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Micro - Streptococci (ii) S Pyogenes (group A) (Presentations (necrotising…
Micro - Streptococci (ii) S Pyogenes (group A)
Intro
beta-haemolytic
many presentations
commonly colonises oropharynx of children + young people
transient
influenced by acquired immunity + competition
from here can invade epithelial cells
droplet transmission, esp in overcrowded places (e.g. schools)
bacitracin susceptible
causes suppurative infections
V virluent due to several factors...
capsule
immunogenic peptidoglycan cell wall (activates alternative complement pathway)
M proteins
over 80 types
mediate attachment
antiphagocytic
increase disease severity
M-like proteins
bind to IgG+M
Cytolysins
cause lysis
streptolysin
we make anti-streptolysin O Ig (measured on ASOT - titre)
leucocidin
haemolysin
streptokinase
hyaluronidase (tissue destruction, facilitates spread)
pyogenic / erythrogenic exotoxins
rash
fever
overactivate immune system
activate T cells
increase ck secretion
TSS
post infection immune-mediated complications (sequelae)
rheumatic fever
may lead to rheumatic heart disease
after recovery heart valves may thicken + become deformed
prophylactic antibiotics need to be given before a a procedure that increases endocarditis risk
main reason we use antibiotics to Tx strep infections is this
type 2 hypersensitivity
anti-group A strep Igs cross-react with heart tissue
ICD on heart
molecular mimicry
associated with pharyngitis, NOT SKIN INFECTIONS
occurs 2-3 wks later
fever, joint pain, carditis, Syndenham's chorea (neurological involvement)
Dx based on Jones criteria (GAS infection + clinical findings)
glomerulonephritis
type 3 hypersensitivity
associated with pharyngitis + sometimes skin infections
NA+ + H2O retention
oedema (puffy face + swollen limbs)
albumin + blood in urine
hypertension
most young patients recover completely
may lead to permanent renal damage
transplant
dialysis
death
affect only a minority of people
Presentations
Strep pharyngitis
1 of most common childhood bacterial infections
occasionally also caused by group C/G
incubation = 2-4 days
sore throat, fever, headache, nausea, vomitting, enlarged tender lymph nodes, enlarged hyperaemic tonsils with exudate (white spots, dDx = EBV), oedema
lab tests: throat culture, ASOT
sequelae
suppurative
otitis media
acute mastoiditis
acute non-suppurative
glomerulonephritits
rheumatic fever
Scarlet fever
delayed skin reaction to pyrogenic toxin produced by S pyogenes
pharyngitis
rash
diffuse erythema, small elevations that blanch with pressure (glass test)
can occur after impetigo
Impetigo
confined infection of superficial layers of skin
honey-crusted lesions
multiple septic spots (white or yellow pus-filled) on exposed sites (usually face, arms, legs)
can also be caused by group C/G or S Aureus
erysipelas + cellulitis
skin infections
caused when bacteria enter via breaches in skin barrier
predispositions
insect bites
abrasions
wounds
IV drug user
inflamm (e.g. eczema)
preexisting skin infection
erysipelas involves upper dermis + superficial lymphatics, and skin involved can be differentiated from uninvolved
cellulitis involves deep dermis + subcut fat, unclear distinction between affected + unaffected skin
erythema, oedema, warmth
necrotising fasciitis
cellulitis that spreads to muscle + fat, causing destruction
surgical emergency
bullae form (large blisters containing serous fluid)
gangrene
systemic signs
mortality > 50%
type 1 = polymicrobial
type 2 = monomicrobial (just S Pyogenes)
due to toxin production determined by M protein
TSS
soft tissue inflamm
pain, fever, chills
rapidly progresses to multi-organ failure
due to pyrogenic endotoxin
BSI
mortality nearly 40%