12.2 Common Infectious Diseases: Skin/Soft Tissue Infections, Tick-Borne Illnesses & Viral Syndromes
Key Takeaways
- Skin and soft tissue infections (SSTIs) are stratified into purulent (abscess, furuncle, carbuncle) vs. non-purulent (cellulitis, erysipelas); purulent abscesses require incision and drainage (I&D) as the definitive primary therapy, with oral MRSA coverage (TMP-SMX, doxycycline, clindamycin) added for systemic signs, surrounding erythema >2 cm, or high-risk locations.
- Necrotizing fasciitis is a surgical emergency characterized by pain out of proportion to physical exam findings, rapid progression, wooden-hard edema, crepitus, hemorrhagic bullae, and systemic shock; clinical suspicion mandates emergent surgical debridement without delaying for imaging or laboratory confirmation.
- Early Lyme Disease (Stage 1) presents with pathognomonic Erythema Migrans (expanding annular erythematous plaque >=5 cm with central clearing); diagnosis is purely clinical and serologic testing is NOT recommended in Stage 1 due to high false-negative rates during early antibody development; treat with oral Doxycycline 100 mg BID for 10-14 days.
- Rocky Mountain Spotted Fever (RMSF) is a lethal tick-borne vasculitis caused by Rickettsia rickettsii presenting with sudden fever, severe headache, and a centripetally spreading petechial rash starting on the wrists/ankles and moving to the trunk, palms, and soles; Doxycycline must be initiated IMMEDIATELY on clinical suspicion, as awaiting serological confirmation increases mortality tenfold.
- Herpes Zoster (Shingles) with vesicles on the tip or side of the nose (Hutchinson's sign) indicates involvement of the nasociliary branch of the ophthalmic nerve (CN V1), denoting Herpes Zoster Ophthalmicus with imminent risk of corneal ulceration and permanent blindness, requiring emergency ophthalmology consultation and high-dose oral antiviral therapy.
Last updated: August 2026
Common Infectious Diseases: Skin/Soft Tissue Infections, Tick-Borne Illnesses & Viral Syndromes
Infectious syndromes spanning cutaneous, vector-borne, and viral etiologies are among the most common presentations in adult and geriatric primary care. For the Adult-Gerontology Primary Care Nurse Practitioner (AGPCNP), diagnostic competence requires discriminating benign superficial infections from rapidly progressive, limb- and life-threatening emergencies (such as necrotizing soft tissue infections or fulminant tick-borne vasculitides), executing targeted antimicrobial stewardship, and recognizing pathognomonic physical signs.
1. Bacterial Skin & Soft Tissue Infections (SSTIs)
The Infectious Diseases Society of America (IDSA) categorizes SSTIs into non-purulent (erysipelas, cellulitis, necrotizing fasciitis) and purulent (furuncles, carbuncles, cutaneous abscesses).
+---------------------------------------------------------------------------------------------------+
| IDSA SSTI STRATIFICATION & ANTIMICROBIAL LADDER |
| |
| [NON-PURULENT INFECTIONS] [PURULENT INFECTIONS] |
| (Erysipelas, Cellulitis) (Abscess, Furuncle, Carbuncle) |
| | | |
| v v |
| * Mild Non-Purulent: * Mild Purulent: |
| - Oral Cephalexin (500 mg QID), - INCISION AND DRAINAGE (I&D) ALONE! |
| Cefadroxil (500 mg BID), or No antibiotics required if healthy. |
| Amoxicillin-clavulanate (875/125 BID). | |
| | v |
| v * Moderate Purulent (Systemic signs, |
| * Moderate Non-Purulent (Systemic signs, surrounding erythema >2 cm, immunocompromised):|
| fever, tachypnea, tachycardia): - I&D + ORAL MRSA COVERAGE: |
| - IV Cefazolin (1-2 g q8h) or * TMP-SMX (1-2 DS tabs BID) or |
| Ceftriaxone (1-2 g daily). * Doxycycline (100 mg BID) or |
| | * Clindamycin (300-450 mg TID - check D-test)|
| v | |
| * Severe Non-Purulent / NECROTIZING: v |
| - EMERGENT SURGICAL DEBRIDEMENT + * Severe Purulent (Hemodynamic instability, |
| Vancomycin + Piperacillin-Tazobactam extensive necrotizing soft tissue infection):|
| + Clindamycin (suppresses toxin synthesis!). - I&D + IV Vancomycin / Daptomycin. |
+---------------------------------------------------------------------------------------------------+
Cellulitis vs. Erysipelas: Differential Diagnosis
+---------------------------------------------------------------------------------------------------+
| CELLULITIS vs. ERYSIPELAS COMPARATIVE MATRIX |
| |
| FEATURE ERYSIPELAS CELLULITIS |
| --------------------------------------------------------------------------------------------- |
| Primary Etiology Streptococcus pyogenes (GAS) Beta-hemolytic Strep (Group A/B/C/G) |
| (rarely S. aureus) and MSSA / MRSA |
| Anatomical Depth Superficial dermis and upper Deeper dermis and subcutaneous adipose |
| dermal lymphatic channels tissue |
| Margin Demarcation SHARPLY DEMARCATED, distinct, ILL-DEFINED, indistinct, non-elevated |
| raised, fiery red border blending border |
| Onset & Onset Signs Abrupt onset; high fever, chills, Indolent onset over days; low-grade |
| early systemic toxicity fever or afebrile |
| Classic Locations Lower extremities, bridge of nose / Lower extremities (unilateral), arms, |
| malar facial cheeks (Milian's sign) abdominal wall |
| First-Line Therapy Penicillin VK, Amoxicillin, Cephalexin, Cefadroxil, Dicloxacillin |
| or Cephalexin PO; Cefazolin IV (add MRSA coverage if purulent) |
+---------------------------------------------------------------------------------------------------+
Animal, Human, and Marine Bite Infections
- Cat & Dog Bites (Pasteurella multocida, Capnocytophaga canimorsus): Cat puncture wounds inoculate bacteria deeply into tendon sheaths and periosteum, with infection rates of 50%–80%. First-line antimicrobial prophylaxis/treatment is Amoxicillin-clavulanate (Augmentin 875/125 mg PO BID) for 3–5 days (prophylaxis) or 7–10 days (established infection). In penicillin-allergic patients: Doxycycline 100 mg BID, or Ciprofloxacin/Levofloxacin + Metronidazole. Never use cephalexin alone, as it lacks activity against Pasteurella!
- Human Bites ("Clenched Fist Injury" / Fight Bite - Eikenella corrodens): High risk of tenosynovitis and septic arthritis of MCP joints. Treatment: Augmentin PO or Ampicillin-sulbactam (Unasyn) IV + urgent hand surgery evaluation.
- Saltwater / Raw Seafood Exposure (Vibrio vulnificus): Fulminant necrotizing cellulitis with hemorrhagic bullae in patients with chronic liver disease/hemochromatosis. Treatment: Ceftriaxone IV + Doxycycline PO/IV.
Necrotizing Fasciitis: The Life-Threatening Surgical Emergency
- Microbiology: Type I (Polymicrobial: aerobes + anaerobes, Bacteroides, Peptostreptococcus, Enterobacteriaceae; common in diabetics/post-op) vs. Type II (Monomicrobial: Streptococcus pyogenes / Group A Strep +/- MRSA; occurs in otherwise healthy young adults).
- Cardinal Clinical Features: PAIN DISPROPORTIONATE TO PHYSICAL FINDINGS, rapid progression over hours, tense wooden-hard edema, skin anesthesia (caused by cutaneous nerve thrombosis), crepitus (subcutaneous gas), bullae with violaceous or hemorrhagic fluid, and refractory septic shock.
- Management: EMERGENT SURGICAL CONSULTATION FOR OPERATIVE DEBRIDEMENT. Do not delay surgery for imaging (CT/MRI) or laboratory testing! Empiric antimicrobial regimen: Vancomycin (15–20 mg/kg IV q8–12h) + Piperacillin-tazobactam (3.375–4.5 g IV q6h) + Clindamycin (900 mg IV q8h). Clindamycin is mandatory in Group A Strep necrotizing infections because it suppresses bacterial ribosome protein translation, halting the synthesis of streptococcal pyrogenic exotoxins (SpeA, SpeC) that drive toxic shock syndrome (Eagle effect).
2. Vector-Borne Tick Illnesses
Tick-borne pathogens produce overlapping acute febrile illnesses across North America. Precise geographical, morphological, and hematological discrimination is vital for board examination and clinical practice.
+---------------------------------------------------------------------------------------------------+
| TICK-BORNE INFECTIONS COMPARATIVE TAXONOMY |
| |
| DISEASE ORGANISM & VECTOR CLINICAL HALLMARKS LABS & DIAGNOSTICS |
| --------------------------------------------------------------------------------------------- |
| Lyme Disease Borrelia burgdorferi Stage 1: ERYTHEMA Stage 1: CLINICAL |
| Vector: Ixodes scapularis MIGRANS (bull's-eye). DIAGNOSIS (No labs!). |
| (blacklegged / deer tick). Stage 2: CN VII palsy, Stage 2/3: 2-Tiered |
| Endemic: Northeast, Mid- AV block carditis. ELISA + Western Blot. |
| Atlantic, Upper Midwest. Stage 3: Knee arthritis.Doxycycline 100mg BID. |
| |
| Rocky Mountain Rickettsia rickettsii Abrupt fever, severe Thrombocytopenia, |
| Spotted Fever Vector: Dermacentor variabilis frontal headache. Rash hyponatremia, high LFTs|
| (RMSF) (American dog tick). starts DAY 2-5 on DOXYCYCLINE IMMEDIATELY|
| Endemic: SE / South Central WRISTS/ANKLES -> CENTRI-for ALL patients |
| (NC, TN, MO, AR, OK). PETAL TO TRUNK/PALMS. (including pregnancy!).|
| |
| Human Granulocytic Anaplasma phagocytophilum Acute fever, rigors, LEUKOPENIA, THROMBO- |
| Anaplasmosis Vector: Ixodes scapularis. myalgias, headache. CYTOPENIA, high LFTs. |
| (HGA) Same vector/region as Lyme. RASH IS RARE (<10%). MORULAE in Granulocytes|
| Doxycycline 100mg BID. |
| |
| Human Monocytic Ehrlichia chaffeensis Acute fever, malaise, LEUKOPENIA, THROMBO- |
| Ehrlichiosis Vector: Amblyomma americanum myalgias, confusion. CYTOPENIA, high LFTs. |
| (HME) (Lone Star tick; SE / Midwest).Rash in ~30% of adults. MORULAE in Monocytes. |
| |
| Babesiosis Babesia microti (Protozoan) Malaise, fever, fatigue.HEMOLYTIC ANEMIA, high |
| Vector: Ixodes scapularis. HEMOLYSIS, JAUNDICE, LDH, low haptoglobin. |
| Asplenic patients = lethal! splenomegaly. "MALTESE CROSS" on smear|
| Atovaquone+Azithromycin|
+---------------------------------------------------------------------------------------------------+
+---------------------------------------------------------------------------------------------------+
| LYME DISEASE: STAGES & ANTIMICROBIAL REGIMENS |
| |
| [STAGE 1: EARLY LOCALIZED (Days 3 to 30)] |
| - Solitary Erythema Migrans (EM) plaque expanding >=5 cm with central clearing + viral symptoms.|
| - Diagnostic Rule: Clinical diagnosis! Serologies are negative in 50% (do not test!). |
| - Treatment: Doxycycline 100 mg PO BID x 10-14 days. |
| (Alternatives: Amoxicillin 500 mg TID x 14 days; Cefuroxime axetil 500 mg BID x 14 days). |
| | |
| v |
| [STAGE 2: EARLY DISSEMINATED (Weeks to Months)] |
| - Multiple secondary annular EM lesions, Lymphocytic meningitis, Cranial Neuropathy (Unilateral |
| or Bilateral Cranial Nerve VII / Bell's Palsy), Lyme Carditis (PR prolongation, AV block). |
| - Treatment: |
| * Isolated Cranial Nerve VII Palsy: Oral Doxycycline 100 mg PO BID x 14-21 days (No IV needed!)|
| * Lyme Carditis (1st/2nd/3rd degree AV block with PR >=300ms) or Meningitis: |
| IV Ceftriaxone 2 g daily x 14-21 days until resolved, then stepdown to oral. |
| | |
| v |
| [STAGE 3: LATE PERSISTENT (Months to Years)] |
| - Chronic asymmetric monoarticular or oligoarticular large-joint arthritis (KNEE with massive |
| effusion), mild encephalopathy, distal axonal polyneuropathy. |
| - Treatment: Oral Doxycycline 100 mg PO BID x 28 days (or Amoxicillin 500 mg TID x 28 days). |
+---------------------------------------------------------------------------------------------------+
Lyme Post-Exposure Prophylaxis (PEP) Criteria
Administer a single prophylactic dose of Doxycycline 200 mg PO if and only if ALL four criteria are satisfied:
- Attached tick can be reliably identified as an adult or nymphal Ixodes scapularis (deer tick).
- Tick is estimated to have been attached for $\ge 36$ hours (based on engorgement or time exposure).
- Prophylaxis can be initiated within 72 hours of tick removal.
- Local ecological infection rate of ticks with B. burgdorferi is $\ge 20%$ (parts of New England, Mid-Atlantic, Minnesota, Wisconsin).
3. Cutaneous Viral Syndromes & Exanthems
+---------------------------------------------------------------------------------------------------+
| VIRAL CUTANEOUS SYNDROMES: CLINICAL DIFFERENTIATION |
| |
| VIRAL SYNDROME ETIOLOGY & MORPHOLOGY COMPLICATIONS & WARNINGS MANAGEMENT PROTOCOL |
| --------------------------------------------------------------------------------------------- |
| Herpes Zoster Varicella-Zoster Virus HUTCHINSON'S SIGN: Valacyclovir 1000 mg |
| (Shingles) (VZV) reactivation in Vesicles on nose tip = PO TID x 7 days or |
| sensory ganglia. Painful CN V1 Nasociliary branchFamciclovir 500 mg |
| grouped vesicles on involvement -> RISK OF PO TID x 7 days |
| erythematous base in a BLINDNESS! Emergent Ophth!(start within 72h). |
| unilateral DERMATOME. Postherpetic Neuralgia. Shingrix vaccine (>=50)|
| |
| Ramsay Hunt VZV reactivation in Ipsilateral peripheral Oral Valacyclovir + |
| Syndrome (Herpes Geniculate Ganglion facial paralysis + ear High-dose oral |
| Zoster Oticus) (Cranial Nerves VII & VIII).pain + hearing loss/ Prednisone taper. |
| Vesicles in ear canal/pinna.vertigo. |
| |
| Herpes Simplex HSV-1 (orolabial > genital);Eczema herpeticum Oral Valacyclovir |
| Virus (HSV-1 / 2) HSV-2 (genital > orolabial). (widespread HSV in (Primary: 1000 mg BID |
| Painful grouped vesicles atopic dermatitis; x 7-10d; Recurrent: |
| that ulcerate and crust. dermatologic emergency).500 mg BID x 3d). |
| |
| Infectious Epstein-Barr Virus (EBV). AMPICILLIN / AMOXICILLIN Supportive; avoid |
| Mononucleosis Exudative tonsillitis, causes 90% MORBILLIFORM contact sports for |
| posterior cervical LAD, RASH (not true allergy!)>=3-4 weeks to prevent |
| fever, splenomegaly, Splenic rupture risk. SPLENIC RUPTURE! |
| atypical lymphocytosis. |
| |
| Measles (Rubeola) Paramyxovirus. 3 C's: Subacute Sclerosing Airborne isolation; |
| Cough, Coryza, Conjuncti- Panencephalitis (SSPE); supportive; Vitamin A |
| vitis + KOPLIK SPOTS severe pneumonia. supplementation. |
| -> Cephalocaudal rash. |
+---------------------------------------------------------------------------------------------------+
Herpes Zoster & Postherpetic Neuralgia (PHN)
- Diagnostic Pearls: Dermatomal burning, dysesthesia, or lancinating allodynia precedes the cutaneous eruption by 48–72 hours (pre-eruptive prodrome), frequently misdiagnosed as acute angina, pleurisy, cholecystitis, or radiculopathy.
- Postherpetic Neuralgia (PHN): Defined as dermatomal neuropathic pain persisting $>90\text{ days}$ after the resolution of the acute zoster rash. Driven by neurotoxic injury, central sensitization, and deafferentation in dorsal horn neurons.
- First-Line Pharmacotherapy for PHN: Gabapentin (titrated 300 to 1800–3600 mg/day in divided doses), Pregabalin (75–300 mg BID), Tricyclic Antidepressants (Nortriptyline 10–75 mg QHS; avoid in elderly with cardiac arrhythmias/fall risk), and Topical Lidocaine 5% patches (12 hours on, 12 hours off).
- Vaccine Prevention (Shingrix): Recombinant, adjuvanted zoster vaccine administered in a 2-dose series (0 and 2–6 months) for all immunocompetent adults $\ge 50\text{ years}$ and immunocompromised adults $\ge 19\text{ years}$, regardless of previous episodes of herpes zoster or receipt of the discontinued live Zostavax vaccine.
4. Geriatric & Lifespan Infectious Disease Considerations
- Atypical Presentation of Severe Sepsis: Older adults frequently fail to mount fever, tachycardia, or leukocytosis due to immunosenescence. Presenting manifestations are commonly non-specific: acute delirium/confusion, functional decline, anorexia, falls, or hypothermia ($<36.0^\circ\text{C}$). Hypothermia in an infected geriatric patient correlates with higher mortality than fever.
- Renal Dosing of Antivirals and Antibiotics: Valacyclovir, acyclovir, and beta-lactams are excreted renally. In older adults with decreased GFR, failure to adjust acyclovir/valacyclovir dosing causes acyclovir-induced neurotoxicity (visual hallucinations, confusion, agitation, myoclonus, encephalopathy).
- Fluoroquinolone Black Box Warnings: Avoid empiric ciprofloxacin/levofloxacin for simple cellulitis or SSTIs in older adults due to severe risks of Achilles tendon rupture/tendinitis, aortic aneurysm dissection, QTc prolongation/arrhythmia, and Clostridioides difficile colitis.
5. Board-Yield Summary & Clinical Pearls
+---------------------------------------------------------------------------------------------------+
| ANCC AGPCNP CLINICAL EXAM PEARLS |
| |
| - Severe Cellulitis with Pain Out of Proportion to physical exam, rapid progression, wooden- |
| hard swelling, and crepitus is NECROTIZING FASCIITIS until proven otherwise! Consult surgery |
| IMMEDIATELY and start Vancomycin + Pip-Tazo + Clindamycin (to stop toxin production). |
| |
| - Stage 1 Lyme Disease (Erythema Migrans >=5 cm) is a CLINICAL DIAGNOSIS! Serologies (ELISA/ |
| Western Blot) are false-negative in 50% of early cases; do NOT order tests—treat with |
| Doxycycline 100 mg BID for 10-14 days immediately. |
| |
| - Suspected Rocky Mountain Spotted Fever (fever + headache + wrist/ankle petechial rash) mandates|
| immediate DOXYCYCLINE therapy. Never withhold doxycycline pending serology; delays cause death.|
| |
| - Vesicles on the tip of the nose in Herpes Zoster (HUTCHINSON'S SIGN) involves the nasociliary |
| branch of CN V1—mandates an IMMEDIATE OPHTHALMOLOGY CONSULT to prevent permanent blindness. |
| |
| - Cat and Dog bites must be treated with AMOXICILLIN-CLAVULANATE (Augmentin) to cover |
| Pasteurella multocida! Cephalexin is ineffective against Pasteurella. |
| |
| - Prescribing Ampicillin or Amoxicillin in patients with Epstein-Barr Virus (EBV) Mononucleosis |
| induces a non-allergic morbilliform rash in >90% of patients. Advise abstaining from contact |
| sports for >=3-4 weeks to prevent fatal splenic rupture. |
+---------------------------------------------------------------------------------------------------+
Test Your Knowledge
A 52-year-old male with a history of poorly controlled type 2 diabetes mellitus presents to the emergency department with severe, rapidly progressive left lower leg pain, erythema, and swelling that began 12 hours ago. Vital signs: BP 86/52 mmHg, HR 124 bpm, RR 24 bpm, Temp 39.2°C (102.6°F), SpO2 93% on room air. On physical examination, the left calf is tensely edematous, erythematous, and exquisitely tender to light palpation across areas extending beyond the erythema margin. The skin feels wooden and indurated, with two scattered violaceous fluid-filled bullae and focal palpable crepitus. Which of the following is the most critical and urgent next step in clinical management?
A
B
C
D
Test Your Knowledge
A 34-year-old female presents to the primary care clinic in rural Connecticut in late June reporting a 4-day history of low-grade fever, severe fatigue, headache, and a rapidly expanding skin lesion on her left thigh. She recalls finding and removing a small tick from that area 10 days ago after hiking. On physical examination, the AGPCNP observes a solitary, non-tender, non-pruritic, annular erythematous plaque measuring 9 cm in diameter with distinct central clearing on the left lateral thigh. Neurological and musculoskeletal examinations are completely normal. What is the most appropriate diagnostic and therapeutic approach for this patient?
A
B
C
D
Test Your Knowledge
A 68-year-old male presents with a 2-day history of a painful, burning, vesicular rash over his left forehead, upper eyelid, and the tip and left lateral ala of his nose. He reports moderate left eye photophobia, foreign body sensation, and blurred vision. Vital signs are normal. Physical examination reveals grouped vesicles and pustules on an erythematous base strictly limited to the left V1 dermatome, with distinct vesicular lesions on the nasal tip. Fluorescein staining reveals punctate epithelial keratitis on the left cornea. What is the AGPCNP's immediate diagnostic classification and clinical action?
A
B
C
D