Lecture 2: Staph aureus & Strep

Question 1

75 year old diabetic with acute onset of rigors and chills and red, warm anterior shin

Physical Exam

• Crackles in both lower lobes, JVP elevated at 5 cm above sternal angle. Heart sounds S4 no murmurs.
• Raised red rash anterior shin right leg with clear fluid blisters
• Not excruciatingly painful. No hemorrhage or darkening of skin
• Moderate pitting edema on both legs
• Tinea pedis (fungus infection) noted between toes

Labs

• WBC 15 with 80% neutrophils and 10% bands, Hg 123,
• platelets 120
• Creatinine 175, electrolytes normal
• Serum glucose 15
• Chest x-ray small bilateral pleural effusions with fluid in minor fissure and Kerley B lines
• Blood cultures return positive for GBS

Answer 1

Streptococcal Skin Infection

• Cellulitis in adults and erysipelas (distinct raised border at edge of infection) in children
• Impetigo is a pustular infection common world wide usually involving young children (age 2-5) involving Beta hemolytic strep and/or S. aureus
• GBS: diabetics, alcoholics, pregnancies
  
  • Drug of choice: Vancomycin

Question 2

Strep skin infection: Risk factors

Answer 2

• Tinea pedis and toe web intertrigo are the most common portal of entry in leg cellulitis
• Chronic edema (venous stasis, volume overload)
• Obesity
• Lymphedema post surgical procedure (lymph node dissection, bi- pass surgery vein graft site)

Question 3

Strep skin infection: Etiology

Answer 3

Most commonly Beta hemolytic streptococci (A, B also known as Streptococcus agalactiae, C and G) and Staphylococcus aureus

– Other organisms from specific circumstances

• Pastuerella multocida from cat bites
• Capnocytophaga canimorsus from dog bites
• Vibrio vulnificus from salt water wounds
• Aeromonas hydrophila from fresh water wounds; Gram-negative, straight rods
• Pseudomonas aeruginosa from hot tubs, burns, neutropenic patients, chronic wet wounds/ulcers
• Erysipelothrix rhusiopathiae infection in workers with domestic animals
• Cryptococcus neoformans in immunocompromised hosts

Question 4

Strep skin infection: DDx

Answer 4

• Abscess. May have erythema and pain. Boggy or fluctuant. Requires incision and drainage. Purulence is more commonly associated with S. aureus
• Deeper infection than skin: necrotizing fasciitis, myonecrosis. Patients severely ill (septic) requires surgical debridement of dead tissue. High mortality
• Bursitis over joints. Also boggy and requires drainage if inflamed
• Deep venous thrombosis. Warm, swollen, red lower limb. If involves femoral veins can embolize to lungs
• Contact dermatitis. Lesions pruritic but not painful, often shiny and weepy.
• Gout. Erythema and pain over joint

Question 5

Strep skin infection: investigations

Answer 5

Mild cases

• Bacteremia occurs in less than 5 %
• Aspirates and punch biopsies also less helpful
• Swabs of intact skin not helpful and should not be done
• Infecting organism can be grown from intertrigo but in absence of pus cells most labs won’t work it up
• Clinical appearance guide stherapy

Severe cases

• Blood culture
• Aspirates of bullae or culture of wounds in specific exposures, large surface areas
• Imaging techniques (CT, US, MRI) useful in looking for underlying abscess that requires drainage, necrotizing fasciitis and myonecrosis

Question 6

Cellulitis: Management

Answer 6

In cases of repeat cellulitis

1. treat edema
2. treat portal entry
3. prophylax

Question 7

Strep skin infection: DDx

Answer 7

• Abscess. May have erythema and pain. Boggy or fluctuant. Requires incision and drainage. Purulence is more commonly associated with S. aureus
• Deeper infection than skin: necrotizing fasciitis, myonecrosis. Patients severely ill (septic) requires surgical debridement of dead tissue. High mortality
• Bursitis over joints. Also boggy and requires drainage if inflamed
• DVT: Warm, swollen, red lower limb. If involves femoral veins can embolize to lungs
• Contact dermatitis. Lesions pruritic but not painful, often shiny and weepy.
• Gout. Erythema and pain over joint

Question 8

Strep skin infection: Management

Answer 8

• Assess severity of illness and presence of systemic toxicity (fever, hypotension, confusion, underlying conditions) Admit for further investigations and IV therapy if present
  
  • SIRS criteria
• Non purulent cellulitis can be treated with oral anti- staphylococcal/ streptococcal beta lactams (cephalexin, cloxacillin, nafcillin)
• Purulent cellulitis should be cultured and therapy aimed at underlying risks including consideration of MRSA

Question 9

Staph skin infection: Risk factors

Answer 9

• Similar portals of entry to those just discussed for Streptococci
• Carbuncles, furuncles and skin abscesses with S. aureus can occur without specific portal of entry or predisposing condition other than colonization
• Association with others with these conditions increases the risk
• Dialysis and diabetic patients have increased carriage rates

MRSA

• Risk for MRSA increased in Hospitalized and health care associated patients: Hospital based strains MRSA type I and II

– Prior surgery
– Invasive devices, central lines
– ICU admission
– Dialysis, diabetes, long term care – Antibiotic exposure

• Community Associated MRSA USA 300 strain or Canadian MRSA IV increased in certain demographic groups
• Epidemics are overlapping

Question 10

Staph toxin: PVL

Answer 10

• Causes leukocyte destruction and tissue necrosis
• Carried on staphylococcal cassette chromosome mec (SCCmec) IV which also carries the altered penicillin binding protein 2a that is responsible for methicillin resistance
  
  • Ability to grow on oxacillin containing medium indicates oxacillin resistance that correlates with the presence of mecA gene
• May be present in some MSSA strains
• CA has PVL; whereas hospital acquired stains do not

Question 11

Staph skin infection: Signs and Sx

Answer 11

• Onset can be small, red inflamed papule “spider bite”
• Surrounding erythema
• Initially indurated. Can become fluctuant suggesting accumulation of pus
• May be accompanied by systemic signs of fever, hypotension
• May have findings of disseminated spread to bone, joint, central nervous system, lung, heart valves.

Question 12

Staph: Toxic shock syndrome

Answer 12

• Classically associated with use of tampons in menstruating women. May occur as complication of postoperative wounds
• Fever, hypotension, skin and mucosa membrane erythema, vomiting , diarrhea
• S. aureus exotoxins (including TSST-1) act as superantigens and activate cytotoxins
• Multi-organ involvement (Brain, liver, kidneys)
• Skin desquamation occurs after infection (like peeling of sun burn)

Question 13

Staph skin infection: management

Answer 13

• MSSA treated best with beta lactams effective against S. aureus
• Vancomycin for MRSA effective but blood levels predose are run higher for invasive or deep seated infection
• Know local susceptibility patterns for use of macrolides and clindamycin in beta lactam allergic patients
• Remove infected prosthetic material (joints, lines, valves etc.) to increase cure rates
• Invasive disease requires high dose IV therapy

Question 14

Staph: issues w management

Answer 14

• Best drugs against S. aureus are beta lactams. Ineffective against MRSA
• Vancomycin does not have same bactericidal effect.
  
  • Kills more slowly
  • Known to be inferior to beta lactams for treatment of MSSA endocarditis
  • Higher relapse rates
  • Increased mortality rates
  • Only effective systemically when given IV
  • Long term therapy can give rise to Vancomycin Intermediate susceptibility strains
• Linezolid
  
  • Bacteriostatic
  • Available IV and PO
  • Current cost 150.00 (Canadian dollars)/ day either route
  • Significant toxicities
  • – Thrombocytopenia in up to 10% of patients. Duration dependant.
  • Possibly increased in renal failure
  • – Lactic acidosis, peripheral neuropathy, optic neuritis
  • – Serotonin syndrome with SSRI’s (antidepressant class), methadone and others
  • Use still being evaluated. Has a selective role in management MRSA infections