Swine Influenza is a respiratory disease of pigs caused by Influenza A viruses. Swine influenza viruses circulate among swine throughout the year, but most outbreaks occur during the late fall and winter months similar to influenza outbreaks in humans. The classical swine flu virus Influenza A H1N1 was first isolated from a pig in 1930 and was the primary cause of swine influenza until 1998.
Pigs can be infected by avian influenza, human influenza, and swine influenza viruses. When influenza viruses from different species infect pigs, the viruses can reassort (swap genes) and new viruses that are a mix of swine, human and/or avian influenza viruses can emerge. Pigs are often referred to as "mixing vessels" because of this process. The multiple strains and subtypes of triple reassortant swine influenza viruses containing combinations of avian, human and swine influenza virus gene segments have emerged and become predominant among North American pigs since 1998.
Four main Influenza A virus subtypes have been isolated in pigs: H1N1, H1N2, H3N2, and H3N1. The H3N2 viruses initially were introduced into the pig population from humans. The current swine flu H3N2 viruses are closely related to human H3N2 viruses. Most of the recently isolated influenza viruses from pigs have been H1N1 viruses and these are genetically different from the 2009 H1N1 influenza virus, which is also a triple reassortant virus. The MN State Fair pig is the first U.S. pig to test positive for 2009 H1N1. The 2009 H1N1 virus has also been found in swine in Canada, Norway, Ireland , Japan, Iceland, China, and Australia.
The 2009 H1N1 virus has also been detected in turkeys in Chile and most recently turkeys in Ontario, Canada. Thus at this time humans, swine, and poultry have all been reported to be infected with genetically compatible strains of the 2009 H1N1 virus.
November 17, 2009 US Department of Agriculture researchers reported 2009 H1N1 virus does not easily infect poultry or spread among them. The researchers inoculated chickens, turkeys, ducks, and quail with the virus and found most of the birds showed no sign of infection; some quail were infected but did not pass the virus to other quail. The authors note that two turkey flocks in Chile were infected earlier this year, but those may have been isolated event
Influenza viruses can be directly transmitted from pigs to people and from people to pigs. Human infection with flu viruses from pigs are most likely to occur when people are in close contact with infected pigs. The infection of humans, swine and poultry with 2009 H1N1 is an indication of how these viruses can be shared and reassortant occur for the development of new viruses that may be a combination of human, swine and/or avian influenza viruses.
The 2009 H1N1 virus is a triple reassortant virus for which there is little or no immunity, there is sustained human-to-human transmission, and rapid worldwide spread. June 2009, the World Health Organization (WHO) declared 2009 H1N1 a pandemic influenza.
Influenza C viruses infect both humans and pigs, but do not infect birds. Transmission between pigs and humans have occurred in the past, but transmission is rare. Unlike Influenza A, Influenza C does not cause pandemics in humans, simply due to its lack genetic diversity and limited host potential.
In addition, all types of birds and several non-ruminant mammals (dogs, ferrets, pigs, and horses) are susceptible to influenza viruses. Cases of H1N1 influenza virus infection “spilling over” into these animals may occur if they come into close contact with an H1N1-infected human. On October 9, 2009, an USDA laboratory confirmed 2009 H1N1 infection in a ferret. The ferret's owner had previously been ill. November 4, 2009 another case of 2009 H1N1 was confirmed in a pet ferret in Nebraska. Also reported the same day by the Iowa State Department of Health was a confirmed case of 2009 H1N1 in a domestic (indoor) cat. In both of these cases the owners has been ill. Reports continue of infected dogs, ferrets, cats, and pet birds.
The best advice is to always follow common sense guidelines when dealing with animals (eg, washing your hands). In addition, it's more important than ever that pet owners keep a good eye on their pet's health and consult a veterinarian if their pet is showing any signs of illness. Keeping your pets healthy reduces their risk of becoming ill. Companion animals including , pot-bellied pigs, and birds should be monitored closely for signs of flu-like illness. Just like in people, treatment by your veterinarian will include efforts to treat the symptoms and/or prevent secondary bacterial pneumonia. It is unknown at this time whether an infected pet will harbor enough virus to spread the infection to a cage mate or uninfected humans. See the American Veterinary Association FAQs for more information.