What makes influenza so changable?

An influenza virion (single particle of virus)
An influenza virion (single particle of virus)

With all the publicity over the new swine flu strain, people who teach kids biologymay be getting questions from their students that are hard to answer. I have found the information in children’s literature to be limited. It doesn’t do a good job of explaining the influenza virus and its ability to change so rapidly.

First, let’s look at what is in an influenza virion – a single particle of the virus. In the center of the virion, there is a coil of RNA complexed with protein. This coil is wrapped in a viral envelope, which is a membrane the virus modifies and takes from its former host cell. There are viral proteins on the outside of the membrane. They look like little knobs sticking out from the virion surface in the photomicrograph.

This illustration, which is a public domain image from the Centers for Disease Control, was made by spreading the virus on a transparent film and adding a stain that blocks electrons. The stain filters down into the low spots and makes them look dark in the photomicrograph, which was made with an electron microscope. Virions of almost all viruses are too small to see with a light microscope. Influenza virions are about 1/20 to 1/10 of a micrometer in diameter. Their host cells have diameters that are thousands of times larger.

The RNA carries the instructions for how to take over a host cell and turn it into a virus factory. Viruses can have RNA instead of the DNA that cells use to hold information. That’s not what makes influenza unusual. Most viruses have their RNA or DNA (they have one or the other, not both, like cells have) in a linear strand or a closed circle. Influenza has its RNA in eight separate pieces.

When influenza replicates, its RNA-protein complex enters a host cell and uses the cell’s resources and machinery to make many copies of the molecules that make up its virion. As the cell fills with viral RNA and proteins, the eight pieces of RNA that make a complete set of viral genes form a complex with protein. Then this nucleic acid-protein complex moves to the cell membrane, to areas where viral proteins have been inserted into the membrane. It buds out of the cell and gets its envelope in the process.

Now if only one virus has infected the cell, then the virions will have the RNA from that virus alone. It is possible, however, for two different virions to infect the same cell. When that happens, the resulting virions can hold an assortment of RNA segments from either virus. The RNA segments are packaged randomly, so they can have many new combinations of genes. Instantly new strains of influenza arise that may have different proteins on the outside. If a person has not been vaccinated against those proteins or has not been infected by a strain of influenza that has those proteins, then the new virion may be able to spread very well in its host. That’s good for the virus, but bad for the person.

Another complication with influenza is that there are strains that infect birds and mammals such as pigs, as well as people. This new influenza virus was called swine flu because some of its genes appear to have jumped from pig influenza virus to a strain that infects people. This may have happened if a sick person and sick pigs were in close company. Note that pork meat cannot carry influenza virus, and eating pork certainly can’t infect people with influenza.

Now back to the little knobs on the outside of the influenza virus – they are called hemagglutinin and neuraminidase. When a person has antibodies to the neuraminidase, they keep the virion from entering a host cell. This means that the antibodies protect the person from the disease. Strains of influenza are named for the type of their hemagglutinin and neuraminidase. The new swine flu strain is called H1N1, because its hemagglutinin and neuraminidase both type 1. Human influenza viruses usually have H1, H2, or H3, and N1 or N2. There are many other types of these outer proteins in the virions that infect other species (and occasionally jump to humans). The H1N1 designation doesn’t tell everything about the virus. Within this type, there are mild and severe virus strains. After all, there are several more genes in the virions.

The big question is “What can we do to protect ourselves from the flu?” One thing that is seldom mentioned is to get more sun exposure or take a vitamin D supplement. There is increasing evidence that high vitamin D levels are important for good immune function and that influenza infections increase as people’s vitamin D levels drop in winter. With summer on its way, it will be easier to get some sun – just don’t get sunburned. Enjoying the outdoors and nature while getting sun exposure ought to be a fun, easy, healthy thing to do for yourself.


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