A 38-year-old dairy farmer from rural Karnataka who regularly assists cows during calving presents with 5 weeks of evening fevers that come and go, drenching sweats, low back pain and tender sacroiliac joints. He has mild hepatosplenomegaly. A blood culture in biphasic (Castañeda) medium becomes positive only after prolonged incubation. The isolate is a small, faintly staining Gram-negative coccobacillus that is non-motile, oxidase positive, catalase positive and urease positive. On primary isolation it grows only in an atmosphere with added CO2 and produces H2S. Which organism is most likely?
Answer: B. Brucella abortus. Brucellosis is a zoonosis acquired from cattle, goats, sheep or pigs through contact with birth products, aerosols or unpasteurised milk, so dairy farmers, veterinarians and abattoir workers are at risk. It causes undulant fever, sweats, hepatosplenomegaly and focal disease, with sacroiliitis and spondylitis common. Brucellae are small, poorly staining, non-motile Gram-negative coccobacilli that are oxidase, catalase and urease positive and grow slowly, so blood cultures need prolonged incubation (biphasic Castañeda medium is the classic method). B. abortus (cattle) characteristically needs added CO2 on primary isolation and produces H2S; species are separated by CO2 need, H2S, urease and dye sensitivity. Handle cultures in containment, as laboratory-acquired infection is common.
A. Francisella tularensis is oxidase negative and needs cysteine-enriched media; it is linked with rabbits and ticks.
C. Pasteurella multocida grows overnight on blood agar, is indole positive and urease negative, and typically follows cat or dog bites.
D. Bordetella bronchiseptica is rapidly urease positive but is motile and grows readily on MacConkey agar within 1 to 2 days.
