Brucellosis

Clinical & Epidemiological Context

Key Concept: A granulomatous zoonosis that acts as a "Great Mimicker" of Tuberculosis and Sarcoidosis.

• Pathogen: Brucella spp. (B. melitensis is the most virulent).
• Thailand/SE Asia Relevance:
  • Endemic in goat/cattle farming regions.
  • The Diagnostic Trap: Clinically indistinguishable from Tuberculosis (TB) without imaging/serology. Misdiagnosis leads to inappropriate anti-TB therapy.
• Clinical Triad: Undulant fever + Arthralgia + Hepatosplenomegaly.

1. Spinal Brucellosis (Brucellar Spondylitis)

The most common osteoarticular manifestation (~50%).

Distribution

• Region: Lumbar (L4-L5) > Thoracic > Cervical.
• Pattern: Typically involves two contiguous vertebrae and the intervening disc.

Imaging Morphology: The "Pedro Pons" Sign

The radiological hallmark is the anterosuperior vertebral corner lesion.

• Radiography/CT:
  • Pedro Pons Sign: Focal erosion of the anterosuperior vertebral endplate.
  • Reactive Sclerosis: Significant bony sclerosis surrounding the erosion (more prominent than in TB).
  • Osteophytes: "Parrot-beak" or bridging osteophytes develop early (an attempt at healing).
  • Intradiscal Gas: Vacuum phenomenon may occur due to ischemic necrosis of the disc (rare in TB/Pyogenic, where proteolytic enzymes cause liquefaction).

MRI Findings (Gold Standard)

• T1: Hypointense disc and adjacent vertebral bodies.
• T2/STIR:
  • Disc: Hyperintense (morphology often preserved longer than in pyogenic infection).
  • Bone Marrow: Diffuse edema in the involved vertebrae.
  • Abscesses:
    • Morphology: Unlike the large, flowing "cold abscess" of TB, Brucella abscesses are small, spherical, and focal.
    • Location: Usually limited to the level of the lesion (paravertebral).
    • Enhancement: Homogeneous or thick peripheral enhancement.
• Epidural extension: Common but usually causes less cord compression than TB.

2. Sacroiliitis (High Yield MSK)

Often overlooked but highly specific when present with fever.

• Frequency: High prevalence (up to 40–50% of osteoarticular cases).
• Laterality: Typically unilateral (crucial differentiation from Spondyloarthropathies like Ankylosing Spondylitis, which are bilateral/symmetric).
• Imaging:
  • Blurring of articular margins.
  • Joint space widening (early) $\to$ narrowing/ankylosis (late).
  • Pearl: Unilateral sacroiliitis in a patient with fever $\to$ Think Brucella.

3. Neurobrucellosis

Rare (<5%) but a high-yield exam topic due to mimicry.

Patterns of Involvement

1. Meningitis (Most Common): Thickening and enhancement of the basal meninges (mimics TB, Fungal, or Sarcoidosis).
2. White Matter Changes:
   • Multiple T2/FLAIR hyperintensities in periventricular white matter.
   • The Mimic: Can appear identical to Multiple Sclerosis (MS), ADEM, or Neuro-Behçet's.
3. Vascular: Arteritis leading to lacunar infarcts or venous sinus thrombosis.
4. Cranial Nerves: Enhancement of CN VIII (Sensorineural hearing loss).

4. Hepatosplenic Brucellosis

Hepatosplenomegaly is the most common radiographic finding overall.

• Acute: Non-specific hepatosplenomegaly.
• Chronic (Brucelloma):
  • "Bull's Eye" Lesion: Central calcification with a surrounding hypodense halo (CT) or hypointense rim (MRI).
  • Snowflake Calcification: Diffuse, flocculent calcifications in the spleen/liver (late stage).

5. Scrotal/Genitourinary

• Brucellar Orchiepididymitis:
  • Usually unilateral enlargement.
  • US: Heterogeneous echotexture, thickened epididymis, +/- testicular abscess.
  • Note: Indistinguishable from standard bacterial epididymo-orchitis on imaging alone; requires clinical correlation.

Differential Diagnosis: The "Thailand Trap" (TB vs. Brucella)

In Thailand, distinguishing these two is the primary diagnostic challenge.

Guidelines & Management (NICE / RCR / Thai Context)

Diagnostic Pathway

1. Suspect: Back pain + Fever + Animal exposure/Unpasteurized dairy.
2. First Line: MRI Spine (contrast essential).
3. Confirmation:
   • Serology is King: Standard Agglutination Test (SAT) > 1:160.
   • Biopsy Caution: Avoid if possible. If serology is positive, biopsy is contraindicated due to aerosolization risk to lab staff (Level 3 Biohazard).

Thai Guideline Nuance

• In equivocal cases (TB vs. Brucella):
  • Assess for concomitant pulmonary findings (favor TB).
  • Look for "Snowflake" splenic calcifications (favor Brucella).
  • Do not start empiric anti-TB treatment solely based on spinal granulomas without ruling out Brucella.

Radiological Pearls