There may be some confusion, so it is worth clarifying from the beginning: avian flu – which has been talked about so much in recent months – and influenza A – which is currently punishing the Spanish population – are not the same, although both diseases are caused by influenzavirus.
The large family of influenza viruses
There are four types of influenza viruses: A, B, C and D. Some influenza A and B pathogens can cause seasonal influenza Pandemics in people, although all known pandemics have been caused by influenza A type influenza viruses, which They are the only ones responsible for bird flu, in addition to causing common flu in humans and swine and horse flu.
Influenza type A viruses can be divided into different subtypes depending on the genes that make up the surface proteins: hemagglutinin (H) and neuraminidase (N). Both are the antigens recognized by the host’s immune system.
There are 18 hemagglutinin subtypes and 11 neuraminidase subtypes, classified from H1 to H18 and N1 to N11, respectively. This implies that there may be dozens of combinations of the influenza A subtype. Within each subtype there is considerable variability, which affects the strain’s ability to cause illness.
All known subtypes of influenza A viruses can cause infections in birds, except for subtypes A(H17N10) and A(H18N11), which have only been detected in bats. Mutations and recombinations can occur among influenza type A viruses and, occasionally, new strains that are very lethal to birds can appear. These are responsible for highly pathogenic avian influenza (HPAI), popularly known as bird flu.
Bird flu: rare but deadly
People can be infected by avian influenza type A viruses, although it is relatively rare. There are five known subtypes (H5, H6, H7, H9 and H10) of this pathogen capable of causing infections in humans, although the most frequently identified are the H5N1 and H7N9 subtypes.
Globally, between January 1, 2003 and December 1, 2023, 882 cases of human infection with the H5N1 avian influenza virus were reported in 23 countries. Of these, 461 were fatal, which represents a fatality rate of 52%. From the beginning of 2013 to date, a total of 1,568 confirmed human infections of the H7N9 virus have been reported to WHO, including 616 fatal cases (39% mortality rate). People often contract the disease by handling or being in close contact with sick or dead birds.
Those responsible for the human flu
In humans, two subtypes of the influenza A virus are responsible for causing seasonal flu outbreaks: A(H1N1) and A(H3N2). Influenza B pathogens, classified into lineages and strains, can also cause influenza outbreaks in humans. Those currently circulating among people belong to the B/Yamagata lineage and the B/Victoria lineage.
Every February, the WHO publishes the composition of the influenza vaccine in the northern hemisphere. The quadrivalent vaccines recommended for the 2023-2024 season have different components:
Those produced from embryonated eggs (inactivated or attenuated) contain a strain analogous to A/Victoria/4897/2022 (H1N1) pdm09, a strain analogous to A/Darwin/9/2021 (H3N2), a strain analogous to B/ Austria/1359417/2021 (B/Victoria lineage) and a strain analogous to B/Phuket/3073/2013 (B/Yamagata lineage).
Those produced from cell cultures contain a strain analogous to A/Victoria/67/2022 (H1N1) pdm09, a strain analogous to A/Darwin/6/2021 (H3N2), a strain analogous to B/Austria/1359417/2021 (lineage B/Victoria) and a strain analogous to B/Phuket/3073/2013 (lineage B/Yamagata).
That 2009 pandemic
In the 2009-2010 winter season, an H1N1 flu pandemic occurred that affected more than 70 countries, although the majority of deaths occurred in Mexico. The pathogen was a combination of swine, avian and human influenza viruses and was named influenza A(H1N1)pdm09 to distinguish it from seasonal H1N1 strains and the 1918 H1N1 pandemic strain.
Since 2009, influenza A (H1N1) pdm09 has circulated in the form of seasonal flu, and being a relatively new virus, to which we are not very accustomed, it still has a significant impact on the general population. The flu viruses that make us sick are spread primarily through droplets produced when infected people cough, sneeze, or talk. Less commonly, someone can get the flu by touching a surface or object contaminated with the virus and then touching their mouth, nose, or even their eyes. Despite the differences, the symptoms caused by influenza A are similar to those of the common seasonal flu.
Currently, rapid influenza diagnostic tests, sold in pharmacies, allow the identification in human samples of the presence or absence of influenza A and B viral antigens that cause influenza in people. However, at the moment there is no commercial rapid diagnostic test that can detect the presence of avian flu.