What is the difference between bird flu and swine flu?

In April 2009, residents of Mexico City began wearing face masks and even respiratory filters to keep from contracting a swine flu.

In April 2009, residents of Mexico City began wearing face masks and even respiratory filters to keep from contracting a swine flu. Sarihuella photo. CC-BY.

You may remember that a few years ago there was much attention given to the bird or avian flu — it was seen a potential source of a worldwide pandemic. But now the pandemic threat isn’t a bird flu, but a swine flu. So the question naturally arises: What is the difference between them?

The quick answer is that there is no basic difference. All influenza viruses come from the same family of viruses (Orthomyxoviridae), and furthermore both swine flu and bird flu are caused by the same species of virus, known as influenzavirus A or the influenza A virus.

However, just as there are genetic differences among humans, who are all of one species, so are there genetic differences among the influenza A virus. And just as genetic differences may make some people more susceptible to certain illnesses or create certain physical differences, so can the genetic differences in the virus create various characteristics. The only real differences between swine flu and bird flu is that the swine flu subtypes tend to be more suitable to infecting pigs while the avian flus tend to be more suitable to infecting birds.

Swine flus are actually quite common. In the United States, from 30 to 50 percent of pigs have been infected with a swine flu. They most often get the infection from other pigs, although they can also get it from birds and even from people. Avian flus aren’t as widespread among birds, although they do tend to infect poultry more than other types of birds. The differentiation among infected species isn’t strict — it is possible for viruses to spread from pigs to birds and vice versa.

The differences among influenza A viruses are classified primarily according to two proteins, hemagglutinin and neuraminidase, that are part of the virus. These proteins types have been numbered, so that, for example, if we talk about a virus with a type 2 hemagglutinin and a type 3 neuraminidase, we call it an H2N3 virus.

The avian flu virus subtype that has caused the most concern has been the H5N1 subtype, partly because among humans there is little immunity to it. Other subtypes, such as the H2N3 found in some ducks, also infect birds. The “classic” swine virus is of the H1N1 subtype, although three other subtypes — H1N2, H3N2 and H3N1 — have also been found in pigs. The flus most common among humans are of the H1N1, H1N2 and H3N2 subtypes.

The good news has long been that while it is possible for humans to catch swine and avian viruses from pigs and birds, respectively, simple sanitation measures are often effective in keeping that from happening. Also, these viruses normally don’t spread easily from human to human, so their impact is limited. The bad news is that viruses can mutate and otherwise change in form, so that there could develop a strain of the H1N1 swine virus or H5N1 avian virus that would spread easily from human to human. The worldwide Spanish flu pandemic of 1918-20 that killed millions was of the H1N1 subtype common in pigs. The most recent flu pandemic, the Hong Kong flu pandemic of 1968-9, which killed probably just under 1 million people, was of subtype H3N2, which could have had its start in either a bird or pig population.

And once a strain develops that spreads easily among people, it becomes a human flu, and it becomes advisable to take precautions to help prevent its spread.

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