allow 

detection of animals that were in the incubation period 

of the disease when acquired. 

Q. What about vaccination? 

A. For cattle and bison in infected areas or for 

replacement animals added to such herds, officials

recommend vaccinating heifers with an approved 

Brucella vaccine. Brucella Strain RB51 is the vaccine 

used in the United States. The vaccine is a live product and must be administered only by an accredited 

veterinarian or State or Federal animal health official. 

For best results, female calves should be vaccinated 

when they are 4 to 6 months old. At the time of vaccination, a tattoo is applied in the ear, which identifies 

the animal as an “official vaccinate.” The tattoo identifies the type of vaccine used and the year in which 

vaccination took place. 

Q. How does the vaccine work? 

A. B. abortus vaccine produces an immune response 

that increases the animal’s resistance to the disease. 

However, vaccination is not 100 percent effective 

in preventing brucellosis; it typically protects about 

70–80 percent of the vaccinated cattle from becoming 

infected by an average exposure to Brucella. 

Q. Previously, Strain 19 was the only approved 

Brucella vaccine. What is the difference between 

Strain 19 and RB51?

A. In March 2003, USDA granted full licensure for the 

Brucella vaccine called Strain RB51, for use in cattle. 

Strain RB51 is as efficacious as Strain 19 vaccine but 

virtually eliminates adverse postvaccination reactions 

in cattle, such as abortions and localized inflammation 

at the vaccine injection site. Most importantly, unlike 

Strain 19, Strain RB51 does not stimulate the same 

type of antibodies that can be confused on standard 

diagnostic tests with those antibodies produced by 

actual infection. 

Q. Is Strain RB51 vaccine approved for use in 

bison?

A. B. abortus Strain RB51 has been approved for use 

in bison in an official USDA program. 

Q. Where or when is vaccination in calves most 

important? 

A. Owners whose herds are located in areas of relatively heavy infection or who ship replacement cattle or 

bison to, or receive animals from, such areas should 

carry out a vigorous calfhood vaccination program. 

Every cattle or bison owner, regardless of location, 

should discuss the advantages and disadvantages of 

vaccination with his or her veterinarian. In addition, 

owners should be sure to check with individual States 

regarding the age and vaccination requirements for 

import of cattle and bison for breeding purposes. 

Q. Where is vaccination less important? 

A. In many areas of the country where infected herds 

have not been found for several years and where the 

detection of early infection through BMST (also known 

as BRT), MCI, and other surveillance systems is in 

effect, the need to continue calfhood vaccination is not 

recommended. Vaccination should be reduced in such 

areas, provided that adequate regulatory measures are 

in effect to prevent reintroduction of the disease. 

Q. How does brucellosis affect humans? 

A. People infected with the brucellosis organism usually develop symptoms similar to a severe influenza, 

but this disease, called undulant fever, persists for 

several weeks or months and may get progressively 

worse. Farmers, ranchers, veterinarians, and packing 

plant workers are more vulnerable to becoming infected 

because they come into direct contact with infected 

animals. The initial symptoms are fatigue and headaches, followed by high fever, chills, drenching sweats, 

joint pains, backache, and loss of weight and appetite. 

Undulant fever does not often kill its victims, but the 

disease is too serious to be dealt with lightly. For additional information on brucellosis in humans, visit the 

Centers for Disease Control and Prevention Web site at 

<www.cdc.gov>. 

Q. What are the main sources of human infection? 

A. In years past, prior to pasteurization, raw milk from 

infected cattle was considered the prime source of 

brucellosis in humans. Today, most humans contract 

the disease by coming into direct contact with aborted 

fetuses, afterbirth, and uterine discharges of diseased 

animals or with infected carcasses at slaughter. 

However, one 1994 study suggests that human 

brucellosis in California is most likely to be a foodborne illness (unpasteurized milk or cheese products) 

acquired in Mexico or from Mexican products consumed in California. Rarely, if ever, does a human 

contract the disease from another human. 

Q. How common is human brucellosis in this country? 

A. Fortunately, the combination of pasteurization of 

milk and progress in the eradication of the disease in 

livestock has resulted in substantially fewer human 

cases than in the past. Ninety-eight cases of human 

brucellosis were reported in 1997, a fraction of the 

6,400 cases reported in 1947. Currently, about 80–100 
cases of human brucellosis are reported annually. 

Many of those are people who have traveled internationally and ingested unpasteurized dairy products. 

Q. Can people get brucellosis by eating meat? 

A. There is no danger from eating properly cooked 

meat products because the disease-causing bacteria 

are not normally found in muscle tissue and they are 

killed by proper cooking temperatures. The disease 

may be transmitted to humans when slaughtering 

infected animals or when processing contaminated 

organs from freshly killed animals. 

Q. How can people be protected from brucellosis? 

A. Ranchers, farmers, or animal managers should 

clean and disinfect calving areas and other places 

likely to become contaminated with infective material. 

All individuals should wear sturdy rubber or plastic 

gloves when assisting calving or aborting animals, and 

scrub well with soap and water afterward. Precautions against drinking raw milk or eating unpasteurized 

milk products are also important. Ultimately, the best 

prevention is to eliminate brucellosis from all animals 

in the area. 

Q. Where can I find more information on brucellosis?

A. For additional information, contact:

USDA, APHIS, Veterinary Services

National Center for Animal Health Programs

Ruminant Health Programs Staff

4700 River Road, Unit 43

Riverdale, MD 20737-1231

Phone: (301) 734-6954 or visit our Web site at 

<http://www.aphis.usda.gov/vs/>.

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