Influenza A :
Antibodies to H1N1:
Influenza A, H1N1
Swine Flu and You
With the recent outbreak of Influenza A/Mexico/09 (H1N1) aka Swine flu A/Mexico/09 (H1N1) initially in Mexico, in the US, and around the world, it may be useful to summarize the features of the influenza virus and its avenues for vaccination and treatment.
The influenza A virus particle is ~100nm in diameter and usually roughly spherical in shape that is coated in a lipid bilayer derived from the plasma membrane of its host. The influenza virus is composed of a viral envelope containing two main types of glycoprotein, wrapped around a central core. Studded over the surface of the virion are the two glycoproteins, hemagglutinin (H or HA) and neuraminidase (N or NA). HA is a lectin that is involved in binding of the virus to target cells and entry of the viral genome into the target cell, while NA is involved in the release of the virus from infected cells, by cleaving sugars that bind the mature viral particles.
Influenza viruses bind through Hemagglutinin (HA) onto sialic acid sugars on the surfaces of epithelial cells; typically in the nose, throat and lungs of mammals and intestines of birds. The virus is imported into the cell by endocytosis. The acidic conditions in the endosome cause the M2 ion channel to allow protons to pass through the viral envelope and acidify the core of the virus. This causes the virus core to disassemble and to release the viral RNA and core proteins. One of the anti-viral drugs on the market, amantadine, blocks the action of the M2 ion channel, thus preventing infection. Since 2006, the Center for Disease Control found rates of amantadine resistance to be much higher than in previous seasons. For some strains and in some areas the resistance has reached 100% signaling the need for additional anti-viral treatments.
After their release into the nuclease, the core proteins and vRNA and the RNA-dependent RNA polymerase begin transcribing complementary positive-sense vRNA. The vRNA is either exported into the cytoplasm and translated, or remains in the nucleus. Newly synthesized viral proteins are either secreted through the Golgi apparatus onto the cell surface (in the case of NA and HA) or transported back into the nucleus to bind vRNA and form new viral genome particles.
Negative-sense vRNAs that form the genomes of future viruses, RNA-dependent RNA polymerase, and other viral proteins are assembled into a virion. The mature virus buds off from the cell. The mature viruses detach once their neuraminidase has cleaved sialic acid residues from the host cell. Drugs that inhibit neuraminidase, such as zanamavir (Relenza) and oseltamivir (Tamiflu), prevent the release of new infectious viruses and halt viral replication. After the release of new influenza viruses, the host cell dies.
Influenza Strains A, B, C
Antigenic drift is due to mutation. Antibodies to the HA protein are the most important in protection, although those to NA also play a role. Both proteins undergo antigenic drift (i.e. accumulate mutations) and accumulate changes such that an individual immune to the original strain is not immune to the drifted one. This is one of the main reasons why people can get the flu more than one time. In most years, one or two of the three virus strains in the influenza vaccine are updated to keep up with the changes in the circulating flu viruses. People, who want to be protected from flu, need to get a flu shot every year.
A description of antigenic shift is as follows: Pigs can be infected with both human and avian influenza viruses in addition to swine influenza viruses. Infected pigs get symptoms similar to humans, such as cough, fever and runny nose. Because pigs are susceptible to avian, human and swine influenza viruses, they potentially may be infected with influenza viruses from different species (e.g., ducks and humans) at the same time. If this happens, it is possible for the genes of these viruses to mix and create a new virus. For example, if a pig were infected with a human influenza virus and an avian influenza virus at the same time, the viruses could mix (reassort) and produce a new virus that had most of the genes from the human virus, but a hemagglutinin and/or neuraminidase from the avian virus. The resulting new virus would likely be able to infect humans and spread from person to person, but it would have surface proteins (hemagglutinin and/or neuraminidase) not previously seen in influenza viruses that infect humans. This type of major change in the influenza A viruses is known as antigenic shift.
The 2009 H1N1 flu virus is a mix of antigens and consists of North American swine influenza viruses, North American avian influenza viruses, human influenza viruses and swine influenza viruses found in both Asia and Europe.
Antigenic shift results when a new influenza A subtype to which most people have little or no immune protection infects humans. If this new virus causes illness in people and can be transmitted easily from person to person, a pandemic can occur. In a normal influenza season, the greatest mortality is typically seen in the most susceptible populations; in infants and the elderly. A pandemic can be recognized because of the greater mortality of adults in mid-life (25-50 age range). Because adults in mid-life do not have as robust an immune systems that young adults have and because they have not lived long enough to have seen a similar flu strain, and develop antibodies such as the older age groups, they often become the hardest hit segment of the population.
Influenza was responsible for the most devastating plague in human history: the “Spanish flu” that swept around the world in 1918 killing 675,000 people in the U.S. and an estimated 20-50 million people worldwide. The timeline for the 1918 breakout involved an initial illness that occurred in the spring of 1918. Over the summer, the flu outbreak lessened, which is typical for influenza. The exception to this was for military personnel who lived in close quarters who continued to fall ill. In September of 1918, when flu typically makes a comeback in the northern hemisphere because of the change in weather and humidity, a second wave infected all segments of the population. This was the most serious outbreak. Again, in the late spring of 1919, a third wave took place. Pandemics can be comprised of a complex series of outbreaks over many population groups worldwide.
Influenza A Antigens
Influenza A, Caledonia (H1N1)
Influenza A, Taiwan (H1N1)
other strains available