Brucella species

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Overview

  • Important zoonoses worldwide
  • Cause chronic granulomatous diseases
  • 6 species
  • Target reproductive organs of certain species
  • Infected animals act as reservoir of infection
  • Organisms can remain viable in moist environment for months
  • Cause undulant fever in humans


Characteristics

  • Small, non-moltile, Gram negative coccobacilli
  • Facultatice intracellular pathogens
  • Modified Ziehl-Neelsen positive - clusters of red coccobacilli on smears
  • Aerobic and capnophilic
  • Catalase positive; oxidase and urease positive except for Brucella ovis
  • Some species require enriched media for growth
  • Non-haemolytic
  • Smooth colonies of B. abortus, B. melitensis and B. suis are small, glistening, blue and translucent after incubation for 3-5 days, and become opaque with age
  • Rough colonies of B. ovis and B. canis are dull, yellow, opaque and friable
  • Slide agglutination with speicific antisera detect important antigens
  • B. abortus lysed by specific bacterophages
  • Oxidative metaboloic rates can differentiate species


Pathogenesis and pathogenicity

  • Brucellae that lack outer membrane LPS (rough colonies) are less virulent than those which possess it
  • Penetrate nasal, oral or pharyngeal mucosa
  • Phagocytosed and carried to regional lymph nodes
  • Smooth organisms survive and multiply in cells of the reticulo-endothelial system
  • Inhibit lysosome-phagosome fusion
  • Superoxide dismutase and catalase production may resist oxidative killing
  • Lymph nodes enlarge (lymphatic and lymphoreticular hyperplasia) and inflammation is induced
  • Surviving organisms spread to other organs (liver, spleen, placenta) and cause granulomatous reactions
  • Eythritol is a growth stimulant and attracts the bacteria to the placenta of cattle, sheep, goats and pigs; also found in mammary gland and epididymis, targets for brucellae
  • Infection of foetus and abortion
  • May localise in joints or intervertebral discs in chronic infections


Clinical disease

  • Bovine brucellosis:
    • Caused by Brucella abortus
    • Eradicated in many countries including UK
    • Infection usually by ingestions but also venereal, skin abrasions, inhalation, transplacental
    • Abortion storms in susceptible herds
    • Abortion after fifth month of gestation due to placentitis
    • Brucellae excreted in foetal fluids for 2-4 weeks following abortion and at subsequent parturitions without abortion
    • Infection of mammary glands and lymph nodes persists for years
    • Excreted intermittently in milk for years
    • Seminal vesicles, ampullae, testicles and epididymus infected in bulls; necrotising orchitis
    • Decreased fertility and milk production


Diagnosis

  • Serological testing of milk (Milk Ring Test) and beef cattle (Rose Bengal Plate Test)
  • Serological tests detect anti-lipopolysaccharide antibodies
  • LPS antigen present in virulent as well as some vaccine strains therefore vaccination may confuse serological testing
  • False positives due to cross-reaction with LPS in other bacteria
  • Modified Ziehl-Neelson stains reveal organisms in samples from cotyledons, uterine discharge and foetal abomasal contents
  • PCR for detection in tissue
  • Enriched media for isolation


Control

  • Vaccination:
    • Strain S19, a live vaccine, stimulating a cell-mediated immune response; vaccination of young animals; interferes with serological testing
    • Newer RB51 vaccine has no LPS O-antigen therefore not detcted by serological tests
    • Live attenuated Brucella melitensis vaccine to protect sheep and goats against B. melitensis
  • Test and slaughter program has eradicated Brucellosis in the UK