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Explained: Why bird flu virus has so many strains, and what it means for humans

Here's a look at the nature of the influenza A virus, which causes the bird flu as well as human flu pandemics, and why it has so many variants.

Written by Gagandeep Singh Dhillon , Edited by Explained Desk | Shimla |
Updated: January 22, 2021 2:34:24 pm
At least 131 different subtypes of influenza A virus have been detected in nature, all but two of which can infect birds. (Express photo: Nirmal Harindran)

Two different subtypes of the bird flu virus or avian influenza, have been detected in Himachal Pradesh – the H5N1 avian influenza in migratory water birds at Pong Dam Lake and the H5N8 subtype in dead poultry birds found dumped near the Chandigarh-Solan highway. Here’s a look at the nature of the influenza A virus, which causes the bird flu as well as human flu pandemics, and why it has so many variants.

How many different subtypes or strains of the flu A virus are out there?

At least 131 different subtypes of influenza A virus have been detected in nature, all but two of which can infect birds, according to the Centers for Disease Control and Prevention (CDC), United States.

The influenza A virus has two proteins on its surface – hemagglutinin (H) and neuraminidase (N) – both of which have 18 and 11 different subtypes respectively, leading to different combinations such as H3N2 and H7N9. There are some strains which only infect birds, while others can infect birds as well as mammals such as pigs, dogs, horses and also humans.

Wild aquatic birds are the natural hosts for most of these subtypes, but the infection generally does not cause sickness in these birds. Poultry birds such as chickens are more adversely affected.

How many of these strains can infect humans?

Mostly, humans have only experienced infections by three different H types (H1, H2 and H3), and two different N types (N1 and N2). Presently, two subtypes, H1N1 and H3N2, circulate among human beings, causing the seasonal flu epidemics. Since these strains are well adapted to humans, they are referred to as human flu rather than bird flu.

Whenever a new flu A virus establishes itself in humans, it can cause a pandemic, and four such pandemics have occurred since 1918, including the Spanish flu (H1N1), the 1957-58 Asian flu (H2N2), the 1968 Hong Kong flu (H3N2) and the 2009 swine flu (caused by a newer version of the H1N1).

Other flu A virus strains typically affect wild birds and poultry, but occasionally infect humans as well, such as the H5N1 strain which has killed hundreds of people in various countries since 1997. But it is not known to transmit from human to human, and is primarily is a bird flu virus. According to the CDC, the most frequently identified subtypes of bird flu which have caused human infections are those with H5, H7 and H9 proteins, but none of them has so far established a stable lineage among humans.

Currently in India, the H5N8 strain has been detected in most of the states, and it is not known to have infected humans so far. The H5N1, which can infect humans, was detected among wild birds at the Pong Lake in Himachal, but the outbreak now appears to be contained according to the wildlife authorities.

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Why does the flu A virus have so many strains?

Influenza A virus mutates constantly. This is because firstly, it is an RNA virus with a segmented genome, i.e. it has eight separate strands, which makes its copying prone to errors or mutations. This ‘antigenic drift’ results in slight but continuous mutations in the surface proteins, which is the reason why flu vaccines have to be updated regularly.

Secondly, when a cell happens to be infected with two different flu A viruses, their genes can easily get mixed up. “This mixing, known as reassortment, is a viral version of sex,” writes Carl Zimmer in ‘A Planet of Viruses’.

“And sometimes, on very rare occasions, reassortment can combine genes from avian and human viruses, creating a recipe for disaster,” he adds. According to him, the virus which caused the swine flu pandemic in 2009 was a result of reassortment of four different strains, including a strain which had been infecting pigs since 1918.

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