What is swine flu?
Like humans, pigs get the flu. They develop a sudden fever, a barking cough, sneezing, lethargy and typically lose their appetite.
Pigs usually don't die from swine flu; their flu viruses cause high levels of illness but low death rates.
Swine influenza viruses circulate among pigs throughout the year, but most outbreaks occur during the late fall and winter, just like with outbreaks in humans.
Most swine flu viruses belong to the Influenza A H1N1 and H3N2 subtypes.
Can humans catch swine flu?
Normally, swine flu bugs don't infect people. When they do, it's been in people who have direct contact with pigs; historically, there's such a case every year or two in the U.S.
Now, we have two clusters of cases identified in San Diego County and Imperial County, California, as well as in San Antonio, Texas. All the illnesses were easily treated and all have recovered.
Even before these clusters, there had been a surge of cases of swine flue in humans in recent years. Since December 2005, there have been 12 human swine flu infections - about four a year.
It's possible this uptick was due to improved reporting systems, but the U.S. Centers for Disease Control says "genetic changes in swine flu viruses and other factors might also be a factor."
Can humans pass swine flu?
Usually no. But what makes this new outbreak in California and Texas worrisome is that in all the recent cases, none had any direct contact with pigs.
Two of the new cases were among 16-year-olds at the same school in San Antonio and there's a father-daughter pair in California, said CDC's Dr. Anne Schuchat.
Is this a new kind of swine flu?
Yes. The CDC reports that the virus in these latest cases is a never-before-seen mixture of viruses typical among pigs, birds and humans. The influenza A H1N1 virus contains DNA typical to avian, swine and human viruses, including elements from European and Asian swine viruses.
Although it's called swine flu, this new strain is not infecting pigs and has never been seen in pigs.
Why would a new strain be worrisome?
Epidemiologists have been warning for years that it's just a matter of time before a new strain of the flu emerges that has the potential to kill millions. Flu pandemics have historically occurred about three times per century and the world hasn't seen one 40 years.
If an influenza virus changes and becomes a new strain against which people have little or no immunity -- and this new strain is easily spread from person to person --, many people around the world could become ill and die.
The World Health Organization estimates that in the best case scenario, the next pandemic could kill two to seven million people and send tens of millions to hospital.
Is there a vaccine?
There is a vaccine available that can be given to pigs to prevent swine influenza. But there is no vaccine to protect humans from swine flu.
I got the flu shot this year. Am I protected?
No. H1N1 swine flu viruses are very different from human H1N1 viruses and, therefore, vaccines for human flu would not provide protection from H1N1 swine flu viruses.
Can people catch swine flu from eating pork?
No. Swine influenza viruses are not transmitted by food; you cannot get swine influenza from eating pork or pork products.
What are the symptoms of swine flu in humans?
Based on the cases seen in the U.S., symptoms of swine flu are similar to those of our regular flu: sudden onset of fever, coughing, aches and pains, and extreme fatigue. Swine flu appears to cause diarrhea and vomiting, symptoms that are not usually found in regular flu in adults.
Can we treat swine flu in humans?
Yes. Most swine influenza viruses have been treated with antiviral medications.
The virus from the most recent U.S. swine flu cases appeared to be resistant to amantadine and rimantadine but were susceptible to zanamivir and oseltamivir (Tamiflu).
Have there been swine flu outbreaks before?
Yes. Most famously, there was an outbreak in 1976 at Fort Dix, N.J., among military recruits that grabbed big headlines at the time.
Worried that they had the beginning of a pandemic on their hands, U.S. officials ordered the manufacture of swine flu vaccine and the country launched a mass immunization program that saw about 40 million people vaccinated.
But the outbreak didn't turn into a pandemic and went away as mysteriously as it appeared.
Sources: The Canadian Press, Public Health Agency of Canada and the U.S. Centers for Disease Control
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