Brucellosis
Brucellosis
Disease History, Characteristics, and Transmission
Introduction
Brucellosis (broo-sell-OH-sis) is a disease that is caused by a variety of bacteria in the genus Brucella. Swine, cattle, and sheep can be directly infected by brucellosis. Humans can develop brucellosis indirectly by contact with infected animals (brucellosis is a zoonotic infection) or by consuming milk or dairy products that are contaminated with the bacteria.
Vaccination of animals born and raised in the United States against brucellosis is required, which helps protect both the nation's livestock and humans most at risk of being secondarily infected. However, monitoring of imported livestock is necessary to prevent introducing brucellosis into a population of animals, as vaccination programs are not in effect in every country.
Disease History, Characteristics, and Transmission
Brucellosis was named after David Bruce, a researcher who isolated the organism in 1887 from five sick British soldiers stationed on the island of Malta. The designation of brucellosis as Malta fever recognizes this origin as well as the 1905 description of human brucellosis cases in Malta from Brucella-contaminated unpasteurized milk. The disease is also known as undulant fever, as the fever tends to increase and decrease with time. Brucellosis dates back much further than these formal descriptions. Descriptions from the time of Hippocrates (Greek physician and philosopher born around 460 BC) are now thought to refer to brucellosis.
A number of different species of the bacterium are responsible for the disease in various livestock. Brucella melitensis infects goats and sheep, B. suis infects pigs and in caribou, B. abortus causes the disease in cattle, bison, and elk, B. ovis also infects sheep, and B. canis causes the disease in dogs.
Brucella are shaped somewhat like a football. In contrast to many disease-causing bacteria that have an outer coating called a capsule, Brucella lack a capsule. A capsule can help shield a bacterium from host defenses such as antibodies. Lacking a capsule, Brucella would be exposed to the body's defenses if not for its infection strategy. Instead, the bacteria cause infection by entering host cells. Within host cells, the bacteria are shielded and are able to grow and multiply.
The species of Brucella that are capable of causing brucellosis in humans are B. melitensis, B. abortis, and B. suis. The infection that develops in dogs is not transmitted to humans. Humans acquire the infection indirectly, usually by handling infected animals or even a carcass; if a person has a cut or abrasion in the skin, especially on the hands, the bacteria easily gain access to the bloodstream. However, entry is possible even in the absence of a wound, as the bacteria are able to invade skin cells and reach the bloodstream. Another route of infection is via contaminated moist soil and hay. In these environments, the bacteria can remain alive and capable of infection for months. As well, people are infected by drinking unpasteurized milk, or eating cheese or ice cream that has been made from unpasteurized milk. Finally, the organism can be inhaled and the bacteria spread to the bloodstream following invasion of lung cells.
Person-to-person spread via breastfeeding and during sex can occur, but is rare. It is possible that the transplantation of contaminated tissue could cause brucellosis.
When the bacteria enter the bloodstream, they migrate to lymph nodes. Normally, lymph nodes such as those located in the neck and the armpit function to destroy invading bacteria and viruses. However, Brucella circumvents this and invades the lymph node cells. From there, the bacteria can spread to the spleen, bone marrow, and liver. Tissue irritation and organ damage occurs. In severe cases, the lining of the heart can be infected.
The time from exposure to the appearance of symptoms is usually around three weeks. Symptoms include general feelings of weakness and tiredness, muscle pain, chills, and fever. The fever and chills can subside and recur during the illness. Brucellosis is lethal in about 10% of cases, usually because of heart infection.
Scope and Distribution
The prevalence of human brucellosis is related to the prevalence of the infection in domestic and wild animal populations. In countries such as the U.S. and Canada, where stringent monitoring and infection control measures are in place and where vaccination programs have been operating for years, brucellosis in both livestock and humans is rare. Culling (slaughtering) of infected animals in some North American wild elk and bison populations has been carried out to ensure that the infection does not spread from the wild populations to livestock.
Infection is most common in those who come into frequent contact with domestic and wild animals; veterinarians, cattlemen, and workers in slaughterhouses. In the U.S., there are about 100 of human brucellosis cases per year, representing one out of every three million Americans.
Elsewhere in the world, brucellosis is more frequent in countries where agriculture involves more people in closer contact with unvaccinated livestock, and where infection control precautions are not as stringent. Areas considered to be high risk according to the U.S. Centers for Disease Control and Prevention (CDC) are China, India, Peru, Mexico, Eastern Europe, the Mediterranean, the Caribbean, and the Middle East.
Age and race do not influence the occurrence of brucellosis. In developing countries where mostly women tend livestock, the disease is initially more prevalent in women. In developing countries where mostly men tend livestock, the situation is reversed.
WORDS TO KNOW
CULL: A cull is the selection, often for destruction, of a part of an animal population. Often done just to reduce numbers, a widespread cull was carried out during the epidemic of bovine spongiform encephalopathy (BSE or mad cow disease) in the United Kingdom during the 1980s
NOTIFIABLE DISEASES: Diseases that the law requires must be reported to health officials when diagnosed, including active tuberculosis and several sexually transmitted diseases; also called reportable diseases.
ZOONOSES: Zoonoses are diseases of microbiological origin that can be transmitted from animals to people. The causes of the diseases can be bacteria, viruses, parasites, and fungi.
IN CONTEXT: PERSONAL RESPONSIBILITY AND PROTECTION
The Coordinating Center for Infectious Diseases/Division of Bacterial and Mycotic Diseases states that “direct person-toperson spread of brucellosis is extremely rare. Mothers who are breast-feeding may transmit the infection to their infants. Sexual transmission has also been reported. For both sexual and breastfeeding transmission, if the infant or person at risk is treated for brucellosis, their risk of becoming infected will probably be eliminated within 3 days. Although uncommon, transmission may also occur via contaminated tissue transplantation.”
To prevent infection the CCID and CDC recommend that travelers “do not consume unpasteurized milk, cheese, or ice cream while traveling. If you are not sure that the dairy product is pasteurized, don't eat it. Hunters and animal herdsman should use rubber gloves when handling viscera of animals.”
As of 2007 there is no vaccine available for humans.
SOURCE: Coordinating Center for Infectious Diseases/Division of Bacterial and Mycotic Diseases, Centers for Disease Control and Prevention (CDC)
IN CONTEXT: TRENDS AND STATISTICS
In October 2005, The Coordinating Center for Infectious Diseases/Division of Bacterial and Mycotic Diseases stated that “for previous 10 years, approximately 100 cases of Brucellosis per year have been reported.”
California, Florida, Texas, and Virginia account for most cases.
“In 2001, the National Brucellosis Eradication Program reported only 3 newly affected cattle herds, compared to 14 herds identified in 2000.”
SOURCE: Centers for Disease Control and Prevention (CDC)
Treatment and Prevention
Brucellosis is suspected based on the symptoms and a history of contact with animals. Confirmation of the infection relies on the recovery of the bacteria from blood samples, bone marrow, or liver tissue. The confirmation step can take months, since Brucella grows slowly during laboratory culture. This also poses a hazard for lab personnel, who may be exposed to the bacteria during the incubation period. A quicker means of detecting the bacteria is by the presence of antibodies produced against the infecting bacteria. Antibody production by the host may not be efficient, since the infection takes place inside host cells. But commercially available antibodies can be used to test blood for the presence of the corresponding bacterial component.
Human brucellosis that is caused by B. abortus is usually mild and may not require treatment. In contrast, the disease caused by B. melitensis and B. suis can produce severe, prolonged symptoms if not treated.
Treatment typically involves antibiotics; for adults, different antibiotics are given orally and by injection for several weeks. The intramuscular injections are necessary to allow the antibiotic to penetrate into the host cells to the site of infection.
Prevention is possible because of vaccines. Typically, vaccination of animals is the norm. Control of the disease in animals controls the disease in humans. In fact, two vaccine formulations used for animals contain live but weakened bacteria, and are capable of causing brucellosis if accidentally given to a person.
Multiple episodes of brucellosis among laboratory workers have been reported in the past, mostly from inhaling the bacteria in the confined space of a laboratory. In order to prevent exposure in the laboratory, scientists now study the bacteria using biosafety level three precautions, including gowns, gloves, and performing tests under a biosafety cabinet.
Impacts and Issues
In North America, brucellosis is prevalent in wild elk and bison herds. Trap and slaughter campaigns of affected animals have been accomplished in Montana and in Wood Buffalo National Park, which straddles the Canadian provinces of Alberta and British Columbia. Ironically, the national park was created in the 1920s to protect the declining bison population. The culls, which have been controversial, are aimed at keeping cattle and swine herds free of brucellosis.
While brucellosis in commercial livestock is unusual in North America, the continent is at risk if infected animals or food products are imported. Preventive measures include the vaccination of all animals that are raised for food. An individual can minimize their risk of brucellosis by not eating animal products from suspect countries and not eating unpasteurized diary products.
Brucellosis is also recognized as a potential biological threat because it can be spread through the air. There is a historical basis for this categorization. Following World War II (1939–1945), the United States military developed a weapon that would disperse B. abortus and B. suis upon detonation. The weapon, which was the first biological weapon developed by the U.S., was intended in part to cripple an enemy's livestock-based agriculture. The weapons program was ended by President Richard Nixon in 1967. Today, scientists are working to develop a rapid diagnostic test for brucellosis in the event of a suspected biological attack, and brucellosis remains among the list of nationally notifiable diseases.
See AlsoAnimal Importation; Bacterial Disease; Bioterrorism; Public Health and Infectious Disease; Zoonoses.
BIBLIOGRAPHY
Books
Drexler, Madeline. Secret Agents: The Menace of Emerging Infections. New York: Penguin, 2003.
Hart, Tony. Microterrors: The Complete Guide to Bacterial, Viral and Fungal Infections that Threaten Our Health. Tonawanda: Firefly Books, 2004.
Periodicals
Kozukeev, Turatbek, B., S. Ajeilat, M. Favorov. “Risk factors for Brucellosis-Leylek and Kadamjay districts, Batken Oblast, Kyrgyzstan, January – November, 2003.” Morbidity and Mortality Weekly. 55(SUP01): 31–34 (2006).
Brian Hoyle