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Brucella Abortus: Biological Review Essay

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Updated: Oct 14th, 2021

Abstract

Brucellosis is a disease transmitted from animals to man. Cattle are the main infection reservoir, and infection transmits from animals to humans, rarely from one person to another. The aim of this essay is to review Brucella bacteria and its role in producing human disease briefly.

Introduction

Brucellosis is one of the zoonotic diseases, which are primarily animal diseases but occasionally transmitted to humans (Pappas et al., 2005, p. 2325). Based on 2003 statistics, the highest rates of human infection are in Syria, Iran, and Turkey. UK and USA show low rates of human infection (19 and 93 cases respectively) (Pappas et al., 2005. p. 2326).

The microorganism

Brucella species is a member of proteobacteria (alpha-2-subdivision). They are gram-negative intracellular Cocco-bacilli (or short bacilli). Characteristic biochemical reactions are partially acid-fast and similar to staphylococcus aureus, pseudomonas, and enterobacteria; they are oxidase catalase positive. They are urease positive like proteus mirabilis and nitrate reductase positive on nitrate broth selective medium. There are various species with preferential animal infection; Brucella abortus infects cattle, B. melitensis infects sheep and is the commonest human pathogen, B. suis preferentially infects pigs, and canis species infects dogs. Brucella has two types of lipopolysaccharide surface antigens that are responsible for the host response. The infected host responds by both humoral and cell-mediated immune responses with a predominance of the cell-mediated response (Mantur and Amarnath, 2008. p. 540).

Human Infection

Infection usually does not spread from one person to another, rather from animal to man. The incubation period is ill-defined (5 days to three weeks); commonly, symptoms start to appear after two weeks. Brucellosis, also called Malta fever, Mediterranean fever, or undulant fever is a multiple systems disease characterised by spontaneous remissions and relapses. The episode lasts for two to four weeks, then spontaneous remission occurs. Some cases may give no symptoms; others cases are accidentally discovered misdiagnosed as a hypersensitivity reaction. Brucella has a special likeness to infect the placenta and foetal membranes resulting in congenital brucellosis. Symptoms of congenital brucellosis are variable; preterm labour may occur as a result; alternatively, low birth weight, neonatal respiratory distress may occur early. Later, hepatosplenomegaly, with or without jaundice, and common failure to match growth curve. Neurological signs (personality disorders, meningitis, or encephalitis) may occur in up to 5% of brucellosis infections (the Center of Food Security and Public Health, 2007, p. 3).

The question of diagnosis

The gold standard for diagnosing brucellosis is to isolate the organism in a blood culture; however, the procedure is time-consuming that needs long incubation. Catalase tests must be performed cautiously as they may be a cause of organism nebulization and transmission of infection. The basic difficulty is the organism is intracellular; therefore, bone marrow culture is the best technique yielding results, with pain as the main disadvantage. Specimens are cultured on solid and liquid media (Cutler, 2006, p. 336-337).

Serological diagnosis depends mainly on the presence of antibodies against the lipopolysaccharide surface antigens resulting in agglutination reaction. ELISA tests (Enzyme-Linked Immunosorbent assay) use cytoplasmic proteins as antigens to measure immunoglobulin A, M, and G. A titre of 1:160 is diagnostic, however in endemic countries; a titre of 1:320 is diagnostic (Cutler, 2006, p. 336-337).

Polymerase chain reaction, PCR, detects various targets as IS711 (IS for intragenic spacer) and BCSP311 (a protein of the outer cell membrane) (Cutler, 2006, pp. 336-337).

Animal infection

Cattle are the main infection reservoir; horses, on the other hand, are resistant to infection. The cardinal sign in cattle is recurring abortion at the late months of pregnancy. In bulls, orchitis is the cardinal sign as endometritis may occur in cows. Young cattle remain symptomless until sexual maturity and pregnancy. In other animals, fistulous bursitis with isolation of Brucella abortus from the track is the cardinal sign (Austvitplan, 2005, pp. 8-9).

Prevention and treatment

Pasteurization of dairy products, proper hygiene and protective clothing when working with animals is the mainstay of prevention. Vaccination of humans at risk by living attenuated vaccine or the polysaccharide antigen showed questionable results. Cattle vaccination succeeded in reducing Brucella infection in many countries (Austvitplan, 2005, p. 14).

Treatment includes antibiotics capable of acting in an intracellular environment, preferably in combinations Pappas et al., 2006, pp. 2332-2333).

Conclusions

Brucella infection in humans can be a variant of occupational disease. The disease is difficult to treat as spontaneous remissions and relapses are the rules, diagnosis is difficult as isolation of the organism is difficult (being intracellular). Therefore, prevention is a fundamental line in management.

References

  1. Austvitplan (Australian Veterinary Emergency Plan), 2005. Disease Strategy Bovine brucellosis. 3 rd. Canberra: Primary Industries Ministerial Council.
  2. Cutler, S., 2006. Brucellosis the most common bacterial zoonosis? The Biomedical Scientist, 26 (1), 336-341.
  3. Mantur, B., G. and Amarnath, S. K., 2008. Brucellosis in India – a review. J. Biosci, 33, 539-547.
  4. Pappas, G., Akritidis, N., Bosilkovski, M., and Tsianos, E., 2005. Brucellosis. N. Engl. J. Med, 352 (22), 2325-2336.
  5. The Center for Food Security and Public Health, 2007. [Online]. Web. Iowa State University.
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