How Influenza Viruses are Identified

The influenza (or “flu,” as it is familiarly known) outbreak of 2009, which struck hardest in Mexico, reached alert level 5, which meant it was one level short of being declared a pandemic.  In the first days of the epidemic, the mass media largely used the term “swine flu” in reference to the type of influenza virus involved.  Later however, the World Health Organization (WHO) made known publicly that they would instead refer to the influenza virus as “Influenza A (H1N1) virus, human.”  The WHO said this is necessary to stay away from the misconception that consuming pork products carries a risk of infection.

Major nationwide influenza epidemics take place in cyclic way.  For example, the A strains of the influenza virus commonly follow a two- to three-year cycle;  B strains, on the other hand, follow a three- to six-year cycle.

There are three ways by which influenza may be transmitted:  by direct contact with infected persons; by articles freshly soiled with nasal discharges of an infected person; or by droplet infection.  Its laboratory diagnosis may be made either directly or indirectly.  In the former, diagnosis of influenza is made by the demonstration of the virus in mouth washings, which are injected into any one of the following:  an embryonated egg, tissue culture, or the nose of a ferret.  Indirect laboratory diagnosis of influenza is made by demonstrating complement-fixing, hemagglutination-inhibiting, or neutralizing antibodies in the blood serum of the infected person.

The etiological agent of influenza is a virus, of which there are three known antigenic types based on their ribonucleoprotein antigen.  These types are identified as influenza A, B, and C.  Earlier, it was thought that the bacterium Haemophilus influenzae is the cause of influenza.  Later studies revealed, though, that this bacterium acts only as an adjuvant or secondary agent in some cases of the influenza virus infection.

Until the year 1972, the classification of influenza viruses was based upon designation of type according to the ribonucleoprotein antigen, or the soluble antigen – “viral antigen that remains in solution after the particles of virus have been removed by means of centrifugation; in the case of the influenza viruses, it is the internal helical structure, free of the external envelope” (the mediLexicon online Medical Dictionary).

Hence, all strains of type A share common ribonucleoprotein antigens, while strains of types B and C have distinctively different ones.  Furthermore, the subtype was based on the hemagglutinin antigen (for example, A2).  But another antigen – neuraminidase – undergoes antigenic variation independent of the hemagglutinin variations.

It is important to explain here that hemagglutinin combines specifically with glycoprotein receptors present on the surface of erythrocytes, or red blood cells, of various mammal and avian species, fixing them to each other in agglutinated masses.  At its critical temperature, the enzyme neuraminidase can split this virus-cell connection and the virus is released unharmed.  On the other hand, the glycoprotein receptors are destroyed, and the red blood cells are no longer agglutinable by the virus.

In addition, both the neuraminidase and hemagglutinin antigens on the surface of influenza viruses of human origin may be closely related to certain strains of nonhuman hosts.  Therefore, for the purpose of having adequate and uniform description of influenza viruses, it has become necessary to design a system of notation to compare the nature of the virus strains as they undergo mutation year by year.  The nomenclature includes the strain designation (type A, B, or C), as well as a description of the hemagglutinin and neuraminidase antigens.  Hence, the details included in the notation are as follows:

1. Type A, B, or C

2. Host origin:  This is not indicated if the isolation was made from humans, but other hosts are indicated (for example:  swine, chicken, or horse)

3. Geographical origin

4. Strain number

5. Year of isolation

6. Character of the hemagglutinin subtype (H), for example:  human (H0, H1, H2, etc.); swine (Hsw 1, etc.); avian (Hav 1, etc.); or equine (Heq 1, Heq 2, etc.)

7. Character of the neuraminidase subtype (N), for example:  human (N1, N2); or equine (Neq 1, Neq 2)

With so many details yet to be confirmed, including the actual location of the outbreak and the exact type or types of strain/s, we may in the meantime use the designation A/Mexico City/?/09 (H1N1) for that 2009 influenza epidemic.


1. “Types of Influenza Viruses”, on the Centers for Disease Control and Prevention (online) –

2. Virology – Chapter Thirteen, “Influenza Virus (Orthomyxovirus)” by Dr. Margaret Hunt, on the University of South Carolina School of Medicine – Microbiology and Immunology On-Line –

3. “Influenza”, on the Wikipedia –

4. “2009 Swine Flu Outbreak”, on the Wikipedia –