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CR - Resp Viruses (iii) Influenza (Pathogenesis (droplets on mucosal…
CR - Resp Viruses (iii) Influenza
Classification
into 3 types based on core proteins
A
common in flu season
infects a range of mammalian + avian species
responsible for annual epidemics + occasional pandemics
birds = main reservoir (often doesn't cause disease in them, simply multiply in their GIT leading to a high faecal load)
B
common in flu season (esp @ end)
restricted to humans
outbreaks every 2-4 yrs, not pandemics
C
uncommon, usually milder
restricted to humans
outbreaks every 2-4 yrs, not pandemics
subtypes based on antigens projecting from envelope (HxNx - H3 most severe)
Segmented genome
8 segments
allows reassortment with other strains (antigenic shift)
major change
whole genes swapped when 2 different viruses simultaneously infect a host cell -> novel virus
causes a pandemic (pop vulnerable)
high attack rate
high morbidity + mortality in all age groups
antigenic drift
minor change
point mutations (transcription errors by viral DNAP - AA substitutions)
causes annual epidemics - partial (cross reacting) immunity in pop (v little in infants)
cause outbreaks of variable severity (esp in extremes of age)
Pathogenesis
droplets on mucosal membrane enter via resp epithelial cells + attach to columnar epithelium
haemagglutinin binds to host cell Rs
neuraminidase (drug target) cleaves silica acid in resp epithelium to allow release of virus from cell + prevent clumping
antigenic drift + shift stops antigens from being recognised by host immune system
virus-induced lymphocyte dysfunction
diffuse inflamm of trachea + bronchi (may progress to ulcerative necrotising tracheobronchitis - loss of mucociliary elevator - bacterial superinfection - secondary pneumonia)
acute resp infection
usually uncomplicated, self-limiting
abrupt onset of fever, cough, myalgia, weakness, depression, + lasts only a few days
95% of exposed individuals that become symptomatic do so in 3 days (incubation of 1-2 days)
infectious up to 7 days after symptom onset
about 30% of those infected are asymptomatic (still infectious though)
Complications
primary influenza pneumonia
secondary bacterial pneumonia (S pneumoniae, H influenza, S aureus)
myostitis, myocarditis
CNS involvement (seizures, encephalopathy, encephailitis)
Reye's syndrome
encephalopathy + liver failure
v rare
can be caused by other things
death (can kill a vulnerable patient)
Tx
mainly supportive (antipyretics + analgesia)
if hospitalised give O2 + consider ICU (then advise vaccine in future
antiviral: neuraminidase inhibitors (oseltamivir = PO, Zanamivir = inhaled/IV)
Vaccine
inactivated
should be given yearly in advance of influenza season (Sep/Oct)
for anyone> 6months old @ risk (immunosuppressed, chronic illness, over 65s, pregnant, up to 6 wks post-partum, morbid obesity, long-term aspirin, residents of longterm stay facilities, pig/poultry/waterfowl workers)
for anyone @ risk of transmitting it to a high risk person (health workers, carers, household contacts)