Understanding The Types of Influenza Viruses

Influenza is caused by four types of Ribonucleic Acid (RNA) viruses known as Influenza types A, B, C and D, which all belong to the family Orthomyxoviridae. The viruses A and B, which are spread through aerosols from sick people or through intimate contact with infected animals, are typically the ones that cause the illness, which is commonly known as the ‘flu’ in humans. These viruses usually affect the respiratory tract causing fever and other respiratory symptoms like cold, cough, sore throat, etc. Influenza A and B viral subtypes can be further classified into strains. Flu shots are a safeguard against the disease. There are, however, possibilities for re-infection following a shot or prior viral exposure.

There are numerous different influenza B virus strains and influenza A subtypes, and new influenza virus strains can emerge and displace older strains. Such newly emerging viruses contain Antigenic variations known as Antigenic shift and drift. Both the Influenza virus and the accompanying host defense affect the disease’s clinical outcome. If you find yourself in search of expertise and guidance regarding influenza viruses, there is no better person to contact than Dr. Sheetu Singh. She stands out as a top expert due to her broad knowledge and experience in the field of virology, notably in the area of influenza viruses.

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Influenza A viruses

Influenza type A viruses are known to infect people, pigs, birds, whales, horses, seals and other animals, but wild birds represent the natural hosts for these viruses. Influenza in its envelope a virus has eight viral RNA segments, three proteins (PA, PB1 and PB2), three membrane proteins (HA, NA and M2), a matrix protein (M1) just below the lipid bilayer, and the NEP protein.

The two membrane proteins on the surface of influenza type A viruses can be used to further categorize influenza viruses into subtypes. These proteins are called Neuraminidase (NA) and Hemagglutinin (HA). There are 18 distinct HA subtypes and 11 distinct NA subtypes (correspondingly, HA1 through HA18 and NA1 through NA11).

Only a portion of influenza A subtypes (i.e. H1N1, H1N2, and H3N2) are currently in widespread human circulation, even though many other combinations of HA and NA proteins are feasible. Other subtypes are found in other animal species. For instance, H7N7 makes horses sick, and H3N8 has been observed to make dogs and seals sick.

Subtypes of Influenza A virus are named by their NA and HA surface proteins. For example, an H3N8 virus designates an Influenza A subtype that has an NA 8 protein and an HA3 protein. Likewise, an H5N1 virus has an NA 1 protein and HA 5 protein.

Influenza B viruses

The same range of illnesses as those caused by influenza A viruses are caused by influenza B viruses, however, pandemics are not caused by influenza B viruses. The virus’s restricted host range (just humans and seals), which prevents the emergence of new strains through reassortment, may be the cause of this feature. In addition, they are not divided into subtypes, although can be broken down into strains and lineages. Currently circulating Influenza B viruses belong to one of the two lineages: B/Yamagata and B/Victoria. 

The four envelope proteins found in influenza B viruses are HA, NA, NB and BM2. A proton channel called BM2 is crucial for the uncoating procedure. It is believed that the NB protein is an ion channel that is not necessary for viral replication in cell culture. This virus is responsible for significant Morbidity. For example, around one-third of all laboratory-verified Influenza cases in the US in 2008 were brought on by Influenza B. Due to this, an Influenza B virus element is a necessary integral component of the seasonal trivalent influenza vaccine.

Influenza C viruses

Influenza C viruses are different in comparison to Influenza A and B. The surface of the encased virions is hexagonal and as they emerge from the cell, they develop structures that resemble stretched cords that are about 500 microns long. The core of influenza C viruses is made up of a ribonucleoprotein, which is made up of viral RNA and four proteins, similar to the influenza A and B viruses.

In a manner comparable to influenza A and B virions, the M1 protein is located immediately below the membrane. CM2, a small viral envelope protein, performs the role of an ion channel. The virus does not contain a Glycoprotein called Hemagglutinin-Esterase-Fusion). Therefore the influenza virion contains 7 RNA segments and not 8. Compared to other prevalent respiratory viruses, influenza type C infections only induce a moderate respiratory disease and are not regarded to be the primary source of epidemics.

Influenza D viruses 

They do not infect or make people sick; instead, they mostly affect animals. Influenza D viruses primarily affect cattle and are not known to cause significant illness in humans. Influenza D viruses are unique in their genetic makeup and they are divided into two subtypes D/Swine and D/Bovine. These viruses’ ability to infect and spread among livestock, especially cattle, has caused alarm in the agricultural sector.