Today’s mix of SIV strains has turned swine flu into a year-round problem, according to SIV experts.

Kurt Rossow, Veterinary Diagnostic Medicine, University of Minnesota, reports an above average number of cases of SIV through early December. Recent numbers show 11 percent of cases positive for SIV in September 2003, jumping to 33 percent of cases the first week of December 2003.

“These cases are showing different H1N1 viruses and changes in H3N2 viruses as well,” he states. “While it’s still out there and needs to be considered, the classic H1N1 (cH1N1) is no longer the predominant SIV type.”

Similar patterns are being seen in Iowa. “The SIV samples 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 (rH1N1) strains are adding to the confusion about SIV and contributing to control challenges.

“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.”

From July 1, through Dec. 1, 2003, 66 percent of SIV isolates at the University of Minnesota were serotyped as H1, 20 percent as H3 and 22 percent were not able to be serotyped using reference H1N1 and H3N2 antisera.

“It is very possible for one herd to carry multiple SIV strains including H1N1, rH1N1, H1N2 and H3N2,” stresses Gramer. To identify multiple strains present in a herd the lab must do genetic sequencing of the virus in addition to performing serotyping.

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. 

According to ISU’s Janke, the positive identification of H3N2 in 1998 made SIV “a two-strain disease in many Midwest herds within six months, thus setting the stage for further variations.”

“Today SIV is definitely a moving target,” argues  Gene Erickson, Rollins Laboratory, Raleigh, N.C. “With the emergence of H3N2, it has become very clear that that virus has very 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 (always) offered adequate protection,” acknowledges UMN’s Gramer. “The vaccine didn’t necessarily fail, it just didn’t cover new strains circulating within the herd.”

Recent research from Iowa State University also shows that pigs undergoing active infection with PRRSV or PCV2 at the time of vaccination against SIV may have a compromised ability to respond properly to the vaccine.

Many herds rely on pre-farrowing SIV vaccination programs to offer protection of pigs through late nursery or early finishing stage but neglect to fully immunize incoming gilts.

“Replacement females need to be brought up to an immune status similar to the sows. Otherwise, the differing immune levels in the sows and offspring will encourage the virus to maintain, or even amplify itself, in the herd,” stresses Erickson.

7 steps for controlling swine flu

Experts recommend the following seven-step program to help you and your clients get the upper hand on an increasingly complex swine influenza virus situation today:

1. Assess signs of clinical illness.

2. Select pigs in the early stages of illness for testing.

3. Ask for sequencing to characterize the isolates.

4. Evaluate vaccines for best control of SIV strains on the farm. The broader the coverage, the less likelihood of SIV breaks.

5. Develop a management plan for testing and vaccination.

6. Continue testing to fine-tune vaccination timing to avoid maternal antibody interference.

7. Implement a diagnostic plan to identify the cause of respiratory outbreaks and identify pathogens (Mycoplasma hyopneumoniae, PRRS, PCV2, emerging SIV strains and bacterial co-infections) that may impact perceived vaccine efficacy.