Session 10: Influenza Flashcards
Flu is an acute viral infection of what?
Respiratory tract (nose, mouth, throat, bronchial tubes and lungs)
Give some features of influenza virus.
Highly infectious illness which spreads rapidly in closed communities. Resp. tract infection Even people with mild or no symptoms can infect others Mostly during winter
Structure of influenza virus (orthomyxoviruses)
Spherical, enveloped containing a segment negative ssRNA genome. Include 3 RNA polymerases with high error rates.
Influenza have two surface antigens. Which?
Haemagglutinin (H) 18 types Neuraminidase (N) 11 types
What does H antigen do?
Binds to cells of the host of the infected person.
What does N antigen do?
Releases the virus from the host cell surface
Why is it important to not that the 3 RNA polymerases of influenza virus have high error rates?
It means that mutations are likely to occur.
What types of influenza viruses are there? (3 main groups)
Influenza A Influenza B Influenza C
Natural host range of Influenza A.
Humans, swine, equine, birds, marine, mammals. However animal -> human transmission is not likely.
Natural host range of Influenza B.
Humans only
Natural host range of Influenza C.
Humans and swine
Epidemiology of Influenza A.
Antigenic shift and drift
Epidemiology of Influenza B.
Antigen drift only
Epidemiology of Influenza C.
Antigenic drift only
Clinical manifestations of Influenza A.
May cause large pandemics with significant mortality in young persons.
Clinical manifestations of influenza B.
Severe disease generally confined to older adults or persons at high risk.
Clinical manifestations of Influenza C.
Mild disease without seasonality.
Features of influenza A.
Cause outbreaks most years and are the usual cause of epidemics and pandemics. Live and multiply in different animals and may spread between them.
Features of influenza B.
Tend to cause less severe disease and smaller outbreaks and predominantly found in humans. Specifically in children.
Explain replication of the influenza virus.
It all stems from a negative ssRNA which is read and processed by RNA-dependent RNA polymerase (RdRp). The negative ssRNA can therefore turn into two strands of mRNAs or a positive ssRNA they are effectively the same thing and have two different fates. +ssRNA can turn into 4 strands of -ssRNA to become the new -ssRNA in a new virus. The mRNAs code for viral proteins which will assemble the nucleocapsids. This all happens in the cytoplasm.

Three potential modes of transmission of influenza virus.
Influenza virus is transmitted from person to person via resp- route and therefore coughing, sneezing and inhaling.
1) small-particle aerosols which can remain in air for many hours non-direct contact.
2) larger particles or droplets will typically fall to the ground within 3 m of the infected person and infect individuals in direct contact.
3) Viral particles could land on surfaces where the influenza virus can remain and be infectious for hours and infect others through non-direct contact.
Barriers to prevent entry of the influenza virus via the respiratory route.
Resp. epithelial cells covered by a thick glycocalyx and tracheobronchial mucus that can trap the virus particles.
Mucociliary escalators
In the lungs immunologic defenses include secretory IgA, NK cells and macrophages
How does the influenza virus enter cells?
H antigen binds to host.
NANA residues act as receptors for influenza virus and forms an endocytotic vesicle.
This means that the virus enters via receptor-mediated endocytosis.
How does the influenza virus leave the cells?
Neuraminidase (N antigen) cleaves the binding that H antigen has with the host cells glycoprotein.
So during exocytosis the newly formed vesicle is still in contact with the membrane of the host cell. But the N antigen cleaves and stops this interaction.