Influenza Flashcards
WHat is influenza?
- it is a flu which is an acute viral infection of the respiratory tract ( nose , mouth , throat , bronchial tubes and lungs )
- it is highly infectious which spreads rapidly between closed communities.
- even people with mild or no symptoms can infect others.
- most cases in the UK occur during an 8 to 10 week period during winter.
What is the structure of the influenza virus ?
Influenza virus is an orthomyxovirus.
It is spherical , enveloped virus containing a segmented , negative strand of ssRNA genome.
It has two surface antigens : haemagglutinin ( H) this binds to cells of the infected person.
Neuraminidase ( N ) - which releases the virus from the host cell surface
What are the three types of influenza? And features of each
- Influenza A : this is the one we worry about. This affects humans , horses , pigs , marine mammals , birds. We do not normally see transmission from an animal to humans though. This can cause antigenic drift and drift. This causes outbreaks most years and the usual cause of epidemics and pandemics.
- Influenza B : this can cause severe disease in the elderly. But less severe disease in younger population. Often vaccinate this. This also undergoes antigenic drift. Only found in humans. Doesn’t generally cause pandemics. Burden of this virus is in children.
- Influenza C : exists in humans and pigs. It is a very mild disease - without seasonality. It undergoes antigenic drift where the antigens on the surface ( H , N) undergo point mutations with each replications which means that antibodies cannot recognise them the same way.
How does the influenza virus replicate ?
- The negative sense ssRNA genome in a host cell does not cause infection. It needs to be transcribed into positive sense ssRNA. ( with help of RNA dependant RNA polymerase)
- This positive sense ssRNA can then be translated into multiple negative sense single stranded RNA.
- The initial -ve ssRNA also generates mRNAs. This will be converted into viral proteins. ( with help of RNA dependant RNA polymerase )
- The viral proteins and the multiple - ve ssRNA can come together to form nucleocapsids. This leaves the host cell and can be spread to person to person.
What is the mechanism of transmission of the influenza virus ?
Transmitted from person to person via respiratory route
- Small particle aerosols - remain suspended in air for many hours
- Larger particles or droplets will typically fall to the ground within 3 m of the infected person. Infect individuals in direct contact.
- Viral particles can land of surfaces where influenza viruses remain infectious.
What are the barriers to the transmission of the influenza virus ?
- Respiratory epithelial cells are covered by a thick glycocalyx and tracheobronchial mucus that can trap virus particles.
- Ciliates epithelial cells continuously sweep mucus up from the lower respiratory tract into the upper respiratory tract where it is usually swallowed.
- In the lung , there are immunological defecnces incl secretory IgA , natural killer NK cells and macrophages.
How does the virus enter cells once it gets passed the barriers ?
- Your influenza virus which contains Hemagglutinin on the surface which binds to a host receptor (a NANA residue - which is a glycoprotein/lipid with sialic acid on the top)
- Enters the host cell via receptor mediated endocytosis.
- Then undergoes the replication previously mentioned.
- Virus leaves the cell via neuramindase cleaving the sialic acid which allows the virus to move away from the host cell.
How does the virus leave the cells ?
Using the other protein on the surface of the virus - neuramindase
- The virus attempts to bud out of the host cell ( still remains attached to host cell slightly). In doing so the haemogluttinin binds to the receptor on the surface of the host cell ( NANA residue with sialic acid on the glycoprotein
- neuramindase cleaves the sialic acid which allows the virus to move away from the cell.
What are symptoms of influenza?
- fever
- headaches
- chills
- muscle aches ( this is very characteristic of full blown flu )
- dry cough
- sore throats
- nasal congestion
- more common in children : diorreah, vomiting ( not so much in adults )
- fatigue
- confusion
What are complications of influenza?
- MSK : myosin is , rhabdomyolysis
- Pregnancy : increased maternal complications , increased risk of prematurity , smaller neonatal size , lower birth weight.
- Respiratory : otitis media + croup ( more common in kids) , sinusitis , pneumonia
- Cardiac : pericarditis , myocarditis , exacerbation of CV disease.
- Neurological : meningitis ,
What is the incubation period of influenza virus ?
1-5days
What increases the risk of most serious complications of influenza?
- Children under 6 months
- Pregnant women
- Older people ( aged over 65)
- Those with underlying health conditions such as respiratory disease , CVD ,
- Morbid obesity
How do you diagnose influenza?
There are a number of tests you could take.
But usually diagnosis is made from symptoms and clinical assessment.
How to treat for influenza?
- Antivirals - eg rimantadine and amantadine. But we do not use them now. Because of resistance.
- Neuraminidase inhibitors : eg Tamiflu and Relenza which inhibit viral release from infected cell and cause aggravation of viral particles. Used for influenza A and B.
- PREVENTION : Vaccine A : for example formalin activated vaccine by injection. This is effective against influenza A and B. This can be quadrivalent / trivalent ( Quad - 2 influenza A and 2 influenza B strains). Or Vaccine B : a live attenuated , cold adapated vaccine ( quadrivalent ) - this is given as a nasal spray. Once it gets to the lower respiratory tract the virus dies because it is no longer in a cold environment.
How to neuramindase inhibitors work ?
They block neuramindase which means they cannot cleave the sialic acid off which means the virus will remain attached to the host cell and doesn’t remove away from the cell.
