Brucellosis
A 35-year-old shepherd from Aseer has four months of fever, drenching night sweats and low back pain. He regularly drinks unpasteurised camel milk. The right knee is swollen with an effusion and lumbar movement is limited. Knee aspiration yields 15,000 white cells/µL with no organisms on Gram stain, and two sets of blood cultures held for 5 days are negative. Which of the following is the most appropriate next action?
- A.Serum agglutination test for BrucellaCorrect
- B.Percutaneous liver biopsy
- C.Bone marrow aspirate and biopsy
- D.Repeat peripheral blood cultures
Explanation
Prolonged undulating fever, night sweats, arthralgia, sacroiliac or spinal involvement, and a history of unpasteurised dairy consumption in an endemic region such as Saudi Arabia make brucellosis the leading diagnosis. Brucella is a small, slow-growing, fastidious intracellular Gram-negative coccobacillus, so conventional blood cultures are frequently negative, particularly in chronic or partially treated disease and when incubation is stopped at the routine five days; laboratories must be alerted so cultures are held for up to three weeks. Serology with the standard tube agglutination test is therefore the practical diagnostic test of choice, with a titre of 1:160 or higher considered significant in endemic areas and a fourfold rise between paired sera being confirmatory. Osteoarticular disease is the commonest focal complication, affecting up to 40% of patients, and typically involves the sacroiliac joints, spine, hip, or knee, exactly as seen here. Treatment requires prolonged combination therapy, usually doxycycline for six weeks plus either streptomycin or gentamicin for the first two to three weeks, or doxycycline plus rifampicin; monotherapy leads to relapse. Liver biopsy might show non-caseating granulomas but is invasive and non-specific. Bone marrow culture is more sensitive than blood culture and can be considered when serology is equivocal, but it is invasive and not the first step. Simply repeating standard blood cultures without extended incubation is likely to remain negative and wastes time.
Why each option
- A.
- Correct. Brucella serology is the diagnostic test of choice because the organism is fastidious and blood cultures are often falsely negative in chronic disease.
- B.
- Liver biopsy may reveal non-specific granulomas but is invasive and unnecessary when a simple serological test is diagnostic.
- C.
- Bone marrow culture has higher yield than blood culture but is invasive and reserved for cases with equivocal serology.
- D.
- Repeating routine blood cultures without prolonged incubation will most likely remain negative in chronic brucellosis.
Reference: Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, 9th ed., 2020
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