Causative Agents

Brucella is a gram-negative, facultative, intracellular bacteria infecting many species of animals and humans. It is distinguished from most other pathogens since it has no obvious virulence factors such as capsules, fimbriae etc.. It is the causative agent of brucellosis (a zootonic disease that can also be transmitted to humans) and also recognized as bioterrorism agents. The genus Brucella has at least six species, where Brucella abortus (affecting primarily cattle), Brucella suis (affecting primarily swine) and Brucella melitensis (affecting primarily sheep and goats.) are of the major concern, which are all ‘not host-specific’ and may transmit to other animal species and humans under appropriate conditions. [1], [2]


Brucellosis occurs worldwide, except in countries where bovine brucellosis (B. abortus) has been eliminated (Australia, Canada, Cyprus, Denmark, Finland, the Netherlands, New Zealand, Norway, Sweden and the United Kingdom).The Mediterranean countries of Europe, Northern and Eastern Africa, Near East countries, India, Central Asia, Mexico and Central and South America are especially affected. It is usually either an occupational or a foodborne infection. Sporadic and epidemics both occur in humans; however the disease or infection is most often either unrecognized or, if diagnosed, not reported to the public authorities. [3]

Modes of Transmission

Transmission of infection to humans (with incubation period of generally 1-2 months) occurs; through breaks in the skin, by direct contact with placental tissues or vaginal discharges from infected animals (lesser transmission degree of contact with blood or urine). Foodborne infection occurred via unpasteurized milk and other dairy products are rarely seen. Occupational airborne infection (laboratories and abattoirs) has also been reported. Also cases of venereal and congenital infection in humans are reported. [3]


Culture and serological studies are usually used for diagnosis. Even though general laboratory findings (commonly slight elevation in liver enzymes) may be presumptive for the diagnosis, culture studies (microbiological isolation and identification) are reliable confirmatory methods. But they are also cumbersome (with at least 6 weeks of incubation period), having risk of transmission to laboratory staff and are not always successful. Serological testing based on detection of agglutinating antibodies (RBT SAT) is the common method. ELISA based on detection of antibodies (IgM, IgG, and IgA) has been gaining popularity recently. Anyway they are not highly sensitive or specific (especially where a cross-react with different antigens occurred). Polymerase chain reaction (PCR) provides a promising potential that allow for rapid and accurate diagnosis of brucellosis which is also clearly important to identify the species of Brucella implicated in natural infections. [4], [5], [6]

Annotated Bibliography

  1. M. J. Corbel, Brucellosis: an overview, Emerg Infect Dis. 1997 Apr–Jun; 3(2): 213–221.
  2. A. Robinson, Guidelines for Coordinated human and animal brucellosis surveillance,FAO. 2003
  3. Brucellosis in humans and animals. WHO guidance. Geneva, World Health Organization, 2005.
  4. Baldi PC, Miguel SE, Fossati CA, et al. Serological follow-up of human brucellosis by measuring IgG antibodies to lipopolysaccharide and cytoplasmic proteins of Brucella species. Clin Infect Dis. Mar 1996; 22(3):446-55.
  5. Debeaumont C, Falconnet PA, Maurin M. Real-time PCR for detection of Brucella spp. DNA in human serum samples. Eur J Clin Microbiol Infect Dis. Dec 2005; 24(12):842-5.
  6. Navarro E, Segura JC, Castano MJ, et al. Use of real-time quantitative polymerase chain reaction to monitor the evolution of Brucella melitensis DNA load during therapy and post-therapy follow-up in patients with brucellosis. Clin Infect Dis. May 1 2006; 42(9):1266-73

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