Brucella and Brucellosis
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Direct answer
Brucella is a genus of small, intracellular Gram-negative coccobacilli that causes brucellosis, a zoonosis acquired from infected animals or unpasteurised dairy products. The disease produces undulant fever, sweating, arthralgia and sacroiliitis, and it can involve almost any organ system. Treatment uses doxycycline with rifampicin for several weeks, because combination therapy lowers the risk of relapse.
What you must remember
- Morphology: small non-motile, non-capsulated, non-spore-forming Gram-negative coccobacilli that survive inside macrophages.
- Species: B. melitensis from goats and sheep is the most virulent for humans, while B. abortus, B. suis and B. canis are also human pathogens.
- Biochemistry: oxidase and catalase positive, urease positive, and slow to grow in culture.
- Clinical picture: undulant fever with sweats, malaise, arthralgia and hepatosplenomegaly, with sacroiliitis and spondylitis typical.
- Complications: endocarditis is the commonest cause of death, and orchitis, neurobrucellosis and osteoarticular spread also occur.
- Serology: Rose Bengal plate test for screening and standard tube agglutination for confirmation, with a titre of 1:160 or more commonly taken as significant.
- Treatment: doxycycline with rifampicin for six weeks, with an aminoglycoside added in complicated disease.
- Prevention: pasteurisation of milk and vaccination of livestock, such as strain 19 in cattle, since no human vaccine is available.
From exposure to cure
Brucellosis is often diagnosed late because its fever resembles many other illnesses. The steps below move from the exposure history to the choice of treatment.
- Exposure history: ask about raw milk, soft cheese, contact with goats, sheep or aborting animals, and work in abattoirs or laboratories.
- Blood culture: draw cultures with extended incubation, since the organism is slow-growing and may need lysis-centrifugation to be recovered.
- Serology: the Rose Bengal plate test screens, and the standard tube agglutination test confirms the diagnosis when titres are high.
- Coombs test: used when clinical suspicion is strong but standard agglutination is negative, because blocking antibodies can hide the infection.
- Combination therapy: doxycycline with rifampicin for six weeks is the standard regimen and lowers relapse compared with single-drug therapy.
- Complicated disease: endocarditis, neurobrucellosis or osteoarticular spread add an aminoglycoside for the early weeks and may need surgery or longer treatment.
Where the exam tries to catch you
Brucella questions test species virulence, serology and the drug combination.
- Trap: giving doxycycline alone as the standard treatment Correct: combination with rifampicin for six weeks is preferred because it reduces relapse.
- Trap: treating undulant fever as a specific diagnosis Correct: it describes the intermittent fever pattern, and brucellosis is confirmed by culture or serology.
- Trap: assuming brucellosis spreads readily between people Correct: humans acquire it from animals or their products, and person-to-person transmission is not significant.
Frequently asked questions
Which Brucella species is the most virulent for humans?
Brucella melitensis, which infects goats and sheep, is the most virulent species for humans and is a common cause of severe brucellosis. Brucella abortus, Brucella suis and Brucella canis can also infect people, mainly through contact with infected animals or their products.
What is undulant fever in brucellosis?
Undulant fever is the intermittent, wave-like fever pattern produced by Brucella infection, with periods of fever and sweating alternating with normal temperature. Brucellosis often presents with this pattern together with malaise, arthralgia and hepatosplenomegaly.
Why is the Coombs test used in brucellosis serology?
The Coombs test detects blocking antibodies, which are incomplete antibodies that prevent agglutination in the standard tube test. Brucellosis may therefore be missed by standard agglutination, and the Coombs test is used when clinical suspicion remains high.
Why is rifampicin combined with doxycycline in brucellosis?
Doxycycline with rifampicin for six weeks is the standard combination in brucellosis because it reduces the relapse seen with single-drug therapy. The two drugs act by different mechanisms, and the combination is preferred per current guidance.
Which test is used to screen for brucellosis?
The Rose Bengal plate test is a rapid screening test for brucellosis that uses antigen agglutination on a slide. Positive samples are then confirmed with the standard tube agglutination test, and blood culture remains the definitive investigation when it is available.