Swine influenza is a respiratory disease of swine caused by multiple subtypes of type A influenza viruses. SIV in swine is not a reportable or regulated animal disease in the United States. The virus is endemic in swine populations in North and South America, Asia, and Europe. The influenza virus genome consists of eight distinct gene segments and the subtypes of viruses are generally described by the characterization of two gene segments, the hemagglutinun (HA) gene and neuraminidase (NA) gene.
Classic SIV infection was caused by the H1N1 subtype and remained relatively unchanged while circulating in U.S. swine populations for over 75 years. Since 1998, SIV infections in the United States have evolved from a seasonal disease caused by a single, relatively stable H1N1 genotype to an endemic year-round respiratory disease caused by multiple genetically unstable SIV subtypes (H1N1, H1N2, and H3N2). Advances in molecular diagnostic capabilities have allowed diagnosticians and researchers to establish that recently characterized SIV strains have recombined from gene segments originating from multiple species (swine, humans, and avian), illustrating the shared nature of influenza virus lineages in poultry, swine, and humans.
The USDA, in cooperation with State and industry, conducts voluntary surveillance for SIV in the US. This surveillance is not conducted to define prevalence - the goal is to identify viruses that may be circulating in swine, and gain knowledge to contribute to improved animal health diagnostics and vaccines. When viruses like the H3N2v viruses with the 2009 H1N1 pandemic M gene recently isolated in humans are detected in swine, they are called "H3N2pM." The agency first identified H3N2pM virus isolates collected in late 2010 and have continued to find it across the U.S. since then.
USDA's SIV Surveillance Program has tested 12,662 samples from 3,766 swine diagnostic lab submissions collected from October 1, 2010 through July 31, 2012. Over that time period, 1,488 case submissions have been identified as positive for Influenza A infection. Overall, 73 H3N2 positive submissions were detected in FY2011 (October 1, 2010 to September 30, 2011) and 138 in FY2012 from October 1, 2011 to July 31, 2012. 57 Of the 138 H3N2 cases identified in FY2012 and tested to date contain the pandemic M gene and were classified as H3N2pM. Several additional H3N2-positive submissions remain under advanced diagnostic processes. Additional detailed characterization results will be reported as results are compiled. The USDA SIV Surveillance Program continues to collect and test samples to monitor for the type and distribution of all influenza viruses in swine.