Swine influenza virus is a more complex problem today than it has been in the past. That’s why it’s important to be vigilant against the disease all year long.

“We identify SIV in every month of the year,” says Bruce Janke, with the Iowa State veterinary diagnostic laboratory.

It would be worth your time to work with your veterinarian to conduct an annual review to examine the critical areas conducive to an effective flu vaccination program. These include looking at titer levels in sows and gilts, maternal immunity in nursery pigs, as well as SIV subtypes and their prevalence within the herd.

H1N1 and, more recently, H2N2, are the primary SIV subtypes in U.S. herds. Often, both are present.

The 1998 outbreak of H3N2 took diagnosticians , veterinarians and pork producers by surprise, but it was not entirely unexpected. The H1N1 virus had been the only subtype affecting U.S. swine for nearly 75 years before H3N2 arrived on the scene. Diagnostic lab tests showed that within six months of identifying the first case of H3N2, most SIV cases were due to that subtype.

But H1N1 was hardly on vacation.

Last year, most flu cases again were due to the H1N1 SIV strain. Of the isolates submitted to the National Veterinary Disease Laboratories in Ames, Iowa, through the first nine months of 2001, 67 percent were H1N1 while 31 percent were H3N2.

“Today, the majority of swine herds have been infected with both viruses,” notes Janke, “but, at any given time, one may be circulating more than the other.”

For this reason, specialists generally recommend vaccinating for both subtypesùa process made easier with the introduction of bivalent swine flu vaccines.

SIV experts suggest the following three steps to stay up-to-date on SIV status:

1. Check herd immunity. The first step in an annual flu review is to determine exposure to H1N1, H3N2 or both. Then check titer levels in gilts and sows to be sure that uniform immunity is present throughout the herd. “Incoming gilts or first-litter sows may need an additional vaccination if their titer levels are substantially lower than older sows in the herd,” says Janke. If sow titers are high, additional vaccination for gilts provides more stable immunity throughout the breeding herd.

2. Watch maternal antibody levels. It is important to know the maternal antibody levels in nursery pigs to ensure optimum vaccination timing. “Without a clear reading of when maternal antibodies fall below an H1 level of 10, we can’t successfully vaccinate without fear of interference,” notes Janke.

3. Develop an SIV profile. The mix and prevalence of virus subtypes in the herd is crucial to an effective vaccination program. “By testing the herd at critical life stages, veterinarians and producers get a much clearer picture of the herd’s SIV profile,” says Janke. “The subtype prevalence within a geographical area also may dictate which vaccination regimen to follow.”

By characterizing SIV subtypes found in the herd, you can monitor any changes that occur. Janke says that antibodies against one subtype do not effectively protect against a second type.

“This process of subtyping helps veterinarians prescribe the most effective vaccination program for the herd,” he adds.

So spending a little review time now, could result in long-term preventative benefits. 

Get the Drift?
When the H3N2 swine influenza virus popped up in 1998, veterinarians and pork producers encountered a new facet of SIV infection – different subtypes.

“The H1N1 virus has remained relatively stable antigenically during its 70-year history,” says Bruce Janke, Iowa State University veterinary diagnostician.

But, antigenic drift occurs when a virulent strain of a virus mutates slightly, which allows it to evade immunity within a population and survive.

Antigenic drift is more significant in the human population. Because each individual is exposed repeatedly to the influenza virus over a lifetime, the virus must change to survive our immune system.

Less common, but of greater concern is antigenic shift. This results when a new virus is created from a re-assortment of existing viruses or a shift in a whole virus between species, such as swine, avian and human. Because the population is not immune to the new virus, it quickly becomes a dominant subtype.

Veterinary diagnostic laboratories and independent testing laboratories are constantly monitoring swine flu subtypes. “We are always on the lookout for the development of new SIV subtypes,” says Janke.