In separate instances, A/H3N2 influenza viruses circulating in swine acquired a gene from the pandemic 2009 H1N1 virus and recently infected two young children, one in Indiana and one in Pennsylvania, the Centers for Disease Control and Prevention (CDC) has reported.

Both of the children recovered, though one was briefly hospitalized, and there is no sign that the viruses spread from the children to others, but any evidence of ongoing transmission would require a rapid response, the CDC said. The agency also said one of the children had no direct contact with pigs, which suggests he caught it from another person. Both children are under age 5.

The two viruses are similar to eight other swine-origin H3N2 viruses found in humans in the past two years, but they are unique in that they contain the matrix (M) gene from the 2009 H1N1 virus, the CDC reported in an online posting in Morbidity and Mortality Weekly Report (MMWR).

"I don't think these [viruses] have pandemic potential; it looks like both of these are sort of dead-end transmissions," said Lyn Finelli, chief of the surveillance and outbreak response team in the CDC's influenza division "One of the reasons we publish this data is that reassortment happens in swine viruses and in humans, so we always want to have surveillance in place so we can detect the next emerging reassortant."

U.S. pork producers are actively engaged in influenza surveillance and share data and information with human health officials. “The industry is cooperating with an ongoing influenza surveillance system to the benefit of both animal and human health,” says Paul Sundberg, DVM, vice president science and technology, National Pork Board. “We’ve been leading in facilitating influenza surveillance for animal health and coordinating that information with public health officials for their application to public health.”

The transmission of influenza from swine to humans is typically low. “According to the CDC Web site, as of Jan. 25, 2011, 20 cases of human infection with swine origin influenza viruses (SOIV) have been reported in the United States since 2005 of which seven were triple assortment H3N2 viruses (trH3N2),” said Jennifer Koeman, DVM, NPB director, producer and public health. “All 20 persons infected with swine viruses recovered from their illness.”

Influenza viruses can also be passed from humans to pigs. “Reassortments with the 2009 H1N1 virus have been previously identified in the swine herd after human-to-pig transmission of the pandemic strain,” adds Koeman. “What is noteworthy is that the reassortant viruses did not cause any unusual clinical signs.” 

Andrew Pavia, chief of the Division of Pediatric Infectious Diseases, called the cases noteworthy but not alarming. "We need to wait and see if either strain has spread. We would not predict that addition of the matrix segment from H1N1 would make a swine H3N2 that spreads poorly in people have pandemic potential but our ability to predict is not great.”

Source: CDC, NPB