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Guard Against Both Swine Flu Strains Year-Round

Routine Vaccination Helps Control Disease - Pork producers and veterinarians need to guard against both subtypes of influenza virus circulating in U.S. swine herds — regardless of the season. That’s the opinion of leading swine-disease specialists speaking at a recent veterinary teleconference.
calendar icon 8 September 2001
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"Swine influenza virus infections are detected during every month of the year," said Dr. Bruce Janke of Iowa State University. "In fact, I don’t think we’ve ever had a month where we didn’t have at least some cases of influenza" based on information from the university’s diagnostic laboratory data bank, which receives over 3,000 cases of swine respiratory disease annually.

Even though swine influenza virus (SIV) is a year-round disease, there are seasonal peaks in October or November, with a second lower peak in April or May. Both H1N1 and the more recently identified H3N2 subtype fall into that same seasonal pattern, he said.

The H3N2 subtype apparently spread rapidly through the U.S. swine population before it was even identified, Dr. Janke said. He cited a "sero survey" conducted at Iowa State in June of 1999 on 1,600 samples — only about six months after H3N2 was first isolated in North Carolina — which indicated a 71% herd prevalence of H1N1 and a 92% prevalence for H3N2.

"And 66% of herds had serologic evidence of infection with both viruses," he added.

Researchers have also tracked viruses that were actually isolated at diagnostic laboratories. At Iowa State in 1999, about 54% of SIV isolates were H1N1 and 46% were H3. Other states had similar findings. In 2000 at Iowa State, there was a marked shift towards H1N1 in viruses; 70% of isolates were H1N1 and 30% were H3N2, he said.

H1N1 Virus Stable

Dr. Janke believes it is unlikely that antigenic changes — genetic changes in the viruses — have occurred to the point that currently available SIV vaccines would fail.

"The H1N1 virus has really not changed that much. We’ve done serologic studies looking at cross reactions between viruses and typing serum or hyper-immune serum, and I think the H1N1 virus really has been pretty stable," he said.

Some swine veterinarians and producers, Dr. Janke continued, have expressed concern about the possible emergence of a H1N2 "second generation" subtype, since a case was reported in Indiana in 1998 and in Minnesota in early 1999. However, more than 160 isolates have been analyzed at Iowa State and "so far, we haven’t found any H1N2s."

Such reassortants probably are not common, and preexisting immunity to the parent strains from which they derived should be protective anyway, he said.

Dr. Gene Erickson of the Rollins Animal Disease Diagnostic Laboratory in Raleigh, N.C., said it is always possible for reassortant SIV strains to develop due to the exposure of pigs to farm personnel. Reassortant viruses can occur when two different influenza viruses infect an animal at the same time and a new virus develops that contains genes from each of the two viruses.

"But I don’t think the exposure factor from people for a [variant] H3N2 SIV strain is very significant at this time."

For one, said Dr. Erickson, the H3N2 Hong Kong type of human influenza has a low circulation level in humans and during the last human flu season, H1N1 dominated.

"It is more likely that we now have a risk for the emergence of a new [variant] of H1N1 from the human population, but that is not very likely because the classical swine flu seems to have held up in pigs for such a long time," regardless of challenges in the H1 human influenza subtype.

He added, "It is our good fortune that many of those reassortant viruses are not successful in the pig."

Routine Vaccination

To control SIV and prevent losses to producers, several of the seven panelists at the teleconference advised routine vaccination.

Dr. Brad Thacker of Iowa State University said routine vaccination of sows can be initiated both for direct control of disease or in herds that have a chronic disease problem.

"Where the clinical signs are obvious, it’s fairly straightforward to implement a flu vaccination program," although practitioners must time vaccination to avoid maternal antibody interference, he said.

"I see a lot of people using [SIV vaccination] more as an insurance policy," which is particularly important when gilts are continually being brought into the breeding program, Dr. Thacker said.

He prefers to vaccinate gilts twice before they come into the breeding herd, during the isolation period and to then booster prefarrowing. "We even give it twice to gilts if we think they are coming out of a positive finisher, because we are not monitoring every group and feel strongly about having uniform immunity in gilts with SIV," he reported.

Dr. Dale Mechler, a practitioner in Algona, Iowa, emphasized that when vaccinating finishing pigs in a naturally exposed sow unit, "it’s really important to know the maternal antibody degradation curve. I don’t think we can accurately time vaccination on positive pigs unless we know the sow herd status."

In heavily challenged models with negative pigs coming into positive finisher flows, "getting two doses of vaccine into them is highly important" to establish good immunity, Dr. Mechler said. The first dose should be administered before pigs are exposed.

"In a nutshell, the most important thing to do is actually tag and profile pigs in these systems" so the best opportunity for vaccination can be identified, he said.

Post-Vaccination Titers

Dr. Vicki Rapp-Gabrielson of Elkhorn, Neb., a researcher with Schering-Plough Animal Health, pointed out that when SIV negative pigs are vaccinated, titers to both H1N1 and H3N2 tend to peak two to three weeks after the second vaccination.

"In a negative herd, the titers you see after vaccination aren’t always that high and they don’t stay high for a very long time. We’ve challenged pigs with H3N2 that have subsequently gone antibody negative (hemagglutination inhibition titers of <= 10), and they are still protected," she said.

To receive a taped copy of the SIV teleconference, send name and address to or fax your request to 913-262-2049.

Source: PR Works, September 2001