Posts Tagged ‘Flu virus typing’

Viral Diseases: Influenza, Part 2

Thursday, March 8th, 2012

Homo Habilis, the first member of the genus Homo

I realized, as I wrote my last post, that I was using medical jargon that might make no sense to most readers. So I want to examine how the influenza virus is described by doctors, specialists in epidemics (AKA epidemiologists) and other scientifically-trained groups.

First of all let's briefly talk about how we classify everything that is alive. There's a complex system called taxonomy which is conventionally used to group separate different  groups of dissimilar and similar organisms. It has seven major layers, or taxa. Humans, for example,  belong to the kingdom Animalia, the phylum Chordata, the class Mammalia, the order Primata, the family Hominidae, the genus Homo and the species Homo sapiens. 

Flu viruses fall into three genuses, and those logically enough are called A, B and C.  The A type has only one species, lives in nature in wild aquatic birds (but can infect other animals), and causes the most severe diseases in humans. Subtypes of flu A can be identified by a variety of laboratory tests that determine which kind of two glycoproteins (complex chemicals that contain both carbohydrate and protein constituents) are found on the surface of the virus.

One of those is called hemagglutinin (H for short) and the other neauraminidase (N). There are 16 H types and 9 Ns; Hs bind the virus to a cell and help it insert its genetic information into that cell. Ns get involved later in the infection and help the virus release its "offspring" from the cells they were produced in.

Laboratory tests can show which H and N are present.  Both are antigens, substances that can cause an immune reaction if taken into your body by one route or another (e.g., breathing them in) and cause your body to produce antibodies, chemicals that are produced to combine with and counter the effects of the antigen. Some important influenza viruses are H1N1 which caused the 1918 Spanish Flu and the 2009 Swine flu, H2N2 (Asian flu of 1957), H3N2 (Hong Kong Flu 1968) and H5N1 (Bird Flu in 2004).

The CDC's short article on types of influenza viruses mentions there are seasonal epidemics nearly every winter in the United States; those are caused by type A or B, not by type C. All of the terrible flu pandemics have been caused by type A flu viruses. The B virus types are normally found only in humans (seals and ferrets are the only other animals that can be infected by flu B).

We get ours every year

Why is type A the killer? It mutates much more rapidly than B, usually by minor changes in the H and N  surface proteins, occasionally by sudden major changes. The first kind of change may alter the antigens you can be exposed to so the antibodies you've developed to fight off a flu infection don't work. That's also why the vaccine you get, which contains two A subtypes and one B strain, may not fully protect you. That's not a reason to skip your flu shot.

The other kind of mutation is more serious and I'll write about it next.