During the last week of 2011, human influenza activity increased in the United States, but remains relatively low, according to the U.S. Centers for Disease Control and Prevention. However, the influenza virus is a quick-change artist undergoing constant reassortment.

To monitor the rapidly-changing influenza viruses, surveillance programs are increasingly important to protect human health as well as animal health, “The pork industry has an active, on-going surveillance program in conjunction with USDA’s Animal and Plant Health Inspection Service which closely tracks the changes and movement of influenza viruses,” says Jennifer Koeman, DVM, director of producer and public health, National Pork Board.

The program advances influenza diagnostic capabilities and helps in vaccine development efforts. “Since the inception of the surveillance program, we have seen an increase in participation by pork producers,” according to Koeman. “We encourage producers to participate in the program.”

To keep up with ever-changing influenza viruses, public health officials have adopted a new virus naming system that is currently being used by the CDC as well as international public health organizations. Viruses that normally circulate in animals and sporadically infect humans will now be referred to as “variant” viruses and denoted with a “v,” according to the CDC.

Through the end of 2011, CDC has reported 12 cases of human influenza caused by influenza A(H3N2)v – the latest virus adaptation developed by the rapidly changing influenza virus family.

 “The influenza A(H3N2)v recently identified in humans is a novel virus that contains gene segments from the 2009 pandemic influenza A (H1N1)pdm09 virus, and gene segments from the North American swine-lineage triple reassortant H3N2 virus,” according to Phil Gauger, DVM, Department of Veterinary Diagnostic and Production Animal Medicine, Iowa State University. The novel pH1N1 virus circulated in humans in 2009 and was transmitted from humans to pigs in the United States.

“At this time it has not been reported by the CDC that the A(H3N2)v is more virulent than the pH1N1 or other human seasonal influenza viruses in humans. Eleven of the 12 A(H3N2)v cases were children and five cases had no individual or family members with a history of direct exposure to swine,” according to Gauger. “The lack of swine exposure combined with current epidemiological data suggests limited human-to-human transmission of the A(H3N2)v may have occurred.”

The swine influenza surveillance program functions in cooperation with approved veterinary diagnostic laboratories.  “Swine respiratory cases submitted to the diagnostic laboratory determined positive for influenza A virus are isolated and sequenced on an anonymous basis,” says Gauger. “Influenza isolates and sequences are provided to the USDA and sequences made available through a public database. Three gene segments, the hemagglutinin, neuraminidase and matrix genes, are sequenced allowing the USDA to monitor reassortant viruses that may demonstrate unique gene combinations and compare swine isolates with influenza viruses isolated from humans.”

The anonymous USDA swine surveillance system has identified H3N2 viruses with pandemic H1N1 gene segments in the U.S. swine population confirming reassortment between triple reassortant swine influenza viruses and the human pandemic H1N1. 

According to Gauger, the detection of the novel reassortant H3N2 in swine has not been associated with increased clinical disease in pigs, although the full implication of these viruses in the swine population is unknown.

The National Pork Board recommends pork producers or individuals in close contact with swine consider vaccination of personnel to reduce the risk of transmission of influenza viruses from people to pigs since human influenza viruses may also transmit back into pigs, as well as vaccination of pigs to reduce potential exposure to farm personnel.

Read additional biosecurity recommendations for swine farms.

Read more on influenza surveillance in pigs.