influenza virus Flashcards
how is influenza spread
predominantly through the air from coughing or sneezing
influenza virus
acute, febrile respiratory disease
high fever, coryza, cough, headache, prostration, malaise, and inflammation of the upper respiratory tree and trachea
seasonal, predominantly in the winter
t/f there is a stomach flu
false
more flu factoids
short incubation period (1-3 days, quick onset of symptoms)
Seasonal flue kills 36K/yr
114K hospitalizations in the US
pandemic flu can be disastrous
modern medicine keeps old people and people with other illnesses alive (asthma, cystic fibrosis) - making them vulnerable to death from flu
flu virus classification
-ss RNA 8 segments orthmyxovirus helical nucleocapids enveloped genome segments coiled like a telephone cord virions are pleomorphic
evolution of influenza virus:
antigenic drift
Types A,B,C
a cell infected with 1 virus can only produce viruses identical to the infecting virus, or viruses with point mutations
the point mutation is generated during replication
evolution of influenza virus: antigenic shift
Type A and B
Reassortment occurs when a cell is infected with >1 different viruses
i.e. virus 1 (a human influenza A virus) and virus 2 (an animal influenza virus)
newly produced virions with segments from animal viruses that encode H and N antigens that humans have less immunity against
reassortment is within the type
type A is more dangerous
influenza type A
causes epidemic/pandemic flu in humans.
It mutates (drift), and reassorts (shift) during passage through
humans and animals (birds, pigs, horses, marine mammals).
Aquatic birds are the main reservoir and mixing of avian and
animal flu viruses mainly occurs in animals such as pigs.
influenza type B
causes non-epidemic seasonal flu.
Mutates and reassorts. B/Victoria and B/Yamagata reassort.
Infects (almost) only humans. Seals can be infected.
less pathogenic
influenza type C
does not cause serious disease in humans. Mutates only (antigenic drift), so there are no subtypes. Infects only humans
nomenclature
- influenza type
- species isolated form (unless human)
- place of isolation
- strain designation (strain number)
- year isolation
- H#N# subtypes
e.g. A/Hong Kong/1/68(H3N2)
there are 17 HA classes and10 NA classes
how does the flu virus invade a cell
a gap opens up between the virus and membrane
HA = Hemagglutinin
binds to silica acid on cells for entry
mediates binding and entry into cells
can be used for diagnosis
Elicits protective antibodies that neutralize the virus
H1, H2, etc. are the different forms of HA that represent antigenic shift
minor mutation result in antigenic drift that makes new strains each season
NA= neuraminidase
cleaves silica acid, allows virus to be release from cells
target of antiviral drugs (zanamivir/Relenza) and oseltamivir
N1, N2 represent distance forms with different antigenicities
Antibodies against N do not neutralize virus, but do reduce virus release
flu transmission
10^5-10^6 virions per droplet of aerosol
incubation time
18-72 hours (relatively fast)
replication
Influenza virus replicates in the epithelial cells throughout the respiratory tree, with virus being recoverable from both the upper and lower respiratory tract
no viremia in seasonal flu
severe pandemic flu can cause
viremia.
what makes you vulnerable to secondary bacterial infections
Flu viruses infection of columnar epithelial cells in lower respiratory tract
causes the tracheal mucosal epithelium to lose ciliary cells until recovery.
Pathogenesis
virus kills the cells it infects
disruption of ciliary action
making patient susceptible to bacterial infections
bronchitis and pneumonia can result from influenza directly but usually are a result of secondary infection
interferon response induced by dsRNA intermediate
Virus has mechanisms to counter IFN effects on cells
theses are partially effective and much of the symptoms are a result of IFN responses
Laboratory diagnosis of influenza virus infection
usually done by noting symptoms, time of year, exposures, rather than for diagnostic test
quick lab tests are available and detect viral antigens or nucleic acid but they are error prone, both false (+) and false (-) results
Viral culture tests can also be performed but take longer
Flu Vaccines
most effective way to prevent influenza
Flu vaccination time in the US (October to November)
vaccine grown in eggs
inactivated trivalent vaccine contain purified HA and NA
live, attenuated vaccine, LAIV (flumist) approved for use in healthy and given to people who are allergic to eggs
people 2-49 years of age who are not pregnant
trivalent vaccine composition
recommendations made based on antigenic analyses of recently isolated influenza viruses,
epidemiologic
data and post-vaccination serologic studies in
humans
•! Vaccine is a cocktail of 3 virus strains
–! 2 strains of Influenza A
–! 1 Influenza B strain
The World Health Organization (WHO) meets in February to decide on the
composition of the Northern Hemisphere vaccine, and in September to decide on
the Southern Hemisphere vaccine.
is it possible to have flu like symptoms and it not be from the influenza virus?
yes! similar diseases are called the flu- 1/3-1/2 of samples for flu diagnosis don’t have a flu virus in them
