Skin and soft tissue infection presentation

A skin and soft tissue infection is bacterial, fungal, viral or parasitic infection of the epidermis, dermis, subcutaneous fat, fascia or muscle.

Classification by depth. Epidermis: impetigo and ecthyma. Hair follicle: folliculitis, furuncle and carbuncle. Dermis and superficial lymphatics: erysipelas. Subcutaneous tissue: cellulitis and abscess. Fascia and muscle: necrotising fasciitis, myonecrosis (gas gangrene), pyomyositis.

Classification by purulence. Purulent infections (abscess, furuncle, carbuncle, purulent cellulitis) are caused mainly by Staphylococcus aureus. Nonpurulent infections (erysipelas, cellulitis) are caused mainly by beta haemolytic streptococci (groups A, B, C and G).

Severity. Mild: local signs without systemic features. Moderate: systemic signs (temperature over 38 C, heart rate over 90, respiratory rate over 20, white count over 12 or under 4 x 10^9/L), or extensive disease. Severe: hypotension, organ dysfunction, rapid progression, bullae, necrosis, pain out of proportion, or an immunocompromised host.

Necrotising infection. Type 1 is polymicrobial (anaerobes, Enterobacterales), usually in diabetes, perineum and postoperative sites. Type 2 is monomicrobial (group A streptococcus or Staphylococcus aureus). Type 3 is Clostridium or Vibrio related (marine exposure, liver disease). Mortality rises sharply with each hour of delay to surgery.

Special settings. Bites (human, dog, cat), water exposure (fresh water Aeromonas, marine Vibrio and Mycobacterium marinum), diabetic foot, intravenous drug use, postoperative wound infection, lymphoedema, venous insufficiency and tropical settings.

Key distinctions. Cellulitis is spreading warm erythema without a collection. Abscess is a fluctuant collection that needs drainage. Necrotising infection has pain and toxicity disproportionate to the skin appearance.

History

• Onset, speed of spread (marking the edge with a pen), pain, fever, rigors, drainage and previous episodes.
• Trigger: minor trauma, insect bite, injection, surgery, tattoo, burn, ulcer, tinea pedis, shaving, scratch, animal bite or scratch, water exposure, fish handling, shellfish, travel and gardening (sporotrichosis).
• Risk factors: diabetes, obesity, venous or lymphatic insufficiency, previous cellulitis, peripheral arterial disease, HIV, cirrhosis, kidney failure, immunosuppression, malnutrition, intravenous drug use, MRSA carriage and recent hospital stay.
• Drugs: corticosteroids, immunosuppressants, anticoagulants and NSAIDs (which may mask or promote progression).
• Immunisation: tetanus status. Allergies, especially to penicillin.
• Region specific: tropical ulcer, Buruli ulcer, cutaneous anthrax, cutaneous diphtheria, leishmaniasis and filarial lymphoedema.

Examination

• Vital signs, mental state, perfusion and urine output.
• Local: erythema, warmth, swelling, tenderness, induration, fluctuance, lymphangitic streaks, regional nodes, vesicles, bullae (clear or haemorrhagic), necrosis, crepitus, anaesthesia over the area, skin colour (dusky, violaceous) and wound or ulcer.
• Portal of entry: web spaces, nail folds, wounds, ulcers and injection sites.
• Joint and tendon involvement, bone exposure and neurovascular status.
• Systemic: signs of endocarditis, septic emboli, and metastatic infection.
• Bedside tests: temperature, glucose, urinalysis, mark the margin of erythema, and bedside ultrasound for collection (cobblestoning and a hypoechoic pocket) and for fascial fluid.

Red flags

• Pain out of proportion to the appearance, skin anaesthesia, rapid spread, dusky or violaceous skin, haemorrhagic bullae, crepitus or gas on imaging (necrotising infection).
• Hypotension, tachycardia over 120, altered mentation, lactate over 2 mmol/L (sepsis).
• Erythema on the face, near the eye, nose or upper lip (orbital cellulitis, cavernous sinus thrombosis).
• Hand infection with tendon sheath signs (flexed finger, pain on passive extension).
• Perineal, scrotal or perianal swelling with systemic features (Fournier gangrene).
• Infection over a joint, prosthesis or implant, and postoperative wound infection with deep collections.
• Diabetes with foot ulcer, immunosuppression, neutropenia, cirrhosis, intravenous drug use.
• Marine injury or raw shellfish exposure in liver disease or iron overload (Vibrio vulnificus).
• Bite wounds from humans or from cats, and wounds over joints.
• Failure to improve after 48 to 72 hours of appropriate antibiotics.
• Rapidly expanding erythema with fever and rash (toxic shock syndrome).

Special groups

• Pregnancy: avoid doxycycline, fluoroquinolones and co-trimoxazole near term; use beta lactams and clindamycin.
• Children: impetigo, staphylococcal scalded skin syndrome and periorbital cellulitis need age specific assessment and doses.
• Older adults and nursing home residents: atypical presentation, with falls or confusion.
• Kidney impairment: adjust beta lactams, vancomycin and co-trimoxazole.
• Region specific: tropical settings with Buruli ulcer, yaws, leishmaniasis, cutaneous anthrax and filarial lymphoedema.

Superficial and follicular infection

• Impetigo (nonbullous and bullous).
• Ecthyma.
• Folliculitis (staphylococcal, Pseudomonas hot tub folliculitis, Malassezia).
• Furuncle and carbuncle.
• Staphylococcal scalded skin syndrome.
• Erysipelas.
• Erythrasma.
• Cutaneous diphtheria.

Deeper infection

• Cellulitis.
• Skin abscess.
• Infected bite wound (human, dog, cat, rodent).
• Infected burn and surgical wound.
• Necrotising fasciitis (types 1, 2 and 3).
• Fournier gangrene.
• Clostridial myonecrosis (gas gangrene).
• Pyomyositis.
• Septic bursitis and septic arthritis.
• Osteomyelitis.
• Infected diabetic foot ulcer.
• Infected pressure ulcer.
• Line and implant related infection.

Specific pathogens and exposures

• Toxic shock syndrome (staphylococcal and streptococcal).
• Vibrio vulnificus and Aeromonas infection.
• Mycobacterium marinum infection.
• Erysipeloid.
• Cutaneous anthrax.
• Tularaemia.
• Cat scratch disease and Pasteurella infection.
• Sporotrichosis.
• Mycetoma and chromoblastomycosis.
• Mucormycosis.
• Cutaneous tuberculosis, nontuberculous mycobacterial infection and Buruli ulcer.
• Leprosy and leishmaniasis.
• Herpes simplex and herpetic whitlow, herpes zoster, and eczema herpeticum.
• Myiasis and tungiasis.
• Lymphatic filariasis with acute dermatolymphangioadenitis.
• Lyme disease (erythema migrans) and other tick borne infection.

Mimics

• Stasis dermatitis and venous eczema.
• Deep vein thrombosis.
• Contact and allergic dermatitis.
• Gout and pseudogout.
• Lipodermatosclerosis.
• Erythema nodosum and Sweet syndrome.
• Pyoderma gangrenosum.
• Drug eruptions.
• Insect bite reaction and spider bite.
• Fixed drug eruption.
• Inflammatory carcinoma of the breast and cutaneous lymphoma.
• Cutaneous vasculitis and calciphylaxis.
• Compartment syndrome.
• Haematoma.

Step 1. Triage. Look for sepsis, necrotising infection, deep space collection, and bone or joint involvement. Necrotising infection is a clinical diagnosis, and surgery should not wait for imaging or scores.

Step 2. Clinical differentiation. Cellulitis is warm, poorly demarcated erythema, and erysipelas is sharply demarcated, raised and shiny. Look for a collection. Diagnosis of cellulitis in the leg is often wrong when both legs are involved symmetrically, which suggests stasis dermatitis.

Step 3. First line tests for moderate and severe infection

• Full blood count, CRP, urea, creatinine, electrolytes, glucose, lactate, liver tests, creatine kinase, coagulation, and blood gas.
• Blood cultures, 2 sets (positive in a minority of cellulitis, higher in severe cases, and positive in most patients with necrotising infection and toxic shock syndrome).
• Pus or tissue culture from abscess, bullae or wound (deep specimen preferred over surface swab), including MRSA screen.
• Swabs for anaerobes when the wound is foul smelling or necrotic.
• Thick and thin films when malaria is possible.
• HIV test, HbA1c and hepatitis serology when risk factors exist.

Step 4. Necrotising infection assessment.

The LRINEC score gives points for CRP 150 mg/L or over (4), white count 15 to 25 x 10^9/L (1) or over 25 (2), haemoglobin 11 to 13.5 g/dL (1) or under 11 (2), sodium under 135 mmol/L (2), creatinine over 141 micromol/L (2) and glucose over 10 mmol/L (1), with a maximum of 13.

A score of 6 or more raises concern, and a low score does not exclude necrotising infection.

Surgical exploration (incision with finger test, grey tissue, no bleeding, dishwater fluid and no resistance to blunt dissection) is the definitive test.

Step 5. Imaging

• Ultrasound for suspected collection, foreign body, joint effusion and fascial fluid.
• Plain radiograph for gas, foreign body and osteomyelitis.
• CT or MRI for suspected necrotising fasciitis in stable patients, deep abscess, pyomyositis, bone involvement, perineal infection and uncertain diagnosis, without delaying surgery.
• Venous duplex when deep vein thrombosis is possible.

Step 6. Targeted tests

• Bites: culture of the wound, with tetanus status and rabies risk assessment.
• Marine exposure: culture on request for Vibrio and Mycobacterium marinum (cool incubation).
• Skin biopsy for culture (bacterial, fungal, mycobacterial) and histology when the diagnosis is uncertain or chronic.
• Streptococcal serology (antistreptolysin O, anti-DNase B) in recurrent infection or poststreptococcal complications.
• Staphylococcal carriage swabs and Panton-Valentine leukocidin testing in recurrent abscess.
• Toxic shock syndrome: cultures plus toxin testing where available.
• Eschar for anthrax: swab for PCR and culture, with public health contact.
• Diabetic foot ulcer: probe to bone test, radiograph, ESR and MRI.

Limits. Cultures from cellulitis are positive in a minority. Surface swabs identify colonisers. The LRINEC score has limited sensitivity. Imaging should not delay surgery.

Immediate

• Sepsis or necrotising infection: oxygen, intravenous access, blood cultures, crystalloid 30 mL/kg for hypotension or lactate 4 mmol/L or over, antibiotics within 1 hour, and urgent surgical review for debridement. Noradrenaline for persistent hypotension.
• Mark the erythema edge, elevate the limb and give analgesia.
• Paracetamol 1 g orally or intravenously every 6 hours (maximum 4 g daily; 3 g daily in hepatic impairment, chronic alcohol use or weight under 50 kg).
• Ibuprofen 400 mg orally every 8 hours with food when no contraindication exists (avoid in necrotising infection, kidney impairment and sepsis).
• Morphine 2.5 to 5 mg intravenously titrated for severe pain.
• Check tetanus status. For a tetanus prone wound in a person with incomplete immunisation, give tetanus toxoid and tetanus immunoglobulin 250 IU intramuscularly (500 IU if over 24 hours since injury or heavy contamination).

Mild nonpurulent cellulitis and erysipelas

• Flucloxacillin 500 mg to 1 g orally four times daily for 5 to 7 days, extended to 10 to 14 days when slow to improve. Reduce in severe kidney impairment. Monitor liver tests with prolonged use because cholestatic hepatitis can occur.
• Penicillin allergy: clarithromycin 500 mg orally twice daily for 7 days, or doxycycline 200 mg orally on day 1 then 100 mg daily for 7 days, or clindamycin 300 to 450 mg orally every 6 hours for 7 days (maximum 1.8 g daily).
• With clindamycin, monitor for diarrhoea and Clostridioides difficile infection.
• Elevate the limb, treat tinea pedis and oedema, and review at 48 hours for spread.

Moderate and severe cellulitis

• Flucloxacillin 1 to 2 g intravenously every 6 hours, or benzylpenicillin 1.2 g intravenously every 4 to 6 hours for streptococcal erysipelas. Switch to oral when afebrile and improving.
• Severe or MRSA risk: add vancomycin 15 to 20 mg/kg intravenously every 8 to 12 hours (loading dose 25 to 30 mg/kg in critical illness) with trough or area under the curve monitoring.
• Alternatively use linezolid 600 mg intravenously or orally every 12 hours. Monitor platelets weekly, avoid with serotonergic drugs, and limit to under 14 days.
• Alternatively use daptomycin 4 mg/kg intravenously once daily. Monitor creatine kinase weekly. Give every 48 hours at creatinine clearance under 30 mL/min.
• Facial cellulitis: ceftriaxone 2 g intravenously once daily or flucloxacillin 2 g every 6 hours.
• Orbital cellulitis: ceftriaxone 2 g intravenously every 12 hours plus metronidazole 500 mg intravenously every 8 hours, with urgent imaging and ENT and ophthalmology review.

Abscess, furuncle, carbuncle

• Incision and drainage is the main treatment. Use lidocaine 1% (maximum 4.5 mg/kg plain, 7 mg/kg with adrenaline), and avoid adrenaline in the digits, ear and penis. Send pus for culture. Cover and review at 48 hours.
• Give antibiotics for systemic features, cellulitis over 5 cm, immunocompromise, difficult sites, multiple lesions or incomplete drainage.
• Options: flucloxacillin 500 mg to 1 g orally four times daily for 5 to 7 days; or doxycycline 100 mg twice daily for 5 to 7 days; or clindamycin 300 to 450 mg every 6 hours.
• For MRSA: co-trimoxazole 960 mg orally twice daily for 5 to 7 days. Halve at creatinine clearance 15 to 30 mL/min and avoid under 15 mL/min. Check potassium and creatinine. Avoid in G6PD deficiency and late pregnancy.
• Recurrent abscess: decolonisation with mupirocin 2% to each nostril twice daily for 5 days and chlorhexidine 4% body wash daily for 5 to 7 days, hygiene measures, and a search for diabetes or immunodeficiency.

Impetigo and folliculitis

• Localised impetigo: hydrogen peroxide 1% cream twice or three times daily for 5 days, or mupirocin 2% ointment three times daily for 5 to 7 days (restrict use to prevent resistance), or fusidic acid 2% cream three times daily for 5 to 7 days where resistance is low.
• Widespread or bullous impetigo, or outbreaks: flucloxacillin 500 mg orally four times daily for 5 to 7 days (children 25 to 50 mg/kg daily in 4 doses).
• Pseudomonas hot tub folliculitis usually settles in a week without treatment. Ciprofloxacin 500 mg twice daily for 5 to 7 days is for persistent or immunocompromised cases.
• Exclude children from school until lesions are crusted or 48 hours after treatment starts.

Necrotising soft tissue infection and toxic shock

• Immediate surgical exploration and wide debridement, repeated at 12 to 24 hours until all necrotic tissue is removed, with intensive care.
• Antibiotics (do not delay for surgery): piperacillin with tazobactam 4.5 g intravenously every 6 hours (3.375 g every 6 hours at creatinine clearance 20 to 40 mL/min, 2.25 g every 6 hours under 20 mL/min).
• Alternatively, meropenem 1 g intravenously every 8 hours (1 g every 12 hours at creatinine clearance 26 to 50 mL/min, 500 mg every 12 hours at 10 to 25 mL/min, 500 mg every 24 hours under 10 mL/min).
• Add clindamycin 600 to 900 mg intravenously every 8 hours (to suppress toxin production).
• Add vancomycin 15 to 20 mg/kg every 8 to 12 hours or linezolid 600 mg every 12 hours when MRSA is likely.
• Narrow to benzylpenicillin 2.4 g every 4 hours with clindamycin for group A streptococcus.
• Vibrio vulnificus: doxycycline 100 mg intravenously or orally twice daily with ceftriaxone 2 g intravenously once daily, with debridement.
• Clostridial myonecrosis: benzylpenicillin 2.4 g every 4 hours with clindamycin and urgent debridement.
• Streptococcal toxic shock: intravenous immunoglobulin 1 g/kg on day 1 then 0.5 g/kg on days 2 and 3, with evidence being limited. Hyperbaric oxygen has uncertain benefit and must not delay surgery.
• Treat compartment syndrome by fasciotomy and monitor creatine kinase, potassium and creatinine for rhabdomyolysis.
• Duration: continue until no further debridement is needed, haemodynamic stability, and afebrile for 48 to 72 hours.

Bites

• Wash thoroughly with saline and irrigate, debride devitalised tissue and leave puncture wounds open. Avoid primary closure of high risk wounds (puncture, hand, over 8 hours old, bites in immunocompromised patients).
• Prophylaxis for 3 to 5 days: amoxicillin with clavulanic acid 625 mg orally three times daily for high risk wounds. These are cat bites, hand, deep, crush, over a joint, human bites, immunocompromised patients, asplenia and liver disease.
• Established infection: 7 days, or intravenous amoxicillin with clavulanic acid 1.2 g every 8 hours when severe.
• Penicillin allergy: doxycycline 100 mg twice daily with metronidazole 400 mg three times daily, or co-trimoxazole with clindamycin.
• Tetanus: give toxoid, with immunoglobulin 250 IU intramuscularly (500 IU if over 24 hours since injury or heavy contamination) when immunisation is incomplete.
• Rabies post exposure prophylaxis for mammal bites or licks of broken skin in rabies risk regions: wash for 15 minutes with soap and water, and apply a virucidal antiseptic.
• Give human rabies immunoglobulin 20 IU/kg infiltrated into and around the wound (maximum possible volume, remainder intramuscularly at a distant site).
• Give vaccine intramuscularly on days 0, 3, 7 and 14 (days 0 and 3 plus 7 in some schedules; 28 days for immunocompromised patients). Observe the animal for 10 days when available.
• Human bites: test for HIV, hepatitis B and C; consider post exposure prophylaxis.

Marine and freshwater wounds. Mycobacterium marinum: clarithromycin 500 mg twice daily with ethambutol 15 mg/kg daily for 3 to 6 months. Aeromonas: ciprofloxacin 500 mg twice daily or ceftriaxone 2 g daily.

Diabetic foot infection.

• Mild: flucloxacillin 500 mg to 1 g orally four times daily for 7 days (clarithromycin 500 mg twice daily or doxycycline 100 mg twice daily if penicillin allergic).
• Moderate: amoxicillin with clavulanic acid 625 mg orally three times daily for 7 to 14 days.
• Severe: piperacillin with tazobactam 4.5 g intravenously every 6 hours, adding vancomycin when MRSA is likely, with urgent surgical debridement.
• Osteomyelitis: usually 6 weeks of antibiotics directed by bone culture. Off loading and glycaemic control are essential.

Other

• Cutaneous anthrax: ciprofloxacin 500 mg orally twice daily for 7 to 10 days (60 days when bioterrorism related), with public health notification.
• Herpetic whitlow, zoster and eczema herpeticum: aciclovir 400 mg orally three times daily for 7 to 10 days (herpes), 800 mg five times daily for 7 days (zoster), or 5 to 10 mg/kg intravenously every 8 hours when severe or immunocompromised.
• Erysipeloid: penicillin or amoxicillin 500 mg three times daily for 7 days.
• Cat scratch disease: azithromycin 500 mg once then 250 mg daily for 4 days in adults.
• Lymphatic filariasis acute attacks: rest, limb hygiene, antibiotics and doxycycline 200 mg daily for 4 to 6 weeks for chronic lymphoedema stabilisation (evidence moderate).
• Erythema migrans: doxycycline 100 mg orally twice daily for 10 days (amoxicillin 500 mg three times daily for 14 days in pregnancy and children under 8 years).
• Pyomyositis: drainage and flucloxacillin 2 g intravenously every 6 hours (vancomycin for MRSA) for 2 to 3 weeks, then oral.

Prevention of recurrence. Treat the portal of entry (tinea pedis, eczema, ulcers), reduce oedema with compression, and manage obesity and diabetes. After 2 or more episodes in a year, consider benzathine benzylpenicillin 1.2 million units intramuscularly every 2 to 4 weeks or phenoxymethylpenicillin 250 to 500 mg twice daily for 6 to 12 months.

Monitoring. Review at 48 to 72 hours, with the marked edge, temperature and white count. In severe infection record observations every hour to 4 hours, with lactate, creatinine, creatine kinase and cultures daily.

Referral and escalation. General or plastic surgery for abscess needing drainage, necrotising infection, perineal infection, hand infection and deep collections. Orthopaedics for joint and bone. Infectious diseases for MRSA, recurrent infection, immunosuppression, bites and unusual organisms.

Ophthalmology and ENT for periorbital and facial infection. Intensive care for shock.

Admission criteria. Systemic toxicity, hypotension, rapidly progressing infection, suspected necrotising infection, facial and hand infection, immunocompromise, diabetes with foot infection, failure of oral therapy at 48 hours, inability to take oral therapy, extremes of age and unreliable follow up.

Discharge and follow up. Review in 48 to 72 hours and again at the end of treatment. Safety net advice: return for spreading redness beyond the drawn margin, fever, severe pain, blistering, dark skin, new swelling or no improvement within 48 hours.