Survey of state diagnostic labs confirms swine influenza virus' prevalence in respiratory disease.
Swine influenza virus (SIV) continues to gain stature as a major respiratory disease agent, according to Bruce Janke, DVM, Veterinary Diagnostic Laboratory, Iowa State University (ISU).
At the Leman Swine Conference in mid-August, Janke reported on a survey of SIV cases for 1999 from state diagnostic labs.
The survey confirms that SIV is one of the three primary pathogens found in swine pneumonia cases, he says. Only porcine reproductive and respiratory syndrome (PRRS) and Pasteurella multocida were found more often.
Minnesota and Iowa diagnostic labs found SIV involvement in 20% of all swine pneumonia cases, Janke says.
Swine respiratory diseases, including SIV, are now year-round problems. But SIV still shows the most pronounced problems in spring and fall, he points out.
Janke's survey bears out that an interaction of swine respiratory disease agents, commonly known as Porcine Respiratory Disease Complex (PRDC), is more commonly observed than a single respiratory infection.
Illinois and South Dakota labs reported diagnosing SIV as the sole cause of PRDC a little more than one-fourth of the time.
SIV was identified without one of the other major players in PRDC (PRRS, mycoplasma and porcine coronavirus) in less than one-half of the cases in the Iowa and Minnesota diagnostic labs.
The two subtypes of SIV are H subscript 1N subscript 1 and H subscript 3N subscript 2. The H subscript 1N subscript 1 subtype was the only known subtype in hogs until H subscript 3N subscript 2 was discovered in August 1998 in North Carolina. It quickly spread to other parts of the country. Janke's survey found a nearly equal distribution of both subtype infections. In a few cases, both subtypes were found.
The survey indicates both SIV subtypes are widespread in the hog population and are causing disease in pigs of all ages, Janke says.
Sow Management for SIV The H subscript 3N subscript 2 form of SIV emerged in southern Minnesota in November 1998, says Mark Wagner, DVM, Fairmont, MN, Veterinary Clinic. It first struck sow herds with severe lethargy, temperatures of 104ø F-plus, labored breathing, abortions and sow deaths.
Since then, H subscript 3N subscript 2 has progressed to nursery and finishing pigs. It has died down some, but it is still present in some sow herds, he reports. Meanwhile, the H subscript 1N subscript 1 subtype remains active in nursery and finishing.
Wagner reported on an attempt to control SIV in a 1,400-sow, farrow-to-wean operation that was completely repopulated in October 1999. New animal introductions to the farm were vaccinated for both H subscript 1N subscript 1 and H subscript 3N subscript 2. The first group of replacement gilts entered the herd last June.
Shortly after the herd was repopulated, a respiratory disease outbreak occurred. Wagner says diagnostic testing and serology pointed to the culprit as H subscript 1N subscript 1-infected replacement gilts.
Sow herd serology has been monitored monthly. H subscript 1N subscript 1 serology titers were high at the time of the outbreak. Since then, H subscript 1N subscript 1 serology titers have continued to drop or level out.
In contrast, H subscript 3N subscript 2 serology was low initially, but it spiked twice before the new herd introductions and remains active within the herd, says Wagner.
The H subscript 3N subscript 2 subtype has remained in this herd even though no new animals were introduced until June, despite vaccination three to four months before the outbreak and despite exposure to a natural infection in January.
Speculation is the H subscript 3N subscript 2 infection occurred either from aerosol spread or during the herd repopulation.
Vaccination Justification These types of chronic cases of SIV can have minimal or devastating effects, depending on the involvement of other respiratory complex diseases such as PRRS and mycoplasma, says Brad Thacker, Iowa State's DVM.
SIV pig vaccination can be helpful. But as with any vaccination program, careful evaluation is needed to ensure that costs don't exceed benefits. Treat accompanying disease problems in the complex.
In most cases, however, Thacker prefers sow herd vaccination to control SIV. "Sow vaccination has been uniformly effective in controlling disease in suckling pigs and often protects pigs through the nursery phase." Gilts should be given two injections before breeding and sows one injection prior to each farrowing.
Declares Thacker, "My premise is that if a vaccine is effective, I will tend to use it in the sow herd as long as it is safe and the maternal immunity is not an issue with respect to piglet vaccination."
The swine influenza virus (SIV) subtypes H subscript 1N subscript 1 and H subscript 3N subscript 2 are mutating, says Kurt Rossow, DVM, veterinary diagnostic laboratory at the University of Minnesota.
Cases of mutation or virus reassortment have been reported in Indiana and southern Minnesota, he says. This sort of activity will continue as SIV subtypes become well-adapted to large populations of pigs.
As a result, expect to see new SIV subtypes emerge. To provide protection, new SIV vaccines will need to be developed almost every year. Rossow says better diagnostics are necessary for effective vaccines to be developed.
"We need to be able to look at these viruses in more detail like we do with human influenza viruses," stresses Rossow.
Influenza viruses fall into three groups. Group A influenza viruses infect mammals and avian species and are the main cause of highly contagious, acute respiratory disease, he says in a report at the Leman Swine Conference in Minneapolis.
Group A flu viruses are divided into subtypes based on two membrane proteins known as hemagglutin (HA) and neuraminidase (NA).
All influenza viruses found in mammals appear to be derived from avian influenza viruses. But not all influenza viruses are found in mammals, explains Rossow.
