a) Influenza Type A and its Sub-types

Wild birds are the natural hosts of the Influenza Type A virus. Hence, they are also the sources of the influenza viruses which are spreaded to other organisms especially mammals, which include humans, pigs and horses.

As mentioned before, influenza viruses are differentiated into types A, B and C based on the nucleoprotein (NP) of the virus. For Influenza Type A viruses only, they are also further divided into HA subtypes and NA subtypes based on the two outer glycoproteins of the virus, the hemagglutinin (HA) and neuraminidase (NA). In total, there are 15 HA subtypes and 9 NA subtypes currently discovered. Hence, there are many possible HA and NA combinations of viruses. On top on this, subtypes are also differentiated into strains.

Human Influenza Viruses

All three Influenza Types, Type A, Type B and Type C, can infect humans. Some common subtypes of Influenza Type A virus transmitted among humans are H1NI, H1N2 and H3N2 .

Avian Influenza A Viruses

As mentioned before, wild birds are the natural hosts of the Influenza Type A virus. Most of the Influenza Type A virus only causes asymptomatic or mild infection in birds. The extent of how serious the infection is to birds will depend of the strain of the virus.

Nevertheless, some particular Influenza Type A strains like the subtypes H5 and H7, are deadly to certain vulnerable species of birds like chickens and turkeys. These subtypes may cause a widespread rapidly over different regions in the world and they are fatal to vulnerable birds.

Low Pathogenic Avian Influenza (LPAI) and Highly Pathogenic Avian Influenza A Viruses (HPAI)

Other than classifying Influenza Type A virus based on the subtypes, HA and NA, Influenza Type A virus are also differentiated to Low Pathogenic Avian Influenza (LPAI) and Highly Pathogenic Avian Influenza (HPAI) based on criteria like the genetic combinations of the molecules and the pathogenesis of the virus after specific testing.

Low Pathogenic Avian Influenza (LPAI)

For most of the time, Low Pathogenic Avian Influenza (LPAI) viruses are related to mild disease in poultry. However, they may also evolve into High Pathogenic Avian Influenza (HPAI) viruses. This has been observed in several avian influenza outbreaks in the past. Some common strains of the Low Pathogenic Avian Influenza (LPAI) include H7N7, N9N2 and H7N2 .

Direct human contact with living or dead birds infected with Low Pathogenic Avian Influenza (LPAI) is usually the cause of the transmission of the virus to humans. The symptoms of Low Pathogenic Avian Influenza (LPAI) in humans are very mild, like conjunctivitis, and are akin to Human Influenza viruses symptoms.

Highly Pathogenic Avian Influenza (HPAI)

Highly Pathogenic Avian Influenza (HPAI) viruses tend to cause severe diseases and are usually fatal in poultry. Some common strains of the Highly Pathogenic Avian Influenza (HPAI) include the subtypes H5 and H7 like H5N1, H7N3 and H7N7 . However, recent research found out that some strains of Highly Pathogenic Avian Influenza (HPAI) do not cause any illness to some poultry, like ducks at all.

The symptoms of Highly Pathogenic Avian Influenza (HPAI) in humans depend on the strain of the virus. H7N3 and H7N7 can cause mild symptoms while H7N7 and H5N1 can be a severe and fatal disease.

Common Influenza Viruses Between Birds And Humans

Birds can only be affected by Influenza Type A viruses and birds are susceptible to all subtypes of the viruses. There are several distinct differences in the genetic combinations of Influenza Type A viruses which affect birds only and those which affect both birds and humans.

Three main subtypes of Influenza Type A viruses which affect both birds and humans include:

Influenza A H5

There are nine different strains of the H5 subtype discovered. The Highly Pathogenic Avian Influenza (HPAI), H5N1, currently with the most attention brought to it, is known to have spreaded rapidly in Asia and Europe in humans and it may be very fatal and deadly to humans in some circumstances.

Influenza A H7

There are nine different stains of the H7 subtype discovered. Unlike H5N1, the subtype H7, does not usually infect humans after direct contact with infected birds. However, it is still possible of infection in humans. The symptoms of H7 subtypes are ranged form mild to serious and they include conjunctivitis and upper respiratory problems. H7 subtype can be associated with Low Pathogenic Avian Influenza (LPAI) and Highly Pathogenic Avian Influenza (HPAI). Examples of each type include H7N2 and H7N7 for Low Pathogenic Avian Influenza (LPAI) and H7N3 and H7N7 for Highly Pathogenic Avian Influenza (HPAI).

Influenza A H9

There are nine different strains of the H9 subtype discovered. H9 subtype is even more uncommon to infect humans and on top of this, all records show that the H9 subtype is classified as a Low Pathogenic Avian Influenza (LPAI).

Other Animals Influenza A Viruses

Other than birds and humans, Influenza Type A viruses also infect other animal species. Some examples include H3N8 and H7N7 which infect horses and H3N8 which infect dogs. Other animals which are susceptible to Influenza Type A viruses include pigs, seals and whales.

b) Influenza Type B

Influenza Type B viruses are normally circulated among humans only. Unlike Influenza Type A viruses, Influenza Type B viruses are not differentiated into subtypes but they are also differentiated into strains. In general, Influenza Type B viruses do not causes epidemics as frequently and severely as Influenza Type A viruses. As such, Influenza Type B viruses have not caused pandemics yet. Nevertheless, Influenza Type B viruses can also cause morbidity and mortality in humans.

c) Influenza Type C

Like Influenza Type B viruses, Influenza Type C viruses are not differentiated into subtypes but they are also differentiated into strains. Influenza Type C viruses do not cause epidemics and pandemics. It is also noted that the flu shot does not protect humans against Influenza Type C viruses.