Brucellosis

Basics

= Undulant fever. Obligate aerobic, fastidious, small Gram-negative coccobacillus. Zoonotic (sheep/goat/cattle). Named: Sir David Bruce (1886), isolated B. melitensis (Malta) — “Malta fever.”

Classification

Genetically ~1 species, phenotypically split into nomen species:

B. melitensis — sheep/goat/camel, MOST PATHOGENIC, infects humans readily
B. abortus — cattle/buffalo
B. suis — pigs
B. canis — dogs (abortion), rare human cases

Antigens: M (predominant B. melitensis) and A (predominant B. abortus) LPS types; B. suis = either.

Pathogenesis

Virulence: melitensis > abortus > suis. Other species rare in humans.

Transmission:

Direct contact (COMMONEST) — abraded skin/mucosa + infected tissue/blood/urine/vaginal discharge/placenta
Food-borne — unpasteurized milk/dairy, undercooked meat
Airborne — dust/aerosol in cowshed/slaughterhouse
Person-to-person RARE (breast milk, transplant, transfusion)

Spread: entry site → bacteremia → RE organs (LN, spleen, liver, marrow) + placenta + musculoskeletal + GU system. Tissue response: neutrophilic infiltration → chronic inflammatory cells → GRANULOMA (subacute-chronic course).

Clinical Manifestations

Incubation 1-3 weeks. “Undulant fever” — evening rise, morning fall, recurs over weeks untreated. + night sweats, malaise, arthralgia, myalgia, weight loss. Hepatosplenomegaly, lymphadenopathy common.

Focal complications:

Sacroiliitis/spondylitis — COMMONEST osteoarticular complication
Epididymo-orchitis
Brucellar endocarditis — RARE but LEADING cause of brucellosis mortality

Untreated → chronic, relapsing over months-years.

Laboratory Diagnosis

Culture: blood/bone marrow = gold standard. Bone marrow MORE sensitive (RE system concentration). SLOW growth (fastidious, biphasic Castaneda, weeks). BSL-3 required (high lab-acquired infection risk).

Serology (practical mainstay):

SAT (standard agglutination test) — smooth LPS Ag, titer ≥1:160 supportive
ELISA (IgM/IgG) — better sensitivity, acute+chronic
Coombs/antiglobulin test — unmasks blocking non-agglutinating Ab in chronic cases (SAT alone underestimates)

Molecular: PCR — faster, avoids biohazard of culture.

Treatment

Intracellular organism → needs PROLONGED combination Rx (single short-course fails, relapse risk). Standard adult: Doxycycline + Rifampicin × 6 weeks. OR Doxycycline + Aminoglycoside (streptomycin/gentamicin) × 2-3wk, then doxycycline alone to complete 6wk. Endocarditis: longer multidrug course + often valve surgery.

Prevention

Milk/dairy pasteurization, proper meat cooking, animal vaccination + herd screening/culling, PPE for occupational exposure (vets, abattoir workers, farmers). NO human vaccine available.

What Brucellosis Is

Brucellosis (also called undulant fever) is a highly contagious febrile zoonotic illness caused by Brucella, an obligate aerobic, fastidious, small Gram-negative coccobacillus. It is primarily a disease of domestic animals — sheep, goats, cattle — and human infection is essentially always tied to occupational or domestic exposure to infected animals or their products.

Brucella is named for Sir David Bruce (1886), the British army physician who isolated the first recognized species, B. melitensis (from Melita, the Roman name for Malta), giving the disease its older name, Malta fever.

Classification

DNA hybridization shows the genus Brucella is genetically very tight-knit — essentially variants of a single species — but for practical purposes it is still split into nomen species by phenotype. The clinically important ones: B. melitensis (sheep, goats, camels — also readily infects humans, and is the most pathogenic species), B. abortus (cattle, buffalo), B. suis (pigs), and B. canis (dogs, causing canine abortion, with occasional human cases).

Antigenic structure: two major LPS antigen types, M and A, present in varying proportions across species — M predominates in most B. melitensis biovars, A in most B. abortus biovars, while B. suis biovars carry either.

Pathogenesis

Virulence ranks B. melitensis > B. abortus > B. suis; human infection with other species is very rare.

Transmission occurs by several zoonotic routes: direct contact (the commonest — abraded skin/mucosa touching infected animal tissue, blood, urine, vaginal discharge, or placenta), food-borne (unpasteurized milk/dairy, undercooked meat, occasionally contaminated water/vegetables), and airborne (inhaling dust/aerosols in a contaminated cowshed or slaughterhouse). Person-to-person spread is extremely rare (breast milk, tissue transplant, transfusion).

From the entry site, organisms spread to blood (bacteremia) and disseminate to reticuloendothelial organs — lymph nodes, spleen, liver, bone marrow — plus placenta, musculoskeletal tissue, and the genitourinary system. The local tissue response starts with neutrophilic infiltration, later replaced by chronic inflammatory cells producing granulomas — reflecting brucellosis’s tendency toward a subacute-to-chronic clinical course.

Clinical Manifestations

Incubation is typically 1–3 weeks (occasionally longer). The name “undulant fever” captures its classic pattern — an undulating fever that rises in the evening and falls by morning, recurring over weeks if untreated, alongside night sweats, malaise, arthralgia, myalgia, and weight loss. Hepatosplenomegaly and lymphadenopathy are common. Focal complications reflect the reticuloendothelial and musculoskeletal tropism: sacroiliitis and spondylitis (the commonest osteoarticular complication), epididymo-orchitis, and — most seriously — brucellar endocarditis (rare but the leading cause of brucellosis mortality). Untreated infection can become chronic, with relapsing symptoms over months to years.

Laboratory Diagnosis

Culture: blood or bone marrow culture remains the gold standard for confirming active infection — bone marrow culture is more sensitive, since Brucella concentrates in the reticuloendothelial system. Growth is slow (Brucella is notably fastidious, sometimes needing prolonged incubation up to several weeks on biphasic Castaneda medium), and because of high laboratory-acquired-infection risk, culture should only be attempted with BSL-3 precautions.

Serology is the practical mainstay given culture’s slowness: the standard agglutination test (SAT) detects antibody against smooth LPS antigen, with a rising or high titre (commonly ≥1:160 in a compatible clinical picture) supporting diagnosis; ELISA (IgM/IgG) offers better sensitivity for both acute and chronic/relapsed disease. The Coombs (antiglobulin) test can unmask “blocking” non-agglutinating antibody in chronic cases where SAT alone underestimates true infection.

Molecular methods (PCR) are increasingly used, particularly useful given culture’s slow turnaround and biohazard risk.

Treatment

Because Brucella is an intracellular organism, treatment needs a prolonged combination regimen to prevent relapse — a single short-course agent reliably fails. The standard adult regimen is doxycycline plus rifampicin for 6 weeks, or doxycycline plus an aminoglycoside (streptomycin or gentamicin) for the first 2–3 weeks, with doxycycline continued alone to complete 6 weeks. Brucellar endocarditis needs a longer multidrug course, often alongside valve surgery.

Prevention

Control rests on animal-side and human-side measures together: pasteurization of milk and dairy products, proper cooking of meat, animal vaccination and herd screening/culling programmes, and protective equipment (gloves, masks) for occupationally exposed workers (veterinarians, abattoir staff, farmers). No human vaccine is currently available.