13.3 Infectious Disease Principles & Systemic Pathogens

13.3 Infectious Disease Principles & Systemic Pathogens

Systemic infectious diseases present diagnostic challenges due to overlapping systemic manifestations. USMLE Step 2 CK emphasizes structured clinical evaluations for Fever of Unknown Origin (FUO), emergency management of neutropenic fever, differential diagnosis of vector-borne zoonoses, and prevention of healthcare-associated systemic infections.

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Fever of Unknown Origin (FUO)

Diagnostic Criteria

Classic Fever of Unknown Origin (FUO) is defined by three strict clinical criteria:

1. Body temperature $> 38.3^\circ\text{C}$ ($101.0^\circ\text{F}$) recorded on multiple occasions,
2. Fever duration $> 3 \text{ weeks}$, and
3. No definitive diagnosis established after $1 \text{ week}$ of intensive outpatient or inpatient investigation (or 3 outpatient visits / 3 days of inpatient testing).

Etiologic Spectrum

FUO cases fall into four major diagnostic categories:

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Febrile Neutropenia Protocol

Febrile neutropenia is a medical emergency requiring immediate empiric broad-spectrum intravenous antimicrobial therapy due to high risk of overwhelming bacterial sepsis.

Definitions

• Fever: A single oral temperature $\ge 38.3^\circ\text{C}$ ($101.0^\circ\text{F}$) OR a sustained temperature $\ge 38.0^\circ\text{C}$ ($100.4^\circ\text{F}$) for $> 1 \text{ hour}$.
• Neutropenia: Absolute Neutrophil Count (ANC) $< 500/\text{mm}^3$, or an ANC expected to fall $< 500/\text{mm}^3$ within 48 hours.

$\text{ANC calculation: } \text{ANC} = \text{Total WBC count} \times \left(\frac{\% \text{Neutrophils} + \% \text{Bands}}{100}\right)$

Empiric Antibiotic Selection

Because neutropenic patients lack functional white blood cells to produce classic signs of localized inflammation (e.g., purulence), Pseudomonas aeruginosa and enteric Gram-negative bacilli can cause rapid mortality.

• First-line Empiric Monotherapy: Immediate administration of an antipseudomonal $\beta$-lactam:
  • Cefepime ($2 \text{ g}$ IV q8h), OR
  • Piperacillin-tazobactam ($4.5 \text{ g}$ IV q6h), OR
  • Meropenem ($1 \text{ g}$ IV q8h).
• Indications to Add Vancomycin: Vancomycin is NOT routinely included in initial empiric therapy unless specific indications exist: hemodynamic instability/septic shock, catheter-related infection, skin/soft tissue infection, known MRSA colonization, or pneumonia.

                    FEBRILE NEUTROPENIA EMERGENCY ALGORITHM
                    
       Single Temp >= 38.3°C (101°F) + ANC < 500/mm³
                                 |
                                 v
       Immediate Blood Cultures x 2 (Peripheral + Central Line)
       + Urinalysis, CXR, Physical Exam (DO NOT perform digital rectal exam!)
                                 |
                                 v
         Administer Empiric IV Antipseudomonal Monotherapy < 60 Min
         (Cefepime 2g IV OR Piperacillin-tazobactam 4.5g IV)
                                 |
         +-----------------------+-----------------------+
         |                                               |
   No MRSA / Shock Signs                   Hemodynamic Instability /
         |                                 Catheter Infection / MRSA Risk
         v                                               v
   Continue Monotherapy                    Add IV Vancomycin to 
   Re-evaluate at 48-72 hours              Antipseudomonal Beta-Lactam

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Vector-Borne and Systemic Zoonotic Pathogens

Vector-borne infections present with characteristic cutaneous findings, geographic exposures, and peripheral blood features:

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Healthcare-Associated Systemic Infections

Central Line-Associated Bloodstream Infections (CLABSI)

CLABSI is defined as a primary bloodstream infection occurring in a patient with a central venous catheter in place for $> 48 \text{ hours}$ prior to infection onset, which is not secondary to infection at another site.

• Most Common Pathogens:
  1. Coagulase-negative Staphylococci (Staphylococcus epidermidis) — most common overall (biofilm formation).
  2. Staphylococcus aureus (both MSSA and MRSA).
  3. Candida species (especially in parenteral nutrition, prolonged broad-spectrum antibiotics).
  4. Enteric Gram-negative bacilli (E. coli, Klebsiella, Pseudomonas).
• Prevention Bundle: Strict hand hygiene, maximal sterile barrier precautions during insertion, chlorhexidine skin antisepsis, avoidance of femoral vein insertion site, and daily review of line necessity with prompt removal of unneeded catheters.