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
• Facultative 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 in cows and bulls; decreased milk production
  • Localisation in spleen and lymphatic tissue of non-pregnant animals
  • Occasionally causes arthritis
• Isolated from closed cases of poll evil and fistulous withers in horses
• Caprine and ovine brucellosis:
  • Caused by B. melitensis
  • Goats more susceptible
  • Abortion, orchitis in males, arthritis, hygromas
  • May not have protective immunity following abortion
  • Rose Bengal agglutination test and complement fixation test
  • Test and slaughter where exotic
• Ovine epididymitis
  • Caused by B. ovis
  • Epididymitis in rams and placentitis in ewes
  • Reduced fertility in rams, sporadic abortion and peinatal mortality
  • Venereal transmission
  • Long latent period in rams following infection (present in semen 5 weeks after infection)
  • Premating checks on rams - serological tests and scrotal palpation
  • Testicular atrophy and swollen epididymis in chronically-infected rams
  • Agar gel immunodiffusion test, complement fixation test and indirect ELISA
  • Vaccination of young rams with B. melitensis vaccine or B. ovis bacterin
• Porcine brucellosis:
  • Caused by B. suis
  • Prolonged bacteraemia
  • Chronic inflammation in reproductive organs of boars and sows; also in joints and bones
  • Routes of infection: ingestion or venereal
  • Abortion, still birth, neonatal mortality, temporary sterility
  • Boars excreting bacteria in semen may have testicular abnormalities or be normal
  • Lameness, incoordination and posterior paralysis in joint and bone involvement
  • Rose Bengal and indirect ELISA for diagnosis
  • Test and slaughter where exotic
  • Modified live B. suis vaccine
• Canine brucellosis:
  • Caused by B. canis
  • Rough therefore lower virulence - mild or asymptomatic infections
  • Abortions, decreased fertility, reduced litter sizes, neonatal mortality
  • Orchitis and epididymitis causing infertility in male dogs
  • Infertility may be permanent if chronic infection
  • Slide agglutination, ELISA, agar gel immunodiffusion
  • Neuter infected animals
• Human brucellosis:
  • Susceptible to B. abortus, B. suis, B. melitensis and B. canis
  • Transmission via contact with secretions from infected animals
  • Routes of infection: skin abrasions, inhalation, ingestion
  • Unpasteurised milk source of infection
  • Undulant fever - fluctuating pyrexia, malaise, fatigue, muscle and joint pains, osteomyelitis
  • Can become chronic
  • B melitensis and B. suis cause most severe infections
  • Antimicrobials

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
• Brucellin for intradermal testing for B. abortus
• Enriched media for isolation
• Complement fixation test
• Indirect and competitive ELISA
• Serum agglutination test
• Antiglobulin test

Control

• Test and slaughter program has eradicated bovine brucellosis in the UK
• Vaccination of heifers against B. abortus in endemic regions:
  • Strain S19, a live attenuated vaccine, stimulating a cell-mediated immune response; vaccination of young animals; interferes with serological testing
  • 45/20 bacterin vaccine less effective
  • Newer RB51 vaccine has no LPS O-antigen therefore not detected by serological tests and gives good protection
• Live attenuated Brucella melitensis vaccine to protect lambs and kids against B. melitensis