Skin & Soft Tissue Infections

Question 1

How does the skin serve as a protective barrier to infection?

Answer 1

1. Physical barrier

• barrier from physical, microbial, chemical assaults
• prevent excessive water loss

2. Chemical barrier

• acidic pH 4-5 (FFAs from phospholipids)
• acidic environment keep bacteria and candida low, regulate desquamation
• continuous renewal of epidermal layer, results in shedding of keratocytes and skin microbiota
• sebaceous secretions inhibit growth of many bacteria and fungi
• normal commensal skin microbiome prevents overgrowth and colonization of pathogenic strains

3. Immunological barrier

• innate (physical barrier + AMPs antimicrobial peptides, cytokines, cells with PRR)
• AMP kills pathogens, influences cellular processes to promote wound healing, initiation of adaptive immune response, controls inflammation

Question 2

What factors impair skin barrier function?

Answer 2

Age
Infection
Physical damage
Physical environment
Ischemia - lack of perfusion
Diseases (e.g., DM)
Drugs
pH
Excessive soap and detergent use
Too much moisture and humidity

Question 3

What are the classification of SSTIs that occur at each anatomical site?
(Epidermis, Dermis, Hair follicles, Subcutaneous fat, Fascia, Muscle)

Answer 3

Epidermis - Impetigo

Dermis - Ecthyma, Erysipelas

Hair follicles - Furuncle, Carbuncle

Subcutaneous fat - Cellulitis

Fascia - Necrotizing fasciitis

Muscle - Myositis

Question 4

What are some other protective mechanisms of the skin?

Answer 4

• Continuous renewal of epidermal layer, results in shedding of keratocytes and skin microbiota
• Sebaceous secretions inhibit growth of many bacteria and fungi
• Normal commensal skin microbiome prevents overgrowth and colonization of pathogenic strains

Question 5

What are some risk factors for SSTI?

Answer 5

Disruption of the skin barrier

• Traumatic: lacerations, recent surgery, burns, abrasions…
• Non traumatic: pressure ulcers, tinea pedis, dermatitis, toe web intertrigo, chemical irritants
• impaired venous and lymphatic drainage: obesity, saphenous venectomy
• peripheral artery disease

Conditions / underlying diseases that predispose to infection

• DM, cirrhosis, neutropenia, HIV, transplant, immunosuppression

History of cellulitis

Also look out for: animal exposure, water exposure, travel history

Question 6

How can SSTI be prevented?

Answer 6

Management of predisposing risk factors => identify at time of initial diagnosis to decrease risk for recurrence

Good care to maintain skin integrity

• good wound care
• treatment of tinea pedis
• preventing dry, cracked skin
• good foot care for DM patients to prevent wound and ulcers

Acute traumatic wound should be irrigated, foreign objects removed, devitalized tissues debrided

Question 7

1. Confirm presence of infection

When is culture required?

Answer 7

Bacteria culture is NOT required for mild and superficial infections (no systemic symptoms)

Blood culture is only required for severe cases with marked systemic symptoms of infection or immunocompromised patients

• E.g., lab (TW, CRP, creatinine phosphokinase, lactate, gram stain and culture)
• If sinister pathology suspected: radiography, CT with contrast, MRI, ultrasonography

Question 8

1. Confirm presence of infection

How should cultures of pus, exudates or tissues from wounds be taken?

Answer 8

Avoid wound swabs - colonizers, contaminants => polymicrobial

Cultures should be collected:

• from deep in the wound after surface cleansed
• from base of a closed abscess, where bacteria grow
• by curettage, rather than wound swab or irrigation

Question 9

1. Confirm presence of infection (diagnosis - clinical presentation)

Describe the clinical presentation of Impetigo

Answer 9

Epidermis, superficial infection

Begin as erythematous papules, evolve into vesicles and pustules that rupture with dried discharge forming honey-coloured crusts on an erythematous base

Usually on exposed areas of the body (face, extremities)

Lesions well localized, frequently many, bullous, or non-bullous

Question 10

1. Confirm presence of infection (diagnosis - clinical presentation)

Describe the clinical presentation of Ecthyma

Answer 10

Dermis, deeper variant of impetigo that extend through the epidermis and deep into dermis

Begin as vesicles/pustules, evolve into “punched out” ulcers (ulcerative form of impetigo)

Pruritis is common

Question 11

1. Confirm presence of infection (diagnosis - clinical presentation)

Describe the clinical presentation of Furuncle and Carbuncle

Answer 11

Furuncle: Infection of the hair follicle with purulent material, extends through the dermis into s/c tissue, small abscess forms

Carbuncle: furuncles coalesce, extent into s/c tissues

*PURULENT

Question 12

1. Confirm presence of infection (diagnosis - clinical presentation)

Describe the clinical presentation of cutaneous abscess

Answer 12

Skin abscess: localized collection of pus within the dermis and deeper skin tissues

Manifest as painful, tender, fluctuant, and erythematous nodules

*PURULENT

Question 13

1. Confirm presence of infection (diagnosis - clinical presentation)

Describe the clinical presentation of Erysipelas

Answer 13

Dermis

Affects upper dermis, more superficial, involves lymphatics

Fiery red, tender, painful plaque (raised) with well-demarcated edges

Common on face, lower extremities

*NON-PURULENT

Question 14

1. Confirm presence of infection (diagnosis - clinical presentation)

Describe the clinical presentation of Cellulitis

Answer 14

S/C tissue, involve deeper and subcutaneous fats

Usually acute, diffuse, spreading, non-elevated, poorly demarcated edges
Rapid onset/progression
Unilateral
Fever in 20-70% of patients

Common in lower extremities, though may appear on any area of the skin

*PURULENT or NON-PURULENT

Question 15

1. Confirm presence of infection (diagnosis - clinical presentation)

What are some cellulitis mimickers?

Answer 15

Deep venous thrombosis
Calciphylaxis
Stasis dermatitis
Hematoma
Erythema migrans

*if cant differentiate, may give a short course to monitor for response

Question 16

1. Confirm presence of infection (diagnosis - clinical presentation)

What are some complications that can arise if cellulitis progresses?

Answer 16

Bacteremia
Endocarditis
Toxic shock (overgrowth of staphs/streps => release toxins)
Glomerulonephritis
Lymphedema
Osteomyelitis
Necrotizing soft-tissue infections (Necrotizing fascia)

Question 17

2. Identify pathogens

Name the likely pathogens for Impetigo

Answer 17

Impetigo
- Staphylococci or Streptococci
- Bullous form caused by toxin-producing strains of S. aureus

Question 18

2. Identify pathogens

Name the likely pathogens for Ecthyma

Answer 18

Ecthyma
- Group A Streptococci

Question 19

2. Identify pathogens

Name the likely pathogens for Nonpurulent (Erysipelas, Cellulitis)

Answer 19

Nonpurulent
- Beta-hemolytic streptococcus, usually GAS (Strep. Pyogenes)
- S. aureus less frequent
- Others based on exposure: aeromonas (fresh water), vibrio vulnificus (sea water), pseudomonas (water exposure, hot tub)

Question 20

2. Identify pathogens

Name the likely pathogens for Purulent (Furuncle, Carbuncle, Skin abscesses, Cellulitis)

Answer 20

Purulent
- S. aureus
- Some beta-hemolytic strep
- Gram-negative and anaerobes may be common in pt with skin abscess involving perioral, perirectal, vulvovaginal area

Question 21

2. Identify pathogens

What is the difference between CA-MRSA and HA-MRSA?

What is used to treat CA-MRSA?

Why is CA-MRSA not a concern in Singapore?

Answer 21

CA-MRSA and HA-MRSA are genetically distinct

CA-MRSA

• Panton-Valentine Leucocidin (PVL) - enzyme breaks down WBC, infection has more abscess and pus
• SCCmec IV

CA-MRSA treatment

• Oral non-beta lactams (e.g., clindamycin, co-trimoxazole, doxycyline), IV vancomycin

CA-MRSA has low incidence in Singapore

Risk factors include:

• international schools
• participants of contact sports, military personnel, IV drug abusers, prison inmates
• overcrowded facilities, close contact, lack of sanitation

Question 22

3. Selection of antimicrobial and regimen

Impetigo and Ecthyma

Discuss the antibiotic selection for Impetigo with mild limited lesions

Answer 22

Impetigo - Staph or Strep
Ecthyma - GAS

Impetigo (mild limited lesions):
- Topical Mupirocin BID x5d

Question 23

3. Selection of antimicrobial and regimen

Impetigo and Ecthyma

Discuss the EMPIRIC antibiotic selection for Impetigo and Ecthyma

Answer 23

Impetigo - Staph or Strep
Ecthyma - GAS

Empiric - Cover staph + strep
- PO Cephalexin 500mg q6h
- PO Cloxacillin 500mg-1g q6h
- PO Clindamycin 300-450mg q6h (if allergic to penicillin) - SE: CDAD

*Treatment duration for oral: 7 days

Question 24

3. Selection of antimicrobial and regimen

Impetigo and Ecthyma

Discuss the CULTURE-DIRECTED antibiotic selection for Impetigo and Ecthyma (if Strep Pyogenes)

Answer 24

Impetigo - Staph or Strep
Ecthyma - GAS

Culture directed (GAS):
- PO Penicillin V 500mg q6h
- PO Amoxicillin 500mg-1g q8h
*natural penicillin and aminopenicillin don’t cover MSSA

*Treatment duration for oral: 7 days

Question 25

3. Selection of antimicrobial and regimen Impetigo and Ecthyma Discuss the CULTURE-DIRECTED antibiotic selection for Impetigo and Ecthyma (if Staph aureus)

Answer 25

Impetigo - Staph or Strep Ecthyma - GAS Culture directed (MSSA): - PO Cephalexin 500mg q6h - PO Cloxacillin 500mg-1g q6h *Treatment duration for oral: 7 days

Question 26

3. Selection of antimicrobial and regimen Purulent (Furuncle, Carbuncle, Skin abscesses, Purulent Cellulitis) Mainstay of treatment is _________

Answer 26

Incision and drainage

Question 27

3. Selection of antimicrobial and regimen Purulent (Furuncle, Carbuncle, Skin abscesses, Purulent Cellulitis) When is adjunctive systemic antibiotics required?

Answer 27

- Unable to drain completely - Lack of response after drainage - Extensive disease involving several sites - Extremes of age - Immunosuppressed - Signs of systemic illness (SIRS: temp >38 or <36, HR >90, RR >24, WBC >12 or <4) - IV antibiotics for severe disease

Question 28

3. Selection of antimicrobial and regimen Purulent (Furuncle, Carbuncle, Skin abscesses, Purulent Cellulitis) Discuss the treatment for MILD common purulent cause (S. aureus)

Answer 28

Purulent - mainly S. aureus (some beta hemolytic, gram negative and anaerobe if skin abscess involving perioral, perirectal, vulvovaginal area) Mild infection: incision and drainage, warm compress

Question 29

3. Selection of antimicrobial and regimen Purulent (Furuncle, Carbuncle, Skin abscesses, Purulent Cellulitis) Discuss the antibiotic selection for MODERATE common purulent cause (S. aureus) - Define MODERATE purulent

Answer 29

Purulent - mainly S. aureus (some beta hemolytic, gram negative and anaerobe if skin abscess involving perioral, perirectal, vulvovaginal area) Moderate (w systemic symptoms): I&D + oral antibiotics - PO Cephalexin 500mg q6h - PO Cloxacillin 500mg-1g q6h - PO Clindamycin 300-450mg q6h (if penicillin allergy) *Treatment duration: 5-10 days

Question 30

3. Selection of antimicrobial and regimen Purulent (Furuncle, Carbuncle, Skin abscesses, Purulent Cellulitis) Discuss the antibiotic selection for SEVERE common purulent cause (S. aureus) - Define SEVERE purulent

Answer 30

Purulent - mainly S. aureus (some beta hemolytic, gram negative and anaerobe if skin abscess involving perioral, perirectal, vulvovaginal area) Severe - with severe systemic signs, failed I&D Severe: I&D + IV antibiotics - IV Cefazolin 1-2g q8h - IV Cloxacillin 500mg-1g q4-6h - IV Clindamycin 600mg q8h - IV Vancomycin 15mg/kg q8-12h (if allergic to everyone else, or if pt has hospital associated risk factors for HA-MRSA, or pt come from another country that may have CA-MRSA) *Treatment duration: 5-10 days

Question 31

3. Selection of antimicrobial and regimen Purulent (Furuncle, Carbuncle, Skin abscesses, Purulent Cellulitis) Discuss the antibiotic selection if pt might have MRSA

Answer 31

Purulent - mainly S. aureus (some beta hemolytic, gram negative and anaerobe if skin abscess involving perioral, perirectal, vulvovaginal area) Empiric (MRSA) - if pt has hospital associated risk factors for HA-MRSA, or pt come from another country that may have CA-MRSA Less severe: - Co-trimoxazole (PO 960mg bid, IV 5mg/kg q8h) - Doxycyline (PO 100mg bid) - Clindamycin (PO 300-450mg q6h, IV 600mg q8h) More severe: - IV Vancomycin 15mg/kg q8-12h - IV Daptomycin 4-6mg/kg q24h - IV Linezolid 600mg q12h *Treatment duration: 5-10 days

Question 32

3. Selection of antimicrobial and regimen Purulent (Furuncle, Carbuncle, Skin abscesses, Purulent Cellulitis) Discuss the antibiotic selection if pt might have gram negative or anaerobes

Answer 32

Purulent - mainly S. aureus (some beta hemolytic, gram negative and anaerobe if skin abscess involving perioral, perirectal, vulvovaginal area) Empiric (gram-negative, anaerobes) - PO Amoxicillin-Clavulanate 625mg TDS or 1g BD - IV Amoxicillin-Clavulanate 1.2g q6-8h *Treatment duration: 5-10 days

Question 33

3. Selection of antimicrobial and regimen Non-purulent (Erysipelas, non-purulent Cellulitis) Discuss the antibiotic selection in MILD non-purulent SSTIs - Define MILD non-purulent - What to cover?

Answer 33

Non-purulent - Beta-hemolytic streptococcus, usually Strep. Pyogenes (S. aureus less frequent, Pseudomonas with water exposure) Mild (no systemic signs of infection): cover Strep Pyogenes - PO Penicillin V 500mg q6h - PO Cephalexin 500mg q6h - PO Cloxacillin 500mg-1g q6h - PO Amoxicillin 500mg-1g q8h - PO Clindamycin 300-450mg q6h (Penicillin allergy) *Duration: 5-10 days, 14 days if immunocompromised

Question 34

3. Selection of antimicrobial and regimen Non-purulent (Erysipelas, non-purulent Cellulitis) Discuss the antibiotic selection if MODERATE non-purulent SSTIs - Define MODERATE non-purulent - What to cover?

Answer 34

Non-purulent - Beta-hemolytic streptococcus, usually Strep. Pyogenes (S. aureus less frequent, Pseudomonas with water exposure) Moderate (w systemic signs of infection, some purulence): include MSSA cover, may initiate IV alr - IV Cefazolin 1-2g q8h - IV Clindamycin 600mg q8h (Penicillin allergy) *Duration: 5-10 days, 14 days if immunocompromised

Question 35

3. Selection of antimicrobial and regimen Non-purulent (Erysipelas, non-purulent Cellulitis) Discuss the antibiotic selection if SEVERE non-purulent SSTIs - Define SEVERE non-purulent - What to cover?

Answer 35

Non-purulent - Beta-hemolytic streptococcus, usually Strep. Pyogenes (S. aureus less frequent, Pseudomonas with water exposure) Severe (w systemic signs of infection, failed oral therapy, or immunocompromised): include broader coverage and explore possibility of necrotizing infections => Cover gram -ve, anaerobe, pseudomonas - IV Piperacillin/Tazobactam 4.5g q6-8h (cover gram +ve, gram -ve, anaerobes) - IV Cefepime 2g q8h (cover gram +ve, gram -ve) - IV Meropenem 1-2g q8h (cover gram +ve, gram -ve, anaerobes, ESBL) *Duration: 5-10 days, 14 days if immunocompromised

Question 36

3. Selection of antimicrobial and regimen Non-purulent (Erysipelas, non-purulent Cellulitis) Discuss the antibiotic selection if SEVERE non-purulent SSTIs if there are MRSA risk factor (what to add on?)

Answer 36

Non-purulent - Beta-hemolytic streptococcus, usually Strep. Pyogenes (S. aureus less frequent, Pseudomonas with water exposure) Severe (w systemic signs of infection, failed oral therapy, or immunocompromised): include broader coverage and explore possibility of necrotizing infections ADD - IV Vancomycin 15mg/kg q8-12h - IV Daptomycin 4-6mg/kg q24h - IV Linezolid 600mg q12h (If MRSA risk factor) *Duration: 5-10 days, 14 days if immunocompromised

Question 37

3. Selection of antimicrobial and regimen What are some non-pharmacological management for non-purulent SSTIs?

Answer 37

- Ensure rest and limb elevation (drainage of edema, inflammatory substances) - Treat underlying conditions (e.g., tinea pedis, skin dryness, limb edema)

Question 38

4. Goal and monitoring therapeutic response for SSTI

Answer 38

Resolution of S&S - Improvement by 48-72h after initiation (resolution not necessary) - No progression of lesion or development of complication - Switch to oral when pt is better - If pt does not respond within 2-3 days, reassess indication and choice of antibiotics Bacteriological Clearance - Repeat culture not required for pt who responded Absence of ADRs and allergies

Question 39

Discuss the use of topical antibiotics in SSTI

Answer 39

Controversial Most mild cases are self-limiting and would not require antibiotics Severe cases require oral/IV antibiotics Local wound care is central in treatment Topical antibiotic may add cost and ADR

Question 40

What is Mupirocin coverage, and what is it used for?

Answer 40

Mupirocin Coverage - highly effective against aerobic gram positive cocci, especially S. aureus (bactericidal at 2% ointment) - not effective against enterococci and gram negatives Mupirocin Use - Nasal staphylococcal carriage - Impetigo (mild limited lesions) - BID x5d Mupirocin resistance in MRSA is a major concern - In hospital, reserved for MRSA decolonization and superficial infection (e.g., hemodialysis catheter exist site infection)