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introduction. Influenza viruses are the only members of the orthomyxovirus family. The orthomyxoviruses differ from the paramyxoviruses primarily in that the former have a segmented RNA genome (usually eight pieces), whereas the RNA genome of the latter consists of a single piece.

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  • Influenza viruses are the only members of the orthomyxovirus family.
  • The orthomyxoviruses differ from the paramyxoviruses primarily in that the former have a segmented RNA genome (usually eight pieces), whereas the RNA genome of the latter consists of a single piece.
  • The term "myxo" refers to the observation that these viruses interact with mucins (glycoproteins on the surface of cells).
discovery of influenza virus
Discovery of Influenza Virus
  • First isolated from a pig in 1931 (swine flu)
  • Isolated from human in 1933
epidemiology seasonal variations
Epidemiology:Seasonal variations

Seasonal risk areas for influenza: November–April (blue), April–November (red), and year-round (yellow).


Influenza reaches peak prevalence in winter:Northern and Southern Hemispheres have winter at different times of the year, there are actually two different flu seasons each year.

  • People are more indoors in Winter: this promote person to person spreading
  • Increased travel due to the Northern Hemisphere winter holiday season may also play a role.

 In addition cold temperatures lead to drier air, which may dehydrate mucus, preventing the body from effectively expelling virus particles.

  • The virus also survives longer on surfaces at colder temperatures .
  • An alternative hypothesis to explain seasonality is an effect of vitamin D deficiency on immunity to the virus(no sun or UV radiation). This idea was first proposed by Robert Edgar Hope-Simpson in 1965.
Antigenic shift, or reassortment, can result in novel and highly pathogenic strains of human influenza
  • Antigenic drift creates influenza viruses with slightly modified antigens, while antigenic shift generates viruses with entirely novel antigens.
  • In any given year some strains can die out while others create epidemics, while yet another strain can cause a pandemic.

The morbidity is Typically between three and five million cases of severe illness per hemisphere and up to 500,000 deaths worldwide.

  • more than 200,000 hospitalizations are directly associated with influenza every year.
influenza structure
Influenza Structure
  • 8 segments of single-stranded RNA
  • Segments combine with nucleoprotein (NP) to form the ribonucleoprotein core
  • M1 matrix protein surrounds the core
  • Lipid coat surrounds the matrix
  • Embedded in the lipid membrane are 2 important viral proteins: hemaglutinin (HA) and neuraminidase (NA)
  • Structure: trimer of “lollipops” with fibrous stem anchored in the membrane and globular protein sphere containing the sialic acid receptor site
  • Function: Sialic acid receptor sites bind to host cell’s glycoproteins allowing for infection to occur
  • Structure: Box-shaped tetramer with stalk that anchors it to the membrane
  • Function: Cleaves off sialic acid molecules from the surface of cells thereby preventing infected cells from “recapturing” budding virus molecules (more on this later…)
influenza replication
Influenza Replication
  • Virus is taken into cell through receptor mediated endocytosis
  • pH drops and ribonucleoprotein is released into the cytoplasm and enters the nucleus
  • Negative sense vRNA is transcribed to mRNA and cRNA- to do this, caps from host mRNA are used as primers and PB1, PB2, and PA make up the reverse transcriptase
  • Once packaged (mechanism unknown), the virus exits the cell, with the help of NA
  • Cell dies from lack of viable mRNA
  • A person becomes infected when they inhale microdroplets containing the virus
  • Upper and lower respiratory tract epithelial cells have sialic acid molecules to which the HA binds
  • As the virus causes the cells to die, inflammation occurs – a cough reflex results thereby spreading the virus again
  • More severe infections (i.e. pneumonia) are sometimes associated with Influenza because of the increased susceptibility to other infections as a result of a damaged airway
  • The pathogenicity and virulence of the influenza virus is determined by several interacting factors
Host factors

Viral factors

  • Presence of target receptors on host cells
  • Availability of enzymes in host cells which are essential for viral entry and replication
  • State of immunocompetence of the individual host
  • Specific immunity against certain viral epitopes in the individual host and target population
  • Ability of the immune system to control the viral replication effectively without causing serious collateral damage for the host by its inflammatory response
  • Ability to bind to host cells
  • Ability to bind to host cells
  • Ability of virus shedding
  • Restriction of cytopathogenic effects to allow for an appropriate balance between viral replication and control by the host
  • Escape from immunosurveillance by evolution of antigenic variation driven by selective pressure of the immune response
  • Escape from immunosurveillance by recombination with different virus strains from zoonotic disease
viral entry how does the virion enter the host
Viral entry: How does the virion enter the host?
  • Virus infection is spread via respiratory droplets
  • The virus particles binds to cells of the respiratory epithelium which are rich in viral receptors.
  • Neuraminidase present on the virus particles aid the infectious process by releasing virus particles which have been bound by the mucous present on the surface of epithelial cells
where does the primary replication occur
Where does the primary replication occur?
  • Cellular proteases are often required to cleave viral proteins to form the mature infectious virus particle
  • Additional factors to entry receptors can determine the site of viral replication
  • In humans, the replication of the influenza virus is generally restricted to the epithelial cells of the upper and lower respiratory tract
how does the infection spread in the host
How does the infection spread in the host?
  • Once influenza has efficiently infected respiratory epithelial cells, replication occurs within hours and numerous virions are produced.
  • Infectious particles are preferentially released from the apical plasma membrane of epithelial cells into the airways by a process called budding.
  • This favors the swift spread of the virus within the lungs due to the rapid infection of neighboring cells.
flu incubation period: refers to the time it takes from exposure to the influenza virus until the beginning of observable symptoms.
  • The flu incubation period is usually one to four days.for H1N1, it may be about four to seven days.
  • The flu incubation period is often shorter for a person in poor health since his or her body is already in a weakened state and less able to fight the virus. During this period, the person may show no symptoms.
host defenses
Host Defenses
  • Interferon signals for cells to produce PKR which inactivates eIF2 and inhibits protein synthesis
  • BUT…Influenza’s NS1 protein binds to dsRNA which keeps PKR inactivated
  • Anti-HA antibodies bind and stay with the virus as it makes its way through the cell and somehow interferes with the replication process
  • Anti-NA antibodies stop the molecule from shaving off the sialic acid residues
clinical features

Clinical features

The disease is usually most severe in very young children (under 5 years of age):lack antibodies to the influenza virus because of no prior exposure.

In addition, the small diameter of components of the respiratory tract in the very young also means that inflammation and swelling can lead to blockage of parts of respiratory tract, sinus system or Eustachian tubes.

symptoms and complications
Symptoms and complications

1. Uncomplicated influenza

  • Fever (38 - 40 degrees C)
  • Myalgias, headache
  • Ocular symptoms - photophobia, tears, achea
  • Dry cough, nasal discharge
  • H1N1 strain, the 2009 "swine flu", also gives rise to gastro-intestinal symptoms (e.g. vomiting, diarrhea)
2 pulmonary complications sequelae
2. Pulmonary complications, sequelae:
  • Croup (acute laryngotracheobronchitis) in young children - symptoms include cough (like a barking seal), difficulty breathing, stridor (crowing sound during inspiration)
  • Primary influenza virus pneumonia
  • Secondary bacterial infection: This often involves Streptococcus pneumoniae, Staphylococcus aureus, Hemophilusinfluenzae
  • The build up of fluids and lack of mucociliary clearance in the respiratory tract provide a good environment for bacterial growth.lications often occur in patients with underlying chronic obstructive pulmonary or heart disease. The underlying problems may not have been recognized prior to the influenza infection.
3 non pulmonary complications of influenza
3. Non-pulmonary complications of influenza:
  • Myositis
  • Cardiac complications
  • Encephalopathy-
  • Reye's syndrome:The effects of influenza virus infection on the liver and brain are particularly serious. In the liver fatty deposits are seen while in the brain edema occurs.
  • Guillain-Barré syndrome (acute idiopathic polyneuritis) It is an autoimmune disease that can follow a viral or bacterial infection.
  • The major causes of influenza-associated death are bacterial pneumonia and cardiac failure. Ninety per cent of deaths are in people over 65 years of age.
clinical diagnosis
Clinical Diagnosis
  • The clinical picture of influenza is nonspecific.
  • Influenza-like illness can be caused by many microbial agents other than influenzavirus, such as adenovirus, parainfluenza viruses, coronavirus, Mycoplasma pneumoniae, Chlamydia pneumoniae, beta-hemolytic streptococcus.
laboratory diagnosis
Laboratory Diagnosis
  • Since the clinical picture of influenza is nonspecific, its specific diagnosis must be confirmed by laboratory tests.
  • This is usually made by virus isolation, identification of specific antigens or antibody rise.


Generally, anti-viral drugs work optimally when taken within a few days of the onset of symptoms.

Certain drugs are used in uninfected individuals to guard against infection.

Four influenza antiviral agents are available : amantadine, rimantadine, zanamivir and oseltamivi

These drugs fall into categories as either M2-inhibitors (admantane derivatives) or neuraminidase inhibitors as illustrated in the following table.

con t
  • Rimantadineand amantadin
  • amantadine is effective only against influenza A, not against influenza B. A derivative of amantadine, rimantadine , can also be used as amantadine but has fewer side effects than it. Note that the vaccine is still preferred over these drugs in the prevention of influenza.

Oseltamvir (Tamiflu) and zanamivir (Relenza)

are also used for the treatment and prevention of influenza. that act by inhibiting the release of virus from infected cells. This limits the infection by reducing the spread of virus from one cell to another. These drugs are effective against both influenza A and B viruses in contrast to amantadine, which is effective only against influenza A viruses.

  • Reasonably effective ways to reduce the transmission of influenza include good personal health and hygiene habits such as: not touching your eyes, nose or mouth;

hand washing

covering coughs and sneezes

avoiding close contact with sick people

Avoiding spitting


According to the WHO, you can decrease your chance of contracting the flu virus by taking the following steps:

  • Get yourself (or family members age 6 months and older) vaccinated against current strains of influenza, if possible.
  • Keep your distance from people who show symptoms of influenza-like illness, such as coughing and sneezing (trying to maintain a distance of about 1 metre if possible);
  • Clean your hands thoroughly with soap and water, or cleanse them with an alcohol-based hand rub on a regular basis (especially if touching surfaces that are potentially contaminated);

Avoid touching your mouth, nose and eyes as much as possible;

  • Reduce the time spent in crowded settings if possible;
  • Improve airflow in your living space by opening windows;
  • Practice good health habits (including adequate
influenza vaccines
Influenza Vaccines
  • Whole virus vaccines: inactivated forms of virus with the predicted HA, are grown in embryonated eggs
  • Subunit vaccine: uses both HA and NA subunits extracted from recomibinant virus forms
  • Split-virus vaccines: purified HA (lessens the side-effects)
  • Recommended for health care workers, elderly/ people in nursing homes, asthmatics, chronic lung disease patients, some pregnant women, and anyone who is susceptible to infection
  • Levine, Arnold J. Viruses 1992, New York p. 155-175
  • Sompayrac, Lauren How Pathogenic Viruses Work 2002, Sudbury, MA p. 19-24