Influenza A is a common respiratory virus. It is responsible for seasonal flu epidemics each year, leading to widespread illness. The virus’s ability to constantly change and adapt makes it a persistent health challenge.
Categorizing Influenza A
Influenza A belongs to the Orthomyxoviridae family of viruses. The virus is enveloped, with its genetic material enclosed within a lipid bilayer membrane. The segmented nature of its genome allows for genetic exchange between different viral strains.
Influenza A viruses exhibit a broad host range, capable of infecting a wide variety of animal species. These include humans, various mammals like pigs, horses, and dogs, and numerous bird species, particularly wild aquatic birds. This broad host range is a key factor in the virus’s evolution and its potential to jump between species, contributing to new strains with pandemic potential. Wild aquatic birds are natural reservoirs.
Anatomy of the Influenza A Virus
The Influenza A virus particle has two major surface glycoproteins: hemagglutinin (HA) and neuraminidase (NA). These proteins are embedded within the viral envelope. Hemagglutinin facilitates the virus’s attachment to host cells by binding to sialic acid receptors on the cell surface, initiating the infection process. It also plays a role in the fusion of the viral envelope with the host cell’s membrane, allowing the viral genetic material to enter the cell.
Neuraminidase, the other prominent surface protein, has a distinct function. It cleaves sialic acid molecules, which frees newly formed virus particles from the infected host cell and prevents them from clumping together. This action allows the progeny viruses to spread and infect new cells. The virus’s RNA genome consists of eight separate gene segments. This segmentation enables genetic reassortment, where gene segments are swapped between different co-infecting strains.
How Influenza A Changes
Influenza A viruses continually evolve through two primary mechanisms: antigenic drift and antigenic shift. Antigenic drift involves minor, gradual changes in the HA and NA genes. These small alterations can accumulate over time, leading to changes in the surface proteins that the human immune system recognizes. This process can reduce the effectiveness of existing immunity or vaccines, which is why seasonal flu vaccines need to be updated annually. Antigenic drift is a continuous process that contributes to seasonal flu epidemics.
Antigenic shift represents an abrupt and major change in the Influenza A virus. This occurs when gene segments are exchanged between different Influenza A strains. For instance, if a human influenza virus and an avian or swine influenza virus co-infect the same host cell, their genetic segments can mix, creating a novel virus with a new combination of HA and/or NA proteins. Since people typically have little or no pre-existing immunity to these new combinations, antigenic shift can trigger widespread epidemics or pandemics. The 2009 H1N1 pandemic, for example, resulted from such a reassortment event involving human, avian, and swine influenza viruses.
Influenza A Compared to Other Types
Influenza A is one of four main types of influenza viruses, alongside B, C, and D. Influenza A viruses are unique in their ability to infect a broad spectrum of hosts, including humans, birds, and other mammals. Influenza B viruses primarily infect humans, while Influenza C viruses infect humans and pigs, and Influenza D viruses are mainly found in cattle.
Regarding disease severity, Influenza A is often associated with more severe illness and is the primary cause of flu pandemics. Influenza B can also cause significant seasonal epidemics and severe illness, but it is less likely to lead to pandemics because it does not undergo antigenic shift. Influenza C infections typically result in milder respiratory symptoms and are not known to cause widespread epidemics. Influenza D viruses are not known to infect humans or cause illness in people. The dynamic nature and broader host adaptability of Influenza A underscore why it is often singled out in public health discussions about influenza.