Swine influenza virus used to be a seasonal problem as pigs would get sick in the fall and winter and then get over it. Today’s mix of SIV strains has turned swine flu into a year-round problem for pork producers.
Kurt Rossow, Veterinary Diagnostic Medicine, University of Minnesota, says that SIV cases seen in the past 6 months have shown “different H1N1 viruses and changes in H3N2 viruses as well. While it’s still out there and needs to be considered, the classic H1N1 is no longer the predominant SIV type.”
Similar patterns surfaced in Iowa. “The SIV samples that we’ve looked at from 2002 and 2003, nearly all appear to be the new reassortant H1N1 rather than classic H1N1,” notes Bruce Janke, Veterinary Diagnostic Laboratory, Iowa State University.
According to Marie Gramer, Veterinary Diagnostic Medicine, University of Minnesota, the new reassortant H1N1 strains are adding to the confusion about SIV and contributing to problems surrounding its control.
“Multiple strains active in a herd on a nearly continual basis make SIV testing more complicated,” says Gramer. “It’s difficult to identify strains definitively using only the basic testing procedures.”
It’s possible for one herd to carry multiple SIV strains including H1N1, rH1N1, H1N2 and H3N2, stresses Gramer. To identify multiple strains in a herd the diagnostic lab must do genetic sequencing of the virus in addition to performing serotyping.
“Today, SIV is a moving target,” says Gene Erickson, Rollins Laboratory, Raleigh, N.C., “with the emergence of H3N2, it has become clear that the virus has a broad ability to reassort with other strains of virus co-circulating in the herd, at the same time allowing it to create a new virus.”
“We’re seeing documented cases in well-vaccinated pigs where the SIV vaccine hasn’t offered adequate protection,” acknowledges Gramer. “The vaccine didn’t necessarily fail, it just didn’t cover new strains circulating within the herd.”
Iowa State research also shows that pigs undergoing active infection with porcine reproductive and respiratory syndrome or porcine circovirus at the time of SIV vaccination may have a compromised ability to respond properly to the vaccine.
Many herds rely on pre-farrowing SIV vaccination programs to protect pigs through late nursery or early finishing stages, but then neglect to fully immunize incoming gilts.
“Replacement females need to be brought up to a similar immune status as the sows. Otherwise, the differing immune levels in sows and offspring will encourage the virus to maintain or even amplify itself, in the herd,” stresses Erickson.
An Evolutionary Virus
Since 1998, SIV has moved from a single, stable virus to a virus with the ability to reconfigure itself to the point where it may avoid control by existing vaccines.
“The new H1N1 strain emerged from a process called genetic reassortment, which occurs when a single cell is infected by two different influenza viruses,” explains Robyn Fleck, a technical service veterinarian at Schering-Plough Animal Health. “The result is a ‘progeny virus’ containing genetic material from both ‘parents’.”
“In other words,” she adds, “the outside of reassortment H1N1 looks like a classical H1N1, but its internal genes are derived from H3N2. Reassortant H1N1 also acts differently from the old classical H1N1. It picks up mutations at an increased rate, thereby evading the pig’s immune system.”
According to Bruce Janke, Veterinary Diagnostic Laboratory, Iowa State University, the positive identification of H3N2 in 1998 made SIV “a two-strain disease in many Midwest herds within 6 months, thus setting the stage for further variations.”