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Brucellosis is a zoonotic infection caused by species of the genus Brucella. Brucella are nonencapsulated, nonmotile, aerobic, gram-negative coccobacilli. Human infections are usually caused by 4 species, which are classified based on their animal reservoir: Brucella melitensis (goat, sheep), Brucella abortus (cattle), Brucella suis (pig), and Brucella canis (dog). B melitensis is the most common cause of human infection. Other species rarely causing human disease include Brucella ceti and Brucella pinnipedialis (marine animals). Brucella inopinata has been associated with infection of prosthetic implants.


Brucella are facultative intracellular microorganisms with a tendency to establish latent and chronic infection. Brucella replicate within vacuoles of macrophages, monocytes, dendritic cells, trophoblasts, and epithelial cells. After the organisms penetrate the mucosal barriers and enter the bloodstream, Brucella multiply in the tissues of the reticuloendothelial system and result in granulomas. The organism evades the host immune response by using a wide repertoire of virulence factors and immunomodulatory strategies. The virulence of Brucella may be related to the smooth lipopolysaccharides that cover the bacterium. The development of type 1 (Th1) cellular immune response with production of proinflammatory cytokines such as interferon-γ (IFN-γ) by T cells and natural killer cells results in clearance of the organism.

Human brucellosis is a major public health problem in many areas of the globe, with an estimated 500,000 new cases reported annually. Brucellosis is endemic in Mediterranean countries, the Middle East, Central Asia, India, Mexico, and Central and South America. The disease is rare in the United States and other high-income countries due to effective eradication programs in domestic livestock. In 2013, only 8 of 99 brucellosis cases reported to the Centers for Disease Control and Prevention were in patients younger than 15 years old.

Transmission of Brucella to humans can occur via ingestion of contaminated animal products, inoculation of the skin or conjunctiva during direct contact with infected animals or their products (such as placenta or aborted tissues), and inhalation of contaminated aerosolized particles. Consumption of unpasteurized milk or other dairy products imported primarily from Mexico accounts for the majority of cases of childhood brucellosis in the United States.

Brucella infection has been reported with recreational activities such as hunting feral swine in California, Florida, and Texas. Brucellosis is an occupational hazard for laboratory personnel, abattoir workers, veterinarians, and shepherds. Brucella is considered a potential agent for bioterrorism given airborne transmission. Human-to-human transmission is very unusual. Mother-to-baby transmission during pregnancy and breastfeeding has been reported.


Brucellosis is a disease of protean manifestations and can involve any organ system. Symptoms typically develop 2 to 4 weeks after exposure and can be acute or insidious. Fever is present in almost every patient and may wax and wane over a prolonged period of time, which is how it earned the name “undulant fever.” The ...

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