Influenza, or flu, is a respiratory infection caused by any of several flu viruses, which are classified as type A, B, or C based on their nucleic acids and protein composition. Type A viruses are found in humans and many animals, including ducks, chickens, pigs, and whales. Type A influenza viruses caused the global flu outbreaks of 1918, 1957, 1968, and 2009. Type B viruses circulate widely in humans. Type C viruses are found in humans, pigs, and dogs and cause mild respiratory infections but do not spark epidemics.
The outer coat of the virus features two kinds of protein spikes: hemagglutinin (HA), which allows the virus to stick to a human or animal cell and initiate infection; and neuraminidase (NA), which enables newly formed viruses to exit the host cell. There are 16 HA subtypes and 9 NA subtypes among Type A viruses. Strains are named based on their combination of HA and NA proteins; for example, the virus that caused the 2009 H1N1 flu pandemic contained H1 and N1 surface proteins. Inside a flu virus are eight segments of single-stranded RNA containing the genetic instructions for making new copies of the virus.
Influenza A virus.Credit: CDC
In nature, the flu virus is found in wild aquatic birds, such as ducks and shore birds. It has persisted in these birds for millions of years and does not typically harm them. But the frequently mutating flu viruses can readily jump the species barrier from wild birds to domesticated poultry and swine.
Pigs can be infected by both bird (avian) flu and the form that infects humans. In a setting such as a farm where chickens, pigs, and humans 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. Such a "reassorted" flu virus can sometimes spread from pigs to people. Depending on the combination of avian flu proteins that make it into the human population, the flu may be more or less severe.
In 1997, for the first time, scientists found that a form of avian H5N1 flu skipped the pig step and infected humans directly. Alarmed health officials feared a worldwide epidemic (a pandemic). But fortunately, the virus could not pass from person to person and thus did not spark an epidemic. As of August 2012, there have been 608 confirmed cases of this avian flu virus, which have resulted in 359 deaths.
Influenza virus is under constant evolutionary change. These genetic changes may be small and continuous or large and abrupt.
Small genetic changes happen continuously in Type A and Type B influenza as the virus makes copies of itself. This process is called antigenic drift. Drifting happens frequently enough to make new strains of virus unrecognizable to the human immune system. For this reason, a new flu vaccine must be produced each year to combat the strains circulating that year.
Type A influenza also undergoes infrequent and sudden changes known as antigenic shift. Antigenic shift occurs when two different flu strains infect the same cell and combine portions of their genetic material. The novel assortment of HA and/or NA proteins in a shifted virus may create a new influenza A subtype. Because people have little or no immunity to such a new subtype, the appearance of a new subtype usually coincides with a very severe flu epidemic or pandemic.
Last Updated November 16, 2012