18. Influenza Flashcards
Simply, what do viruses consist of?
Delivery system and a payload
What is the role of the delivery system?
Protects it against degradation in the environment and contains structures used to bind to target cells in the host
What is the payload?
Contains genome and enzymes necessary to initiate the first steps in virus replication
What is the flu?
An acute viral infection of the respiratory tract
What is the structures of the influenza virus?
Orthomyxoviruses are spherical, enveloped viruses containing a segmented, negative strand RNA genome
What is the genetic material in the influenza virus?
(-)ssRNA - 8 genes
Encoding 11 proteins
Include 3 RNA polymerases (high error rates)
What are the 2 surface antigens on the influenza virus?
Haemagglutinin (H) - 18 types - binds to cells of the infected person
Neuraminidase (N) - 11 types - releases the virus from the host cell surface
How many gene segments are there in each of the influenza viruses?
A - 8
B - 8
C - 7
What is the natural host range of influenza A?
Humans Swine Equine Birds Marine mammals (Also mink, dogs, cats)
What is the natural host range of influenza B?
Humans only
What is the natural host range of influenza C?
Humans
Swine
What are the clinical manifestations of influenza A?
May cause large pandemics with significant mortality in young persons
What are the clinical manifestations of influenza B?
Severe disease generally confined to older adults or persons at high risk
Pandemics not seen
What are the clinical manifestations of influenza C?
Mild disease without seasonality
What are the main animal reservoir of influenza A?
Birds, particularly wildfowl
What happens in replication of the influenza virus?
(-)ssRNA converted to (+)ssRNA which is then translated to multiple (-)ssRNA
(-)ssRNA also converted to mRNAs which are translated to viral proteins
These then assemble into nucleocapsids
How is influenza transmitted generally?
From person to person via the respiratory route (coughing, sneezing, inhaling)
What are the 3 potential modes of transmission?
- Small particle aerosols - remain suspended in air for many hours
- Larger particles or droplets will typically fall to the ground within 3m of the infected person - infect individuals in direct contact
- Viral particles could land on surfaces, where influenza viruses remain infectious - infect others through indirect contact
What are the barriers to entry via the respiratory route?
- Respiratory epithelial cells are covered by a thick glycocalyx and tracheobronchial mucus that can trap virus particles
- Ciliated respiratory epithelial cells continually sweep mucus up from the lower respiratory tract into the upper respiratory tract, where it is usually swallowed
- In lung, immunologic defences include secretory IgA, natural killer cells and macrophages
How does the influenza virus enter cells?
Hemagglutinin protein on the surface of the virus
Neu5Ac (NANA) residues - sialic acid on a glycoprotein/glycolipid which acts as a receptor for influenza virus
Entry then occurs via receptor-mediated endocytosis
How does the influenza virus leave cells?
Glycoprotein bound to hemagglutinin
Neuraminidase cleaves bond so virus can leave and infect another cell
What are the symptoms of influenza?
Fever, headache, confusion
Dry cough, sore throat, nasal congestion
Nausea, vomiting, diarrhoea (mainly in children)
Muscle aches
What are the complications of influenza?
Otitis media, croup (mainly children)
Sinusitis, bronchitis, pharyngitis
Pneumonia
What is the incubation period of influenza?
1-5 days
Which groups of the population are most at risk of serious complications?
Children under 6 moths Older people (over 65) Those with underlying health conditions Pregnant women Morbid obesity (BMI>40)
How is influenza usually diagnosed?
From symptoms and clinical assessment
Test only done if admitted to hospital
What are the 3 different treatments for influenza?
Antivirals
Neuraminidase inhibitors
Prevention
How do antivirals work?
Inhibits viral in coating after uptake probably through M2 protein
For influenza A
Why aren’t antivirals used much anymore?
Resistance
How do neuraminidase inhibitors work and give examples?
Inhibit viral release from infected cell and cause aggregation of viral particles
For influenza A and B
E.g. oseltamivir (Tamiflu), Relenza
What are the 2 preventions for influenza?
Formalin-inactivated vaccine (quadrivalent/trivalent) - influenza A and B
Live, attenuated, col-adapted vaccine (quadrivalent) by nasal spray - influenza A and B (mainly used in children)
What is the life cycle of the influenza virus?
6 hours
What leads to mutations in the influenza virus?
High error rate and lack of proofreading ability leads to mutations
What is antigenic drift?
Minor changes (natural mutations) in the genes of flu viruses that occur gradually over time - cause seasonal epidemics
Which influenza shows antigenic shift?
A
What is antigenic shift?
Major changes in the genes of flu viruses that occur suddenly when 2 or more different strains combine
Results in a new subtype - causes widespread epidemics/pandemics
Surface antigens hemagglutinin and neuraminidase from different species
What is an epidemic?
Prevalent among a people or a community at a special time and produced by some special causes not generally present in the affected locality
How does the flu kill people?
a) Immune system overreacts - T cells attack and destroy the tissues in which the iris is replicating, in particular the lungs
b) there is an opportunistic secondary infection usually in the lungs
Who does the WHO recommend annual vaccination for?
Pregnant women at any stage of pregnancy
Children aged between 6 months to 5 years
Elderly individuals (over 65)
Individuals with chronic medical conditions
Health care workers
