Swine influenza virus is a more complex problem today than it has been in the past. That’s why it’s important to keep a vigilance for SIV all year long.

“We identify SIV in every month of the year from samples sent to the Veterinary Diagnostic Laboratory at Iowa State University,” notes Bruce Janke, a veterinary diagnostician and SIV expert with the Iowa laboratory.

Some experts recommend that veterinarians undertake an annual flu review to help clients examine the critical areas conducive to an effective flu vaccination program. These include: titer levels in sows and gilts; maternal immunity in nursery pigs; and SIV subtypes and their prevalence within the herd.

H1N1 and, more recently, H3N2, are the primary SIV subtypes seen in U.S. herds. In many cases, both subtypes may be involved.

The outbreak in H3N2 in 1998 took diagnostic labs, veterinarians and swine producers by surprise but was not entirely unexpected. The H1N1 virus had remained the only subtype affecting U.S. swine for nearly 75 years before H3N2 arrived on the scene. Tests from diagnostic labs showed that within six months of the identification of the first case of H3N2, the majority of SIV cases were due to H3N2.

But H1N1 was hardly on vacation.

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

“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 subtypes – a process made easier with the recent introduction of bivalent swine flu vaccines.

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

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 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,” says Janke. If sow titers are high, additional vaccination for gilts provides more stable immunity throughout the sow herd.

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

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

Characterization of the SIV subtypes found in the herd will show any changes in subtype. Janke says that antibodies against one subtype do not effectively cross-protect against a second type.

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

Jim Carlton is the editor of Swine Practitioner magazine, a supplement to Pork magazine for swine veterinarians.