Swine influenza virus is believed to be spread primarily by close contact between pigs and potentially contaminated objects moving between infected and uninfected pigs. Herds with continuous infection and herds that are vaccinated against swine flu in may have sporadic disease, or may show mild or no symptoms of infection.
The signs of swine influenza in pigs may be the sudden onset of fever, depression, cough (barking), discharge from the nose or eyes, sneezing, difficulty breathing, redness or inflamed eyes and loss of appetite.
H1N1 and H3N2 influenza viruses are endemic among pig populations of pigs in the United States and something that the industry deals with regularly. Outbreaks usually occur in pigs in the cold months and sometimes with the introduction of new pigs into susceptible herds. Studies have shown that swine influenza H1N1 is widespread in pig populations around the world, with 25 percent of animals with antibody evidence of infection. In the U.S., studies have shown that 30 percent of the pig population has antibody evidence of infection with H1N1. Specifically, 51 percent of pigs in the north-central United States have shown antibody evidence of infection with H1N1 swine. Human infections with H1N1 virus of swine influenza were rare. There is currently no way to differentiate antibodies produced in response to vaccination against influenza in pigs from the antibody response to infections of pigs from swine H1N1 influenza.
Although swine H1N1 virus has been known to circulate in pig populations since at least 1930, the influenza virus H3N2 did not circulate among swine in the United States until 1998. The H3N2 virus was first introduced in the pig population by humans. The current H3N2 virus of swine influenza is closely related to H3N2 virus.
Vaccines are available to be given to pigs to prevent swine influenza. There is no vaccine to protect humans from swine influenza. The vaccine against seasonal influenza will likely help to provide partial protection against H3N2 swine, pork, but no H1N1 virus.