Brucellosis

Introduction

Disease History, Characteristics, and Transmission

Scope and Distribution

Treatment and Prevention

Impacts and Issues

BIBLIOGRAPHY

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

Brucellosis

Definition

Brucellosis is a bacterial disease caused by members of the Brucella genus that can infect humans but primarily infects livestock. Symptoms of the disease include intermittent fever, sweating, chills, aches, and mental depression. The disease can become chronic and recur, particularly if untreated.

Description

Also known as undulant fever, Malta fever, Gibraltar fever, Bang's disease, or Mediterranean fever, brucellosis is most likely to occur among those individuals who regularly work with livestock. The disease originated in domestic livestock but was passed on to wild animal species, including the elk and buffalo of the western United States. In humans, brucellosis continues to be spread via unpasteurized milk obtained from infected cows or through contact with the discharges of cattle and goats during miscarriage. In areas of the world where milk is not pasteurized, for example in Latin America and the Mediterranean, the disease is still contracted by ingesting unpasteurized dairy products. However, in the United States, the widespread pasteurization of milk and nearly complete eradication of the infection from cattle has reduced the number of human cases from 6,500 in 1940 to about 70 in 1994.

Causes and symptoms

The disease is caused by several different species of parasitic bacteria of the genus Brucella. B. abortus is found in cattle and can cause cows to abort their fetuses. B. suis is most often found in hogs and is more deadly when contracted by humans than the organism found in cattle. B. melitensis is found in goats and sheep and causes the most severe illness in humans. B. rangiferi infects reindeer and caribou, and B. canis is found in dogs.

A human contracts the disease by coming into contact with an infected animal and either allowing the bacteria to enter a cut, breathing in the bacteria, or by consuming unpasteurized milk or fresh goat cheese obtained from a contaminated animal. In the United States, the disease is primarily confined to slaughterhouse workers.

Scientists do not agree about whether brucellosis can be transmitted from one person to another, although some people have been infected from a tainted blood transfusion or bone marrow transplant. Newborn babies have also contracted the illness from their mothers during birth. Currently, it is believed that brucellosis can also be transmitted sexually.

The disease is not usually fatal, but the intermittent fevers (a source of its nickname, "undulant fever") can be exhausting. Symptoms usually appear between five days and a month after exposure and begin with a single bout of high fever accompanied by shivering, aching, and drenching sweats that last for a few days. Other symptoms may include headache, poor appetite, backache, weakness, and depression. Mental depression can be so severe that the patient may become suicidal.

KEY TERMS

Antibody— A specific protein produced by the immune system in response to a specific foreign protein or particle called an antigen.

Chronic— Disease or condition characterized by slow onset over a long period of time.

Parasite— An organism living in or on, and obtaining nourishment from, another organism.

Pasteurization— The process of applying heat, usually to milk or cheese, for the purpose of killing, or retarding the development of, pathogenic bacteria.

In rare, untreated cases, the disease can become so severe that it leads to fatal complications, such as pneumonia or bacterial meningitis. B. melitensis can cause miscarriages, especially during the first three months of pregnancy. The condition can also occur in a chronic form, in which symptoms recur over a period of months or years.

Diagnosis

Brucellosis is usually diagnosed by detecting one or more Brucella species in blood or urine samples. The bacteria may be positively identified using biochemical methods or using a technique whereby, if present in the sample, the brucellosis bacteria are made to fluoresce. Brucellosis may also be diagnosed by culturing and isolating the bacteria from one of the above samples. Blood samples will also indicate elevated antibody levels or increased amounts of a protein produced directly in response to infection with brucellosis bacteria.

Treatment

Prolonged treatment with antibiotics, including tetracyclines (with streptomycin), co-trimoxazole, and sulfonamides, is effective. Bed rest is also imperative. In the chronic form of brucellosis, the symptoms may recur, requiring a second course of treatment.

Prognosis

Early diagnosis and prompt treatment is essential to prevent chronic infection. Untreated, the disease may linger for years, but it is rarely fatal. Relapses may also occur.

Prevention

There is no human vaccine for brucellosis, but humans can be protected by controlling the disease in livestock. After checking to make sure an animal is not already infected, and destroying those that are, all livestock should be immunized. Butchers and those who work in slaughterhouses should wear protective glasses and clothing, and protect broken skin from infection.

Some experts suggest that a person with the disease refrain from engaging in unprotected sex until free of the disease. The sexual partners of an infected person should also be closely monitored for signs of infection.

Resources

ORGANIZATIONS

Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. (800) 311-3435, (404) 639-3311. 〈http://www.cdc.gov〉.

OTHER

"Bacterial Diseases." Healthtouch Online Page. 〈http:www.healthtouch.com〉.

Centers for Disease Control. 〈http://www.cdc.gov/nccdphp/ddt/ddthome.htm〉.

Brucellosis

Characteristics of Brucella

Brucella species

Symptoms and treatment of brucellosis

Prevention

Resources

Brucellosis is a disease caused by bacteria in the genus Brucella. The disease infects animals such as swine, cattle, and sheep. Humans can become infected indirectly through contact with infected animals or by drinking Brucella -contaminated milk. In the United States, most domestic animals are vaccinated against the bacteria, but brucellosis remains a risk with imported animal products.

Characteristics of Brucella

Brucella are rod-shaped bacteria that lack a diffuse sugary covering (capsule) around their cell membranes. Unlike most bacteria, Brucella cause infection by actually entering host cells. As the bacteria cross the host cell membrane, they are engulfed by host cell vacuoles called phagosomes. The presence of Brucella within host cell phagosomes initiates a characteristic immune response, in which infected cells begin to stick together and form aggregations called granulomas.

Brucella species

Three species of Brucella cause brucellosis in humans: Brucella melitensis, which infects goats; B. abortis, which infects cattle and, if the animal is pregnant, causes the spontaneous abortion of the fetus; and B. suis, which infects pigs. In animals, brucellosis is a self-limiting disease, and usually no treatment is necessary for the resolution of the disease. However, for a period of time from a few days to several weeks, infected animals may continue to excrete brucella into their urine and milk. Under warm, moist conditions, the bacteria may survive for months in soil, milk, and even seawater.

Because the bacteria are so hardy, humans may become infected with Brucella by direct contact with the bacteria. Handling or cleaning up after infected animals may put a person in contact with the bacteria. Brucella are extremely efficient in crossing the human skin barrier through cuts or breaks in the skin.

Symptoms and treatment of brucellosis

The incubation period of Brucella —the time from exposure to the bacteria to the start of symptoms—is typically about three weeks. The primary complaints are weakness and fatigue. An infected person may also experience muscle aches, fever, and chills.

The course of the disease reflects the location of the Brucella bacteria within the human host. Soon after the Brucella are introduced into the bloodstream, the bacteria seek out the nearest lymph nodes and invade the lymph node cells. From the initial lymph node, the Brucella spread out to other organ targets, including the spleen, bone marrow, and liver. Inside these organs, the infected cells form granulomas.

Diagnosing brucellosis involves culturing the blood, liver, or bone marrow for Brucella organisms. A positive culture alone does not signify brucellosis, since persons who have been treated for the disease may continue to harbor Brucella bacteria for several months. Confirmation of brucellosis, therefore, includes being able to grow the bacteria on a suitable food source (generally called a medium), detection of antibodies (proteins produced by the immune system in response to the presence of a foreign proteins

KEY TERMS

Granuloma— An immune response in which cells infected with bacteria clump together. Granuloma formation is typical of tuberculosis and brucellosis.

Lymph nodes— Small, bean-shaped structures located along the lymphatic vessels of the body. Lymph nodes function in the immune response to protect the body against foreign cells.

Pasteurization— The process in which milk or either dairy products are heated to a high temperature in order to kill disease-causing bacteria.

generically termed antigens) to the microorganism in a blood sample, as well as evidence of the characteristic symptoms and a history of possible contact with infected milk or other animal products.

In humans, brucellosis caused by B. abortus is a mild disease that resolves itself without treatment. Brucellosis caused by B. melitensis and B. suis, however, is chronic and severe. Brucellosis is treated with administration of an antibiotic that penetrates host cells to destroy the invasive bacteria.

Prevention

Since the invention of an animal vaccine for brucellosis in the 1970s, the disease has become somewhat rare in the United States. Yet the vaccine cannot prevent all incidence of brucellosis. According to the Centers for Disease Control and Prevention, 100 to 200 cases occur each year in the United States. Most of these were reported in persons who worked in the meat processing industry. Brucellosis remains a risk for those who work in close contact with animals, including veterinarians, farmers, and dairy workers.

Brucellosis also remains a risk when animal products from foreign countries are imported into the United States. Outbreaks of brucellosis have been linked to unpasteurized feta and goat cheeses from the Mediterranean region and Europe. In an outbreak in the 1960s, brucellosis was linked to bongo drums imported from Africa: drums made with infected animal skins can harbor Brucella bacteria, which can be transmitted to humans through cuts and scrapes in the human skin surface.

In the United States, preventive measures include a rigorous vaccination program that involves all animals in the meat processing industry. On an individual level, people can avoid the disease by not eating animal products imported from other countries. If this is not possible or desirable, make sure that imported cheeses have been made with pasteurized milk. If the package does not indicate pasteurization, do not eat the cheese.

As of 2006, there is no vaccine for brucellosis.

Resources

BOOKS

Icon Health Publications. Brucellosis: A Medical Dictionary, Bibliography, and Annotated Research Guide to Internet References. San Diego: Icon Health Publications, 2004.

Lopez-Goni, I. Brucella. Oxford: BIOS Scientific

Publications, 2005.

Prescott, L., J. Harley, and D. Klein. Microbiology 5th ed. New York: McGraw-Hill, 2002.

Kathleen Scogna

Brucellosis

Brucellosis is a disease caused by bacteria in the genus Brucella. The disease infects animals such as swine, cattle, and sheep ; humans can become infected indirectly through contact with infected animals or by drinking Brucella-contaminated milk. In the United States, most domestic animals are vaccinated against the bacteria, but brucellosis remains a risk with imported animal products.

Characteristics of Brucella

Brucella are rod-shaped bacteria that lack a capsule around their cell membranes. Unlike most bacteria, Brucella cause infection by actually entering host cells. As the bacteria cross the host cell membrane , they are engulfed by host cell vacuoles called phagosomes. The presence of Brucella within host cell phagosomes initiates a characteristic immune response, in which infected cells begin to stick together and form aggregations called granulomas.

Brucella species

Three species of Brucella cause brucellosis in humans: Brucella melitensis, which infects goats ; B. abortis, which infects cattle and, if the animal is pregnant, causes the spontaneous abortion of the fetus; and B. suis, which infects pigs . In animals, brucellosis is a self-limiting disease, and usually no treatment is necessary for the resolution of the disease. However, for a period of time from a few days to several weeks, infected animals may continue to excrete brucella into their urine and milk. Under warm, moist conditions, the bacteria may survive for months in soil , milk, and even seawater.

Because the bacteria are so hardy, humans may become infected with Brucella by direct contact with the bacteria. Handling or cleaning up after infected animals may put a person in contact with the bacteria. Brucella are extremely efficient in crossing the human skin barrier through cuts or breaks in the skin.

Symptoms and treatment of brucellosis

The incubation period of Brucella—the time from exposure to the bacteria to the start of symptoms—is typically about three weeks. The primary complaints are weakness and fatigue. An infected person may also experience muscle aches, fever, and chills.

The course of the disease reflects the location of the Brucella bacteria within the human host. Soon after the Brucella are introduced into the bloodstream, the bacteria seek out the nearest lymph nodes and invade the lymph node cells. From the initial lymph node, the Brucella spread out to other organ targets, including the spleen, bone marrow, and liver. Inside these organs, the infected cells form granulomas.

Diagnosing brucellosis involves culturing the blood , liver, or bone marrow for Brucella organisms. A positive culture alone does not signify brucellosis, since persons who have been treated for the disease may continue to harbor Brucella bacteria for several months. Confirmation of brucellosis, therefore, includes a culture positive for Brucella bacteria as well as evidence of the characteristic symptoms and a history of possible contact with infected milk or other animal products.

In humans, brucellosis caused by B. abortus is a mild disease that resolves itself without treatment. Brucellosis caused by B. melitensis and B. suis, however, is chronic and severe. Brucellosis is treated with administration of an antibiotic that penetrates host cells to destroy the invasive bacteria.

Prevention

Since the invention of an animal vaccine for brucellosis in the 1970s, the disease has become somewhat rare in the United States. Yet the vaccine cannot prevent all incidence of brucellosis. In 1989, the Centers for Disease Control reported only 95 total cases in the United States. Most of these were reported in persons who worked in the meat processing industry. Brucellosis remains a risk for those who work in close contact with animals, including veterinarians, farmers, and dairy workers.

Brucellosis also remains a risk when animal products from foreign countries are imported into the United States. Outbreaks of brucellosis have been linked to unpasteurized feta and goat cheeses from the Mediterranean region and Europe . In the 1960s, brucellosis was linked to bongo drums imported from Africa : drums made with infected animal skins can harbor Brucella bacteria, which can be transmitted to humans through cuts and scrapes in the human skin surface.

In the United States, preventive measures include a rigorous vaccination program that involves all animals in the meat processing industry. On an individual level, people can avoid the disease by not eating animal products imported from other countries. If this is not possible or desirable, make sure that imported cheeses have been made with pasteurized milk. If the package does not indicate pasteurization, do not eat the cheese.

Resources

books

Prescott, L., J. Harley, and D. Klein. Microbiology. 5th ed. New York: McGraw-Hill, 2002.

Thimm, Bernhard M. Brucellosos: Distribution in Man, Domestic, and Wild Animals. Berlin: Springer-Verlag, 1982.

periodicals

Kiel, Frank W., and M. Yousouf Khan. "Brucellosis in Saudi Arabia." Social Science and Medicine 29 (1989): 999-1001.

Olle-Goig, Jaime E., and Jaume Canela-Soler. "An Outbreak of Brucella melitensis. Infection by Airborne Transmission Among Laboratory Workers." American Journal of Public Health 77 (March 8, 1987): 335-38.

Wright, Paul. "Brucellosis." American Family Physician 35 (May 1987): 155-59.

Kathleen Scogna

KEY TERMS

Granuloma

—An immune response in which cells infected with bacteria clump together. Granuloma formation is typical of tuberculosis and brucellosis.

Lymph nodes

—Small, bean-shaped structures located along the lymphatic vessels of the body. Lymph nodes function in the immune response to protect the body against foreign cells.

Pasteurization

—The process in which milk or either dairy products are heated to a high temperature in order to kill disease-causing bacteria.

Brucellosis

Brucellosis is a disease caused by bacteria in the genus Brucella. The disease infects animals such as swine, cattle, and sheep; humans can become infected indirectly through contact with infected animals or by drinking Brucella-contaminated milk. In the United States, most domestic animals are vaccinated against the bacteria, but brucellosis remains a risk with imported animal products.

Brucella are rod-shaped bacteria that lack a capsule around their cell membranes. Unlike most bacteria, Brucella cause infection by actually entering host cells. As the bacteria cross the host cell membrane, they are engulfed by host cell vacuoles called phagosomes. The presence of Brucella within host cell phagosomes initiates a characteristic immune response, in which infected cells begin to stick together and form aggregations called granulomas.

Three species of Brucella cause brucellosis in humans: Brucella melitensis, which infects goats; B. abortis, which infects cattle and, if the animal is pregnant, causes the spontaneous abortion of the fetus; and B. suis, which infects pigs. In animals, brucellosis is a self-limiting disease, and usually no treatment is necessary for the resolution of the disease. However, for a period of time from a few days to several weeks, infected animals may continue to excrete brucella into their urine and milk. Under warm, moist conditions, the bacteria may survive for months in soil, milk, and even seawater.

Because the bacteria are so hardy, humans may become infected with Brucella by direct contact with the bacteria. Handling or cleaning up after infected animals may put a person in contact with the bacteria. Brucella are extremely efficient in crossing the human skin barrier through cuts or breaks in the skin.

The incubation period of Brucella, the time from exposure to the bacteria to the start of symptoms, is typically about three weeks. The primary complaints are weakness and fatigue. An infected person may also experience muscle aches, fever, and chills.

The course of the disease reflects the location of the Brucella bacteria within the human host. Soon after the Brucella are introduced into the bloodstream, the bacteria seek out the nearest lymph nodes and invade the lymph node cells. From the initial lymph node, the Brucella spread out to other organ targets, including the spleen, bone marrow, and liver. Inside these organs, the infected cells form granulomas.

Diagnosing brucellosis involves culturing the blood, liver, or bone marrow for Brucella organisms. A positive culture alone does not signify brucellosis, since persons who have been treated for the disease may continue to harbor Brucella bacteria for several months. Confirmation of brucellosis, therefore, includes a culture positive for Brucella bacteria as well as evidence of the characteristic symptoms and a history of possible contact with infected milk or other animal products.

In humans, brucellosis caused by B. abortus is a mild disease that resolves itself without treatment. Brucellosis caused by B. melitensis and B. suis, however, can be chronic and severe. Brucellosis is treated with administration of an antibiotic that penetrates host cells to destroy the invasive bacteria.

Since the invention of an animal vaccine for brucellosis in the 1970s, the disease has become somewhat rare in the United States. Yet the vaccine cannot prevent all incidence of brucellosis. The Centers for Disease Control usually reports fewer than 100 total cases per year in the United States. Most of these were reported in persons who worked in the meat processing industry. Brucellosis remains a risk for those who work in close contact with animals, including veterinarians, farmers, and dairy workers.

Brucellosis also remains a risk when animal products from foreign countries are imported into the United States. Outbreaks of brucellosis have been linked to unpasteurized feta and goat cheeses from the Mediterranean region and Europe. In the 1960s, brucellosis was linked to bongo drums imported from Africa; drums made with infected animal skins can harbor Brucella bacteria, which can be transmitted to humans through cuts and scrapes in the human skin surface.

In the United States, preventive measures include a rigorous vaccination program that involves all animals in the meat processing industry. On an individual level, people can avoid the disease by not eating animal products imported from countries where brucellosis is frequent, and by avoiding foods made with unpasteurized milk.

See also Bacteria and bacterial infection; Food safety; Infection and resistance; Pasteurization