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Brucella is a bacterial genus that encompasses bacterial species that mainly cause infections in animals. Brucella species causes brucellosis, a zoonotic infection in animals. Brucellosis or undulant fever is a genitourinary infection, and the disease causes abortion in animals including pigs, cattle, sheep and goat. Brucella species are Gram-negative, non-motile, non-spore forming; facultative and rod-like obligate intracellular bacteria that majorly parasitize animals and humans by chance. Humans only become infected occasionally especially via exposure to animal reservoirs of the pathogen. Also, the consumption of animal products such as unpasteurized milk and other dairy products can serve as route via which the bacteria spread to man. People who work in animal farms, veterinary workers and those that work in slaughter houses or abattoirs are mostly at risk of an infection with Brucella species. Herdsmen and people who drink animal blood are also prone to Brucella infection. Human-to-human transmission of Brucella species do not occur. However, the bacteria can gain entry into the body through inhalation, penetration through cracked skin and via ingestion of contaminated food products especially those emanating from animal origin.     


Human infection with Brucella species is mainly via ingestion of contaminated food products of animal origin (e.g., raw or unpasteurized milk) or through occupational exposures especially in persons working in abattoirs and animal farms. Contact with carcasses of infected animals can also serve as route of pathogen spread. After entry into the human host, Brucella species migrate to the reticuloendothelial cells of the liver, kidney, bone marrow and spleen which they basically colonize. The incubation period of Brucella infection is about 1-4 weeks. Brucella species cause granulomas in the organs of the reticuloendothelial system (RES) including those of the spleen, liver and kidney where they apparently become intracellular. Brucella species leave the granulomas and become systemic (i.e., enters the bloodstream) to cause febrile or fever-like episodes usually experienced by the infected host mainly in the night hours. The presence of this fever-like episodes is the reason why the name of the disease due to Brucella infection is called undulating fever. Irregular and continuous fever known as undulating fever, arthritis, malaise, persistent night sweats, fatigue and headache which are akin to other bacterial infections are some of the clinical signs and symptoms that characterize human brucellosis. Abortion occurs only in animals (excluding human subjects) following the growth of the organism in pregnant animals especially in the placenta. Growth of Brucella species in the placenta of animals is mediated by erythritol, a growth enhancing factor which is absent in human brucellosis.   


The laboratory diagnosis of Brucella infection is usually based on the isolation of the bacteria in culture. Blood and biopsy materials are required for cultural techniques. Brucella species grow on commonly used culture media including chocolate agar, tryptone soy medium and brain heart infusion medium. A selective agar known as Brucella agar is also available for the differential isolation of the pathogen from specimens. Biochemically, Brucella speciesare oxidase-positive, catalase-positive and urease-positive. Serological tests based on agglutination techniques are also available for the laboratory diagnosis of the disease.


Because Brucella species assume an intracellular position in infected humans, protection against the diseases is usually cell-mediated. However, there is a marked production of antibody following the invasion of host cells by the bacterial pathogen.   


A combination antibiotic therapy that includes tetracycline, gentamicin or sulphamethoxazole-trimethoprim (co-trimoxazole) is usually the drugs of choice for treating human brucellosis. Though sensitive to a wide variety of antibiotics, Brucella species are difficult to eradicate from the infected human host because the organism is an obligate intracellular parasite that thrives and lives inside the cells of the infected human hosts.     


The prevention and control of human brucellosis is based on reducing contact or exposure to risk factors of infection by avoiding the consumption of unpasteurized or raw milk. Infected animals should be eradicated, and humans should try as much as possible to reduce contact with the carcasses of infected animals. No vaccine currently exists for human brucellosis. 


  • B. abortus infect cattle’s
  • B. canis infect dogs
  • B. suis infect swine
  • B. melitensis infect goats

Further reading

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.

Gilligan P.H, Shapiro D.S and Miller M.B (2014). Cases in Medical Microbiology and Infectious Diseases. Third edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Mahon C. R, Lehman D.C and Manuselis G (2011). Textbook of Diagnostic Microbiology. Fourth edition. Saunders Publishers, USA.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA.

Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.

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