A clinically depressed farmer complains of extreme weakness, a daily rise and fall in fever, and night sweats. Small gram-negative rods are isolated from blood cultures after a 2-week incubation period. Which of the following organisms is the most likely etiologic agent?

Brucella are small, aerobic, gram-negative coccobacilli. Of the four well-characterized species of Brucella, only one-B. melitensis-characteristically infects both goats and humans. Brucellosis may be associated with GI and neurologic symptoms, lymphadenopathy, splenomegaly, hepatitis, and osteomyelitis. Susceptibility to dyes (thionin and basic fuchsin) can help in differentiation of the species. None of the remaining options (C. jejuni, S. enteritidis, and S. marcescens) are known to routinely be passed from animals to humans. Francisella tularensis is known to be so (see the table below), but usually in connection with hunting and cleaning game animals. See the table below for a listing of gram-negative rods associated with animal sources.GRAM-NEGATIVE RODS ASSOCIATED WITH ANIMAL SOURCESSpeciesDiseaseSource of Human InfectionHuman Transmission from Animal to humanMode of DiagnosisBrucella speciesBrucellosisPigs, cattle, goats, sheepDairy products; contact with animal tissuesSerology or cultureFrancisella tularensisTularemiaRabbits, deer, ticksContact with animal tissues; ticksSerologyYersinia pestisPlagueRodentsFlea biteImmunofluorescence or cultureFasteurella multocidaCellulitisCats, dogsCat or dog biteWound culture SPIROCHETES OF MEDICAL IMPORTANCESpeciesDiseaseMode of Transmission DiagnosisMorphologyGrowth in Bacteriologic MediaTreatmentTreponema pallidumSyphilisIntimate (sexual) contact; across the placentaMicroscopy; serologic testsThin, tight, spirals, seen by dark-field illumination, silver impregnation, or immunofluorescent stain-Penicillin GBorrelia burgdorferiLyme diseaseTick biteClinical observations; microscopyLarge, loosely coiled; stain with Giemsa stain+Tetracycline or amoxicillin for acute; penicillin G for chronicBorrelia recurrentisRelapsing feverLouse biteClinical observations; microscopyLarge, loosely coiled; stain with Giemsa stain+TetracyclineLeptospira interrogansLeptospirosisFood or drink contaminated by urine of infected animals (rats, dogs, pigs, and cows)Serologic testsThin, tight spirals, seen by dark-field illumination+Penicillin G