Skin and soft tissue infections Flashcards
What is difference between erysipelas, cellulitis, necrotising fasciitis?
Erysipelas - superficial layers
Cellulitis - subcutaneous fat
Necrotising fasciitis - deeper layers including fascia. Often associated with severe pain
What are most common causes of cellulitis?
Staph aureus
Group A strep
Who should be offered antibiotic prophylaxis against cellulitis?
If 2 or more episodes in 12 month period
- Phenoxymethylpenicillin
250 mg twice a day - Erythromycin
250 mg twice a day
re-assess if still required after 12 months
55 year old comes in drowsy, vomiting, hypotensive, pyrexial. Furuncle seen on foot, with surrounding erythema following gardening accident.
What is possible diagnosis?
Toxic shock syndrome
multi-organ failure mediated by bacterial toxins, which act as immunological superantigens, stimulating high levels of cytokine release - hypotension, increase tissue permeability. Desquamation occurs later.
normally antigens presented in MHC by APC. However, superantigens can bind to external MHC, and cross link with T-cell receptors. Thereofre specific MHC-TCR receptor site binding not required. So large number of T-cells are stimulated at once
Which organisms cause toxic shock syndrome?
Staph aureus - TTST-1 toxin. 3% mortality
GAS - streptococcal pyrogenic exotoxins. 30% mortality Some evidence of IVIG having benefit in streptococcal toxic shock syndrome
Chickenpox, complicated by cellulitis.
What is most common cause?
Group A strep
Child with rash, with fever
What are differential diagnoses?
Parvo B19
HHV6/7
measles
Rubella
Vesicular -
Enterovirus
VZV
Foreign travel -
chikungunya
dengue
zika
Child with rash, fever, conjunctivitis. What is possible diagnosis?
Measles
may have respiratory symptoms - cough/ coryza
may have Kopliks spots in mouth
Adenovirus
What is management of measles?
Supportive - fluids/ paracetamol
ribavirin - some evidence but not routinely used
WHO suggests vitamin A, once daily, for two days, in children who are 12 months or older
What are neurological complications of measles infection?
Acute post-infectious meningoencephalitis - occurs weeks after infection. Most recover fully
Inclusion body encephalitis - 75% die. Occurs 1 year after infection
Sub-acute sclerosing pancencephalitis - most die within 1 year. Presents within 20 years of measles infection
Patient with erythema migrans
What are differences in clinical picture of lyme disease acquired in UK v USA?
Borrelia burgdorgeri covers 15 species of bacteria which cause Lyme disease
USA - more fever, headache, fatigue. B burgdorferi sensu stricto is most common species there
UK - Borrelia afzelii, has lower virulence. Most common species in Europe
Apart from erythema migrans, what are other manifestations of Lyme disease?
meningitis
cranial nerve palsy
mononeuritis multiplex
AV block
myocarditis
arthritis
47 year old ma with T2DM has history of foot pain, with worsening redness.
Ulcer over plantar surface right midfoot
CRP 156
WCC 19
Wound swab from GP grew -
pseudomonas
strep agalactiae
strep equisimilis
What are other important parts of examination?
diabetic foot care e.g footwear, recent trauma, any metalwork
diabetic control e.g HbA1c, retinopathy
evidence of neuropathy - charcot arthropathy, claw/ hammer toes, buniones, calluses
check peripheral pulses
47 year old ma with T2DM has history of foot pain, with worsening redness.
Ulcer over plantar surface right midfoot
CRP 156
WCC 19
Wound swab from GP grew -
pseudomonas
strep agalactiae
strep equisimilis
What is significance of wound swab?
Wound swabs can be heavily contaminated, and may not indicate actual infection.
But these organisms are usual culprits -
staph aureus
streptococci
Gram neg - pseudomonas, E. coli, Klebsiella
Anaerobes
47 year old ma with T2DM has history of foot pain, with worsening redness.
Ulcer over plantar surface right midfoot
CRP 156
WCC 19
Wound swab from GP grew -
pseudomonas
strep agalactiae
strep equisimilis
What are next management steps?
X-ray/ MRI - osteomyelitis?
IV antibiotics
surgical debridement if sepsis/ life threatening infection
improved diabetic control
Which pathogens are associated with prosthetic joint infection?
Staphylococci, including CoNS
Enterococci
Diptherioids
Mycobacteria
Fungal
Patient with cellulitis.
Staph aureus grown in pure culture.
What are important antibiotic discs to test for with regards to resistance?
penicillin - likely mechanism is beta-lactamase producer
cefoxitin - potent inducer of mecA gene. Predicts that organism has alteration to PBP2 - which confers pencillin resistance (MRSA)
Patient with cellulitis.
Staph aureus grown in pure culture.
On resistance testing, you notice it is resistant to erythromycin and clindamicin.
What is significance of this?
if erythromycin and clindamicin discs are close together, there may be formation of a D-shaped zone of inhibition around the clindamicin.
Erythromycin can induce production of erm gene. erm gene can either be “always on”, or “inducible”. D-zone test is assessing whether inducible form is present
Bacteria closest to erythromycin re in presence of an inducer, and therefore will be resistant to clindamicin. This causes a flattening of the zone between the discs
If bacteria had erm gene always on - would have no resistnace on plate.
Can explain why some patients might fail on clindamicin
Cat bite
Blood culture grows gram neg bacilli
oxidase pos
fail to grow on MacConkey
What is the organism?
Pasteurella multocida
Capnocytopahga - oxidase neg, is other option
if cat scratch (usually kitten) - then more likely to be Bartonella henselae (cat scratch disease)
infections following animal bites are often polymicrobial - one study showed average of 5 different bacteria in anima bite
serious infections more common in cat bites, than dog bites. Possibly because canines are longer and sharper than dogs
Human bite
What are likely causative organisms?
Gram pos
Strep anginosus
Gram neg
Eikenella
Prevotella
Fusobacterium
Patient has cat scratch disease
What is clinical syndrome?
Cat scratch - usually kitten
lesion forms 1-2 weeks later
1-2 weeks more regional lymphadenopathy develops
Caused by Bartonella henselae
difficult to diagnose - serology tests available, look for 4-fold rise in acute and convalescent serology. Biopsy is sometimes used
Child with vesicular rash
What is differential diagnosis?
VZV
HSV
Enterovirus
Pox-like lesions cna look vesicular -
molluscum
monkeypox - Central/ West Africa
smallpox
VZV can cause latent infection.
Which part of the immune system keeps in in check?
Cell-mediated immunity is necessary to maintain latency (not VZV antibody)
A loss of CMI, as seen in older age/ transplant/ HIV/ diabetes/ physiologic stress can lead to reactivation
PAtient with varicella infection, admitted to hospital.
What are infection control precautions?
Cutaneous - skin covering and side room (until lesions are crusted)
Disseminated - respiratory and contact precautions in side room (ideally negative pressure)
Non-immune staff or pregnant should not care for patient. If non-immune person has contact, they should not have contact with other patients for 2 weeks, as may be incubating virus
The following groups are considered to be at increased risk from varicella and may require passive immunisation with Varicella Zoster Immunoglobulin (VZIG):
- pregnant women
- neonates
- immunocompromised individuals including those on steroids, chemotherapy
- HIV positive
- haematological malignancy
What do different words mean -
varicella
varicella zoster
herpes zoster
varicella - chickenpox
varicella zoster - name of virus
herpes zoster - shingles
GAS infection of skin can cause streptococcal toxic shock syndrome (STSS)
Which toxin mediates this?
Streptococcal pyogenic exotoxin (SpeA)
Superantigen
Binds to multitude of antigen-presenting cells/ T-cells/ Produces massive amount of proinflammatory cytokines. Activates complement, coagulation and fibronylysis cascades, which are responsible for abnormal physiological response
M1 streptococcal serotype is most common strain which has this toxin
What is link between VZV and GAS?
VZV outbreaks in children cause itchy blisters which are scratched by children. They then inoculate GAS, can lead to necrotising fasciitis
This then leads to an outbreak of GAS
GAS suspected necrotising fasciitis.
Sensitive to penicillin.
Why do we not treat with penicillin monotherapy?
High bacterial load means penicillin might not be able to kill all bacteria.
Established infection - bacteria might be in stationary phase. So penicillin less effect
Addition of clindamicin has been shown to reduce toxin production
GAS.
What is D-test used for?
To check for clindamicin susceptibility
GAS may appear fully sensitive to clindamicin.
However. they may have an erm gene, which confers inducibile clindamicin resistance. Means that in vivo, GAS may undergo mutation to resist clindamicin
Add erythromycin near clindamicin disk. This will flatten the zone of inhibition, producing a letter D
Slow indolent progressive skin soft tissue injection
What are possible organisms?
Mycobacteria Nocardia Sporothrix Histoplasma Blastomycosis Leishmania
Mycobacteria which only make pigment when exposed to light, are called photochromogens.
If produce pigment independent of exposure to light, called scotochromogens
What are most common photochromogens?
M marinum
M kansasii
What are symptoms/signs of RMSF?
rash (90%)
fever
myalgia
hyponatraemia
thrombocytopenia
increased coagulation
coagualtion issues are due to DIC which can be fatal
What are most common species of Leishmania in:
Europe/ Africa/ Asia
South/ Central America
Europe/ Africa/ Asia (old world)
- L tropica
- L major
- L donovani (most common cause kala-azar)
South/ Central America (new world)
- L brasiliensis
- L mexicana
- L Infantum (chagasi) - visceral
What is the immunogenic component of smallpox vaccine
Vaccinia virus - not Variola (smallpox)
Live vaccine (not attenuated) - produces very mild self-limiting lesion at innoculation site Protection last approximately 5 years
Vaccinia is an orthopox virus with similar antigenic components to Variola
What is the cause of orf
What are risk factors
Orf Parapox virus
Sheep/ goat farmers, vets, butcher
Parapox viruses - Orf, Milk maid’s module
Orthopox viruses - Cowpox, Monkey pox, Vaccinia, Variola
What is the cause of Milker’s nodule
What are risk factors
(unspecified) Parapox virus
Dairy farmers, vets, butcher
Parapox viruses - Orf, Milk maid’s module
Orthopox viruses - Cowpox, Monkey pox, Vaccinia, Variola
Poxviridae are classified into orthopox and parapox viruses
What are examples of each
Orthopox - Cowpox Monkeypox Vaccinia Variola
Parapox -
Orf
Milk maid’s nodule
What is difference between cowpox and milk maid nodule
Cowpox - orthopox virus rare more extensive distribution of lesions usually acquired from cats/ rodents as opposed to cows
Milk maid module - parapox virus common usually one lesion on hand usually acquired from cows
DDx for maculopapular rash
Measles
Rubella
EBV
CMV
HHV6
HHV7
Parvo B19
S. Aureus
GAS
