Many of you will have heard on the news about the recent spread of a new Influenza outbreak from Mexico. Some have probably been wondering “what’s the big deal?” With 59 deaths reported by the WHO as of April 23, and less than 1000 cases of swine flu, this disease is, so far, a small player. So I’ll do my best to explain why this disease is getting infectious disease experts stirring, and why it will be one to keep a watchful eye on over the next few days and weeks.
From the days of the Ancient Greeks, the Influenza virus has infected people. But the genetic identity of the Influenza virus today isn’t the same as for the virus that caused the flu 100 years ago. In fact, it’s not even exactly the same virus as it was last year. The reason is that the Influenza virus mutates rapidly, generating new strains.
From year to year, the changes in the Influenza virus are known as genetic drift. Because of genetic drift, each flu season there are a new handful of Influenza strains that predominate. Genetic drift explains why you have to get a new flu shot every year. In the Fall, the WHO makes predictions of which five strains of Influenza they think will be the most prevalent, and those are the strains that go into the vaccine.
In addition to genetic drift, every so often there is a more significant mutation in the Influenza virus. We classify Influenza strains according to two proteins that are expressed on the virus capsules: hemmaglutinin (H) and neuroaminidase (N). For example, H5N1 is the subtype name for a strain of bird flu that is being watched globally for pandemic potential. The recent swine flu in Mexico is H1N1. Genetic drift from year to year doesn’t generally change the HxNx classification of the virus, and so over time the world population builds up some immunity to strains that are prevalent.
Every once and awhile, however, there is a chance recombination of the virus that results in a new subtype becoming infectious in humans. Often these recombinations come from animal strains of the flu, because animals suffer from different subtypes than we do. Many animal flu strains can be transmitted to people, but generally you need to have contact with the infected animal, and it won’t spread from person to person. If an animal flu recombines, however, to be transmissible from human-to-human, it might potentially create a new subtype to which we have no global immunity. This sets the stage for a global pandemic. Over history there have been regular, periodic pandemics of newly recombined Influenza strains the most notable being the Spanish Flu, which killed 40% of the global population.
Perhaps now it is clearer why there is so much fuss over the emerging swine flu epidemic. Swine flu has existed in pigs for forever, and there was always pig-to-human transmission. The new event, is that this H1N1 strain of swine flu has recombined to be infectious from human-to-human. It now fits the criteria for what we expect a pandemic-causing virus to look like. Will swine flu be the cause of the next Influenza pandemic? That will depend on many factors including the transmissibility and virulence of the virus itself, and how effectively it is contained by WHO efforts. It will be a disease to keep a very watchful eye on over the next few days and weeks.