Influenza Virus: An Overview
Each winter, millions of people suffer from the flu, which is a highly contagious infection. The
influenza virus spreads easily from person to person, mainly when an infected person coughs or sneezes. The influenza virus causes the flu (which is the short name for influenza).
(Click Flu to read more about the flu.)
Features of the Influenza Virus
The influenza virus is round, but it can also be elongated or irregularly shaped. Inside the influenza virus are eight segments of single-strand RNA (ribonucleic acid) containing the genetic instructions for making new copies of the virus. The most striking feature of the influenza virus is a layer of spikes projecting from its surface. There are two different types of spikes: one is the protein hemagglutinin (HA), which allows the virus to "stick" to a cell and initiate infection, the other is a protein called neuraminidase (NA), which enables newly formed viruses to exit the host cell.
Influenza viruses are classified as type A, B, or C based upon their protein composition. Type A viruses are found in many kinds of animals, including:
- Ducks
- Chickens
- Pigs
- Whales
- Humans.
Type B widely circulates in humans. Type C has been found in humans, pigs, and dogs, and causes mild respiratory infections, but does not spark epidemics.
Type A influenza is the most frightening of the three, and it is believed to be responsible for the global outbreaks of 1918, 1957, and 1968. Type A influenza viruses are subdivided into groups based on two surface proteins, HA and NA. Scientists have characterized 16 HA subtypes and 9 NA subtypes of the influenza virus.
Subtypes of the Influenza Virus
Type A subtypes of the
influenza virus are classified by a naming system that includes:
- The place the strain was first found
- A lab identification number
- The year of discovery
- In parentheses, the type of HA and NA it possesses.
For example, A/Hong Kong/156/97 (
H5N1). If the virus infects non-humans, the host species is included before the geographical site, as in A/Chicken/Hong Kong/G9/97 (H9N2). There are no type B or C subtypes of the influenza virus.
Where Does the Influenza Virus Come From?
In nature, the influenza virus is found in wild aquatic birds, such as ducks and shore birds. The flu has persisted in these birds for millions of years and does not typically harm them. However, the frequently mutating influenza virus can readily jump the species barrier from wild birds to domesticated ducks and then to chickens. From there, the next stop in the infectious chain is often pigs.
Pigs can be infected by both bird (avian) influenza and the form of influenza that infects humans. In a setting such as a farm, where chickens, humans, and pigs live in close proximity, pigs act as an influenza virus mixing bowl. If a pig is infected with avian and human flu simultaneously, the two types of virus may exchange genes. This influenza virus can sometimes spread from pigs to people.
Depending on the precise assortment of bird-type flu proteins that make it into the human population, the flu may be more or less severe. For the first time in 1997, scientists found that bird influenza skipped the pig step and infected humans directly. Alarmed health officials feared a worldwide epidemic (a
pandemic), but, fortunately, the virus could not pass between people and thus did not spark an epidemic.
Drifting and Shifting of the Influenza Virus
The
influenza virus is one of the most changeable of viruses. These genetic changes may be small and continuous or large and abrupt. Small, continuous changes happen in type A and type B influenza as the influenza virus makes copies of itself. The process is called antigenic drift. The drifting is frequent enough to make the new strain of influenza virus often unrecognizable to the human immune system. For this reason, a new
flu vaccine must be produced each year to combat that year's prevalent strains.
Type A influenza also undergoes infrequent and sudden changes, called antigenic shift. Antigenic shift occurs when two different flu strains infect the same cell and exchange genetic material. The novel assortment of HA or NA proteins in a shifted virus creates a new influenza A subtype. Because people have little or no immunity to such a new subtype, their appearance tends to coincide with severe flu epidemics or pandemics.
(Click Pandemic for more information about pandemics.)
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