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College of Veterinary Medicine Experts Explain the Letters and Numbers Behind Flu Strains

Every year, it seems the CDC warns us about a new flu variety, bandying about names that seem more like grid locations in a game of Battleship than actual descriptions:  H1N1, H2N1, and now H5N1 – or technically, H5N1 2.3.2.1. This new form of so-called “bird flu” is a highly pathogenic and dangerous strain of avian flu that first showed up a year ago and appears to be driving a resurgence of the H5N1 virus.

So what’s the difference between an H1N1 and an H5N1? What do these letter/number combos mean? Drs. Mike Martin and Barrett Slenning, epidemiology and population health experts in the NC State College of Veterinary Medicine, explain how flu strains are named.

First of all, the H and the N are abbreviations for two structures found on the surface of influenza viruses. The H is hemagglutinin, which helps the virus to attach to cells, and the N is neuraminidase, which aids the virus in escaping from a host cell once it has reproduced. There are 16 identified H types, and nine N types.

Humans can be infected by influenza carrying the H1, H2, H3, H5, H7, H9 and H10 types. Wild and domestic birds can be infected by all 16 of these types, and pigs can be infected by three of the 7 H-types that also infect humans. So H5N1 is named for the type of hemagglutinin and neuraminidase proteins that the researchers who discovered the strain saw on the surface of the virus.

But wait, there’s more! If you want to get really technical about naming these things, then you have to go back a bit further. There are three major types of influenza: A, B, and C. Animals and humans can be infected by type A influenza, but B and C generally infect only humans. So when naming a flu strain, there’s a particular convention that looks like this: Type/animal or human (if blank it means it came from a human)/location of index – or first recorded – case/number of laboratory where index case was recorded/year found/strain. So an example of a full name for a Type A influenza is A/chicken/Hong Kong/202/97/H5N1, which is one of the first highly pathogenic avian H5N1 strains discovered in 1997.

And if you want to get all biblical about it, and figure out exactly which strain “begat” the current one, you can append numbers that indicate which genetic clade (or sub-family) the strain derived from – that’s where the 2.3.2.1 I mentioned earlier comes from. Slenning clarifies:

“Think of how a legal document is written: The text has different sections comprised of subsections, sub-subsections, and clauses. That is how deep the original 1997 highly pathogenic Asian-strain H5N1 has gone in its evolution. There are at least 10 major clades (the first number), a few of which may now be extinct. Clade 2 emerged around 2005, and is the clade that jumped out of Asia and went west into Europe and Africa that winter. It has developed related groupings within the clade. Some of those groupings have divided into further subgroupings. These are all determined by way of DNA characterizations and statistical models doing lots of comparisons across different strains and samples. So, 2.3.2.1 are all descendants of 2.3.2, which is a descendant of 2.3, which is a descendant of clade 2, which is a descendant of the original H5N1 that emerged as a human pathogen in Hong Kong in 1997.”

I know, right?

The takeaway here is that influenza is an ever-evolving virus, and that epidemiologists are constantly working to pin down where and how it’s changing. The good news? Scary name aside, most of the influenza experts do not seem especially upset about H5N1 2.3.2.1; it’s a fairly predictable result of normal flu evolution. That said, the flu still stinks, no matter what its name is.

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