12.2 Skin & Soft Tissue, STI, and Vector-Borne Emergencies

12.2 Skin & Soft Tissue, STI, and Vector-Borne Emergencies

Skin & Soft Tissue Infection (SSTI) Classification & Management

Skin and soft tissue infections (SSTIs) are among the most frequent infectious diagnoses encountered in the emergency department. The Infectious Diseases Society of America (IDSA) categorizes SSTIs into two broad clinical presentations: non-purulent and purulent, further stratified by severity (mild, moderate, or severe).

Non-Purulent SSTIs (Cellulitis and Erysipelas)

• Pathophysiology & Microbiology: Non-purulent infections involve diffuse dermal and subcutaneous inflammation without macroscopic purulence, fluctuance, or abscess formation. Over 90% of cases are caused by beta-hemolytic streptococci (Streptococcus pyogenes [Group A Strep], Streptococcus agalactiae [Group B Strep], and Streptococcus dysgalactiae [Groups C and G]). Methicillin-susceptible Staphylococcus aureus (MSSA) is a secondary pathogen.
• Clinical Distinction:
  • Erysipelas: Acute superficial cutaneous involvement with fiery red erythema, raised, sharply demarcated margins, and rapid dermal lymphatic involvement.
  • Cellulitis: Deeper dermis and subcutaneous fat involvement with warmth, edema, and indistinct, poorly demarcated advancing borders.
• Empiric Regimens:
  • Mild (Outpatient): Cephalexin 500 mg PO every 6 hours (or Cefadroxil 500 mg PO every 12 hours). In patients with non-severe penicillin allergies: Clindamycin 300-450 mg PO every 8 hours (if local streptococcal resistance is low) or Cefadroxil.
  • Moderate / Severe (Inpatient): Cefazolin 1 g to 2 g IV every 8 hours.
• Antimicrobial Stewardship Pitfall:
  • Reflexive prescription of trimethoprim-sulfamethoxazole (TMP-SMX) or doxycycline to cover community-acquired MRSA (CA-MRSA) for typical non-purulent cellulitis is not recommended. Multiple randomized clinical trials demonstrate that adding TMP-SMX to cephalexin provides no improvement in clinical cure rates for uncomplicated cellulitis. TMP-SMX and doxycycline have unreliable, erratic activity against beta-hemolytic streptococci.

Purulent SSTIs (Cutaneous Abscesses, Furuncles, Carbuncles)

• Pathophysiology & Microbiology: Purulent SSTIs feature a circumscribed collection of pus in the dermis or deeper tissues, primarily driven by Staphylococcus aureus, with CA-MRSA accounting for 70% to 80% of isolates in many emergency departments.
• Definitive Treatment:
  • Incision and drainage (I&D) is the primary definitive therapeutic intervention. Evacuation of pus, probing loculations, and appropriate wound management achieve cure in the vast majority of simple, localized abscesses without antibiotics.
• Indications for Systemic Antibiotics Post-I&D:
  • Systemic signs of toxicity: fever (>38.0°C), tachycardia (>90 bpm), tachypnea (>20 breaths/min), or leukocytosis (>12,000 cells/mcL)
  • Severe immunocompromise, active chemotherapy, or neutropenia
  • Extremes of age (neonates/infants or frail elderly)
  • Multiple abscess sites or rapidly progressive surrounding cellulitis
  • Difficult-to-drain anatomical locations (face, hands, feet, genitalia)
  • Lack of response to prior incision and drainage alone
• Antimicrobial Regimens for Purulent SSTI:
  • Oral (Outpatient): Trimethoprim-sulfamethoxazole (TMP-SMX) 1 to 2 Double Strength (DS) tablets PO twice daily; OR Doxycycline 100 mg PO twice daily for 5 to 7 days.
  • Intravenous (Hospitalized / Severe): Vancomycin 15-20 mg/kg IV every 8-12 hours; OR Daptomycin 6-8 mg/kg IV once daily.

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Sexually Transmitted Infections: CDC 2021 Guideline Paradigms

The CDC Sexually Transmitted Infections Treatment Guidelines establish critical changes in first-line dosing, duration, and co-treatment paradigms.

CDC STI Treatment Matrix

Gonorrhea: The High-Dose Ceftriaxone Monotherapy Shift

In previous guidelines, dual therapy with ceftriaxone 250 mg IM plus oral azithromycin 1 g was standard. The CDC updated this recommendation to ceftriaxone 500 mg IM monotherapy (<150 kg; 1000 mg IM if ≥150 kg) based on:

1. Rapid emergence of macrolide resistance among Neisseria gonorrhoeae isolates globally.
2. Pharmacokinetic/pharmacodynamic data showing that ceftriaxone 500 mg achieves prolonged free concentrations above the MIC in pharyngeal, urogenital, and rectal mucosa, overcoming strains with elevated cephalosporin MICs.
3. Routine azithromycin co-treatment exerted adverse collateral selection pressure on respiratory and enteric flora.

Syphilis Safety Alerts & The Jarisch-Herxheimer Reaction

• Jarisch-Herxheimer Reaction: An acute systemic febrile reaction occurring within 2 to 24 hours after the first dose of penicillin in patients with spirochetal infections (most common in secondary syphilis, ~50-75% of patients). Driven by the massive release of endotoxin-like pyrogens from dying treponemes, patients develop high fevers, rigors, tachycardia, hypotension, flushing, and exacerbation of their cutaneous lesions.
• Emergency Management: This reaction is not an allergic hypersensitivity. Do not discontinue penicillin or label the patient as allergic. Management is entirely supportive with antipyretics (acetaminophen, ibuprofen), IV fluid resuscitation, and patient reassurance.

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Vector-Borne Emergencies: Rocky Mountain Spotted Fever & Lyme Disease

Tick-borne emergencies are seasonal clinical challenges presenting to emergency departments from spring through autumn. Prompt recognition and immediate empiric antimicrobial therapy are life-saving.

Rocky Mountain Spotted Fever (RMSF - Rickettsia rickettsii)

• Transmission & Geography: Transmitted by Dermacentor variabilis (American dog tick) and Dermacentor andersoni (Rocky Mountain wood tick). Endemic across the South Atlantic and south-central states (North Carolina, Tennessee, Oklahoma, Arkansas, and Missouri).
• Pathophysiology: R. rickettsii is an obligate intracellular bacterium with tropism for vascular endothelial cells. Bacterial invasion causes widespread microvascular injury, vasculitis, microvascular thrombosis, increased vascular permeability, non-cardiogenic pulmonary edema, cerebral edema, and multi-organ ischemic necrosis.
• Clinical Presentation:
  • Initial symptoms (days 1-3): Sudden onset of high fever (39.5-40.5°C), excruciating frontal headache, severe myalgias, photophobia, and nausea.
  • The Rash (days 2-4): Begins as small, blanching pink macules on the wrists, forearms, ankles, palms, and soles, spreading centripetally to the arms, legs, and trunk. Over subsequent days, it becomes petechial and purpuric, heralding extensive microvascular thrombosis.
  • Laboratory Hallmarks: Thrombocytopenia (<100,000/mcL), hyponatremia (<135 mEq/L), elevated transaminases, and elevated serum lactate.
• Therapeutic Regimen: Doxycycline 100 mg PO or IV every 12 hours (pediatric dosing: 2.2 mg/kg/dose IV/PO every 12 hours, maximum 100 mg/dose) for 5 to 7 days, continuing until the patient has been afebrile for at least 72 hours.

Lyme Disease (Borrelia burgdorferi)

• Transmission: Transmitted by Ixodes scapularis (blacklegged tick) in the Northeast and Upper Midwest. Tick attachment for at least 36 to 48 hours is required for transmission.
• Stages & Manifestations:
  • Early Localized: Erythema migrans (EM) rash (expanding annular erythematous plaque ≥5 cm, classically with central clearing creating a bull's-eye appearance) appearing 7 to 14 days post-bite, accompanied by fatigue, headache, and arthralgias.
  • Early Disseminated: Multiple secondary EM lesions, cranial neuropathy (unilateral or bilateral Cranial Nerve VII / facial palsy), Lyme carditis (fluctuating high-grade AV block, PR interval >300 ms), and aseptic meningitis.
  • Late: Recurrent monoarticular or oligoarticular arthritis (typically involving the knee).
• Pharmacotherapy:
  • Early Localized / Mild: Doxycycline 100 mg PO twice daily for 10 to 14 days. Alternatives in pregnancy or lactation: Amoxicillin 500 mg PO three times daily for 14 days or Cefuroxime axetil 500 mg PO twice daily for 14 days.
  • Lyme Carditis (Symptomatic, High-Degree AV Block, PR >300 ms) or CNS Disease (Meningitis): Ceftriaxone 2 g IV once daily for 14 to 21 days until clinical stabilization, then step down to oral doxycycline.
  • Post-Exposure Prophylaxis (PEP): Single dose Doxycycline 200 mg PO (pediatric: 4.4 mg/kg up to 200 mg) administered within 72 hours of tick removal, provided all criteria are met: tick is identified as an Ixodes species, attached for ≥36 hours, and local tick infection prevalence is ≥20%.