Source: Lippincott’s Illustrated Microbiology

What is Influenza Virus?

Influenza virus is a type of Negative single stranded RNA virus from family  Orthomyxoviruses.

Classification of viruses:

What is Orthomyxoviruses?

Orthomyxoviruses are spherical, enveloped viruses containing a segmented, negative-strand RNA genome.

Viruses in this family infect humans, horses, and pigs, as well as nondomestic water fowl, and are the cause of influenza.

Orthomyxoviruses are divided into 3 types: influenza A, B, and C.

Only influenza virus types A and B are of medical importance. Type A influenza viruses differ from type B viruses in that they have an animal reservoir and are divided into subtypes.

Influenza virions are spherical, enveloped, pleomorphic particles.

Two types of spikes project from the surface: one is composed of hemagglutinin (H protein) and the second of neuraminidase (N protein).

• Influenza viruses are classified as types A, B, and C, depending on their inner proteins, mainly the M and NP proteins.
• Thus, all type A viruses share common internal antigens that are distinct from those shared by all type B viruses.
• Only the type A viruses are broken down into subtypes.
• The classification into subtypes depends on antigens associated with the outer viral proteins, H and N.
• Taking into consideration animal as well as human influenza viruses, 14 H and 9 N subtypes have been described.
• However, among human influenza viruses, only three H (H1, H2, and H3) and two N (N1 and N2) subtypes are found.
• Human influenza viruses are therefore designated, for example, Swine flu is H1N1 influenza virus while bird’s flu is H5N1 etc..

Both the H and N influenza proteins are integral membrane proteins.

The M (matrix) proteins underlie the viral lipid membrane.

The RNA genome, located in a helical nucleocapsid, is composed of eight distinct segments of RNA, six of which code for a single protein.

Each nucleocapsid segment contains not only the viral RNA but also four proteins (NP, the major nucleocapsid protein, and three P proteins that are present in much smaller amounts than NP and are involved in synthesis and replication of viral RNA).

How to differentiate influenza virus from other viruses?

There are two unusual features associated with synthesis and replication of influenza viral RNAs that distinguish the influenza viruses from the other RNA viruses discussed up to this point.

First, the synthesis of influenza virus mRNAs and the replication of the viral genome, occur in the nucleus. This is in contrast to the replication of other RNA viruses, which occurs completely in the cytoplasm.

Second, compounds such as actinomycin D and amanitin, which inhibit the synthesis of eukaryotic RNA polymerase II1 (Pol II) transcripts (messenger RNAs), inhibit the replication of influenza virus.

How Influenza Virus spreads?

In humans, influenza is spread by respiratory droplets and is an infection solely of the respiratory tract.

There is rarely viremia or spread to other organ systems.

Destruction of respiratory epithelial cells is attributed to the host immune response, specifically cytotoxic T cells.

Typically, influenza has an acute onset characterized by chills, followed by high fever, muscle aches (caused by circulating cytokines), and extreme drowsiness.

The disease runs its course in four to five days, after which there is a gradual recovery.

The most serious problems, such as development of pneumonia, occur in the very young, the elderly, and people with chronic cardiac or pulmonary disease or those who are immunodeficient.

How come the is no definite vaccines for influenza virus?

Influenza viruses have shown marked variation in antigenic properties, specifically H and N proteins.

Two distinct phenomena account for this observation: antigenic drift and antigenic shift.

Antigenic drift:

• Minor antigenic changes in H and N proteins that occur each year.
  
• Does not involve a change in the viral subtype.
  
• Caused by random mutations in viral RNA and single or a small number of amino acid substitutions in H and N proteins.

Antigenic shift:

• Much more dramatic change in the antigenic properties of the H and/or N proteins
• Occurs only infrequently, perhaps every ten or twenty years.
  
• For example, the appearance of a new, extremely virulent H1N1 virus, due presumably to antigenic shift, probably accounted for the pandemic of 1918 that resulted in the death of an estimated twenty million people worldwide, including more than 500,000 in the United States .
  
• In 1957, antigenic shift again occurred, and H1N1 virus was replaced by subtype H2N2
• In 1968, H2N2 was replaced by H3N2
• Since 1977, multiple subtypes of influenza A have been circulating around the world.