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.

The term "myxo" refers to the observation that these viruses interact with mucins (glycoproteins on the surface of cells).

DISCOVERY OF INFLUENZA VIRUS

First isolated from a pig in 1931 (swine flu)

Isolated from human in 1933

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.

EPIDEMIC AND PANDEMIC SPREAD

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 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)

HEMAGGLUTININ

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

NEURAMINIDASE

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 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

PATHOGENESIS

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

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

Host factorsViral factors

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?

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?

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

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•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

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:

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, Hemophilus influenzae

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:

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

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 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.

TREATMENT 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

  Rimantadine and 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.

PREVENTION

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 sneezesavoiding close contact with sick peopleAvoiding 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

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

SOURCES http://www.cdc.gov/ncidod/hip/INFECT/flu_acute.htm#table http://microbiology.mtsinai.on.ca/bug/flu/flu-bug.shtml http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/I/Influen

za.html http://www.sbimc.org/2000/spring/fiddian-slides/ http://microvet.arizona.edu/Courses/MIC419web/Case4flu.html http://www.tulane.edu/~dmsander/WWW/335/Orthomyxoviruse

s.html http://www.psc.edu/science/Herlocher/Herlocher.html http://www.cs.bsu.edu/homepages/dmz/david/replicate.html http://web.uct.ac.za/depts/mmi/jmoodie/welcome1.html http://www.goshen.edu/bio/Biol206/Biol206LabProject/Influenza

/SoniaRachel/page.html Levine, Arnold J. Viruses 1992, New York p. 155-175 Sompayrac, Lauren How Pathogenic Viruses Work 2002,

Sudbury, MA p. 19-24