Human Influenza Viruses
Human influenza viruses are the influenza virus types that circulate among human populations. There are three types of influenza viruses; influenza A virus, influenza B virus, and influenza C virus. Humans can be infected by all three types, although the influenza A virus always present the most serious problems. Influenza A virus may be subtyped according to their composition of two glycoproteins; hemagglutinin (HA, which may be shortened to H) and neuraminidase (NA, which many be shortened to N). Both of these proteins form structures on the surface of the influenza A virus. The HA structure allows the virus to attach to a human body cell and then to enter it. The NA structure helps the newly formed virus to escape the host cell. There are 16 known HA serotypes and 9 known NA serotypes. Thus, there are 153 possible combinations of HA and NA proteins (influenza B and C viruses do not have subtype classifications) based upon this scheme. According to the National Wildlife Health Center, "Within the possible combinations [of HA and NA proteins] are numerous genetic sequences that can determine pathogenicity of the virus to the infected host."
A) All mammals may host the influenza A virus. However, there are large differences between how many and which viral subtypes the many animal species are able to host. The ability of species to host the various influenza virus subtypes is determined by different forms of sialic acid present on the virus HA glycoprotein . Thus, certain species, such as birds, can host all 15 HA serotypes of the influenza A virus, while other species, such as humans, normally can host only three influenza A virus serotypes (H1, H2, and H3).
B) The influenza B virus circulates widely in the human population and can cause both morbidity and mortality among humans. It is usually associated with less severe epidemics than those caused by the influenza A virus, and has not been known to cause a pandemic.
C) The influenza C virus may be found in humans, pigs, and dogs; it causes mild respiratory infection. The influenza C virus is not known to cause epidemics or pandemics.
Presently, there are three types of influenza A viruses circulating in the human population: H1N1, H1N2, and H3N2.
All viruses, including the influenza virus, evolve or undergo mutations that change the character of the surface proteins on the virus. Virus evolution occurs as either antigenic drift or antigenic shift. Antigenic drift happens frequently, usually annually; antigenic shift happens only occasionally, perhaps every 20 - 40 years.
Antigenic drift refers to small, gradual changes that occur through point mutations of the virus' genetic material while the virus replicates itself. In the influenza virus, point mutations occur on the two genes that produce the HA and NA proteins. Mutations are unpredictable and spontaneous, and these changes in the virus surface proteins do not usually result in the appearance of a new virus strain. Because genetic changes occurring as a result of genetic drift tend to be minor, humans who have received earlier flu vaccines or been previously infected by by influenza viruses are likely to retain some degree of immunity against the new virus variant.
Antigenic shift is an infrequent, yet sudden, major change that produces a novel influenza virus, i.e., a virus subtype not previously circulating among the human population. Antigenic shift can occur in a number of ways, one of which is through a process called genetic reassortment. Genetic reassortment happens when different flu virus strains infect the same cell and exchange genetic material.
There are three ways antigenic shift can occur in the influenza A virus.
1) A duck or aquatic bird passes the avian influenza A virus to an intermediate host such as a chicken or a pig. Along those same lines a human passes the human influenza A virus to that same chicken or pig. Genes from both viruses combine and yield a new influenza strain with different H and N proteins. This new strain can pass from the intermediate host to humans who have little to no immunity to the new virus subtype.
2) Without undergoing genetic change, an avian influenza strain can move directly from a duck or other aquatic bird to humans.
3) Without undergoing genetic change, an avian influenza strain can be transmitted directly from a duck or other aquatic bird to an intermediate host such as a chicken or a pig and then to humans.
Usually, the creation of a new influenza virus subtype is followed by severe flu epidemics or pandemics. Given the specificity of the immune response, new influenza viruses are not recognized by antibodies created in response to earlier influenza strains. Humans and other animals who had developed previous immunities to past viruses do not have immunity to new viruses. Consequently, people can get the flu more than once and a new flu vaccine must be produced each year to combat that year’s most prevalent strains.