There may be some confusion, so it’s worth clarifying from the start: bird flu – which has been talked about a lot in recent months – and influenza A – which is currently punishing the Spanish population – although both diseases are different , but they are not the same. Caused by influenza virus.
Influenza virus family
There are four types of influenza viruses: A, B, C and D. Several influenza A and B pathogens can cause seasonal influenza epidemics in humans, although all known pandemics are caused by influenza A viruses, which are the only influenza viruses besides causing common influenza in humans as well as swine. In addition to influenza and equine influenza, it also causes avian influenza.
Influenza A viruses are divided into different subtypes based on the genes that make up their surface proteins: hemagglutinin (H) and neuraminidase (N). Both are antigens recognized by the host immune system.
There are 18 hemagglutinin subtypes and 11 neuraminidase subtypes, divided into H1 to H18 and N1 to N11 respectively. This means there are potentially dozens of combinations of influenza A subtypes, with considerable variability within each subtype, which affects the strain’s ability to cause disease.
All known influenza A virus subtypes can cause infection in birds, except for subtypes A(H17N10) and A(H18N11), which have been detected only in bats. Influenza A viruses can mutate and recombine with each other, sometimes resulting in new strains that are very lethal to birds. They cause highly pathogenic avian influenza (HPAI), commonly known as bird flu.
Bird flu: rare but deadly
People can become infected with the avian influenza A virus, although this is relatively rare. There are five known subtypes of this pathogen (H5, H6, H7, H9, and H10) capable of causing infection in humans, but the most common are H5N1 and H7N9 subtypes.
Globally, between January 1, 2003 and December 1, 2023, 23 countries reported 882 cases of human infection with the H5N1 avian influenza virus. Among them, 461 people died, a mortality rate of 52%. Since the beginning of 2013, WHO has received a total of 1,568 confirmed cases of human infection with the H7N9 virus, including 616 deaths (mortality rate 39%). People often contract the disease through handling or close contact with sick or dead birds.
Those responsible for human influenza
In humans, two subtypes of influenza A viruses cause seasonal influenza outbreaks: A(H1N1) and A(H3N2). Influenza B pathogens, classified by lineage and strain, can also cause human influenza outbreaks. The ones currently circulating among the people belong to the B/Yamagata lineage and the B/Victoria lineage.
Every February, the World Health Organization releases the ingredients for the northern hemisphere flu vaccine. The quadrivalent vaccines recommended for the 2023-2024 season have different components:
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Viruses produced from embryonated eggs (inactivated or attenuated) contain strains similar to A/Victoria/4897/2022 (H1N1) pdm09, strains similar to A/Darwin/9/2021 (H3N2), strains similar to B/ strains of Austria/. 1359417/2021 (B/Victoria lineage) and strains similar to B/Phuket/3073/2013 (B/Yamagata lineage).
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Strains produced from cell culture contain strains similar to A/Victoria/67/2022 (H1N1) pdm09, strains similar to A/Darwin/6/2021 (H3N2), strains similar to B/Austria/1359417/2021 (lineage B/Victoria) and a strain similar to B/Phuket/3073/2013 (lineage B/Yamagata).
The 2009 pandemic
In the winter of 2009-2010, the H1N1 influenza pandemic affected more than 70 countries, but most of the deaths occurred in Mexico. The pathogen is a combination of swine, avian and human influenza viruses and has been named influenza A (H1N1) pdm09 to distinguish it from seasonal H1N1 strains and the 1918 H1N1 pandemic strain.
Influenza A (H1N1) pdm09 has been circulating as a seasonal flu since 2009. As a relatively new virus, we are not very accustomed to it, but it still has a significant impact on the general population. The flu viruses that make us sick spread mainly through droplets produced when an infected person coughs, sneezes or talks. Less commonly, someone can catch the flu by touching a surface or object that is contaminated with the virus and then touching their mouth, nose, or even eyes. Although there are differences, influenza A causes symptoms similar to those of the common seasonal flu.
Currently, rapid influenza diagnostic tests sold at pharmacies can identify the presence in human samples of the influenza A and B virus antigens that cause influenza in humans. However, there are currently no commercial rapid diagnostic tests that can detect the presence of avian influenza.