(Note: this is a spinoff of my Web page on influenza in response to the current outbreak of "swine flu". I am updating frequently with information from the US Centers for Disease Control and Prevention, the World Health Organization, and the news media. This separate page discusses only "swine flu"; for more information on influenza in general, please see my main influenza page.
Influenza viruses actually infect wild and domestic birds, swine (pigs), and other animals, which makes them almost impossible to eradicate -- especially since most birds, including wild birds, can carry the viruses without getting sick. Some of the antigens in swine, avian (bird), and human influenza can trade places with each other and produce new and different strains of viruses. Swine flu, like human flu, is most common during the fall and winter. However, the flu viruses are always present in some swine (as they are in some people) in the off-season.
According to the US Centers for
Disease Control and Prevention, as of August 22, 2009 36,872 cases
of humans infected with an H1N1 influenza A virus have been confirmed
by lab testing in the US, with cases confirmed in all 50 states, the
District of Columbia, Puerto Rico, and the US Virgin Islands, with 8,843
hospital admissions and 556 deaths attributed to this flu strain.
The CDC stopped reporting numbers of cases and deaths on a state-by-state
basis in late July and is now reporting numbers by US Department of Health
and Human Services regions: the most cases (8,314) have been reported in
region V, which includes Illinois, Indiana, Michigan, Minnesota, Ohio,
and Wisconsin, and the second highest (6,528) in Region IX (Arizona and
California). Region VII (Iowa, Kansas, Missouri, and Nebraska) have so
far had 1,078 cases, the lowest of the regions. (These numbers may be
misleading, since the populations of these regions differ widely.)
The map below shows the progression of swine flu cases in the United
States from the earliest cases at the end of April to early June, when
cases were reported in every US state for the first time (darker colours
correspond to higher numbers of confirmed cases).
The Michigan Department of Community Health has stopped reporting confirmed "swine flu" cases by county. As of August 22, MDCH reported 3,338 cases of this "swine flu" in 54 counties in Michigan, with the highest numbers seen in Macomb County (the northeast suburbs of Detroit; 911 cases), Wayne County (62 cases in the city of Detroit and 841 cases in other Wayne County communities), and 640 in Oakland County (the northwest Detroit suburbs). Another largish cluster is found in the southwest Lower Michigan counties of Kent (including Grand Rapids), Kalamazoo (including the city of Kalamazoo), and Ottawa (adjacent to Kent County) and surrounding counties.
Apparently the new strain of flu virus contains genetic material from swine, bird, and human flu viruses found in North America, as well as swine flu viruses found in Europe and Asia. Thus, this virus has been popularly referred to as "swine flu", although there has been only one report of pigs being infected with this strain (in Canada, in a pig exposed to a farm worker who had recently been in Mexico. Remember that influenza can be spread from one person or animal to another without physical contact -- sneezing can be enough). Most people who have died from complications of this flu strain in the US had other medical problems which may have made them more susceptible to influenza. This flu outbreak seems to be affecting mostly children and young adults, while seasonal flu usually hits the elderly hardest, and most reported cases have been mild (the CDC suspects that most people with the "swine flu" are not being tested because their symptoms are so mild, and that there may actually be over 100,000 cases in the US).
As of August 28, 2009, the World Health Organization (WHO) has reported over 209,000 confirmed cases and at least 2185 deaths worldwide, with cases in over 100 countries. On June 11, WHO declared a phase 6 alert (sustained outbreaks in communities in two regions of the world, which constitutes a global pandemic). An explanation of the alert levels can be found at the WHO Web page on pandemic alerts.
A vaccine will be available in the next few weeks for this strain; although the H1 and N1 antigens are similar to those in recent strains of human flu, they are not similar enough that the current flu vaccine is expected to protect people from this swine flu strain. A report from CDC suggests that people in their 50's and 60's have some immunity to this strain, and that the "new" strain is actually a descendant of the 1918 "Spanish" H1N1 flu virus, rather than the H1N1 strains of recent years. The strain appears so far to be resistant to the uncoating antivirals (amantidine and rimantadine), but sensitive to the neuramidase inhibitors (zanamivir and oseltamivir), although there have been recent reports from several countries around the world of people infected with a virus of this strain that is resistant to oseltamivir (but not zanamivir).
People in the United States who have died of this strain each had other medical problems, such as morbid obesity (body mass index of 35 or higher), asthma, diabetes, heart disease, or pregnancy, which made them more susceptible to complications of the flu. My personal speculations on why (and remember that I am not a microbiologist, an infectious disease specialist, or an epidemiologist):
(As I said -- these are my personal speculations...)
The best preventative measures are still good hygiene. The CDC recommends:
For those people who eat pork products: it is impossible to catch swine flu by eating well-cooked pork products. Pork and pork products (such as ham and bacon) should always be cooked thoroughly anyway, to eliminate any possibility of infection with trichinosis, a parasite which is often found in pork products.
For more information on swine flu, see the CDC Web site section on swine flu or the WHO Web page on swine flu. For more information on flu in general, including symptoms, signs, and possible complications, see my main influenza page.