Ever since the H3N2 subtype of swine influenza virus (SIV) was identified in U.S. swine herds in 1999, the virus has continued to change, much like it has previously done in the human population.
The evolution of SIV has frustrated producer and swine veterinary control efforts alike, according to human medicine as well as veterinary laboratory diagnosticians who track prevalence and control efforts.
There has been increased SIV activity and variation in strains in recent years around the world, but especially in the U.S. swine population (Figure 1), notes Richard Webby of St. Jude Children's Hospital, Memphis, TN. Change in human influenza activity has been rather minimal in recent years, he reports.
The upsurge in genetic diversity of swine flu strains is a concern for both human and animal health, Webby explains in a presentation at the Iowa State University Swine Disease Conference for Swine Practitioners in Ames, IA. Chances are the two major strains currently identified — H1N1 and H3N2 — will continue to alter and circulate in swine herds, he says.
The challenge is for industry to develop a surveillance system to monitor these changes, thereby, ensuring continued efficacy of SIV vaccines and for early detection of potentially devastating viruses, he reports.
“For the last few years, we've been finding a lot more variety of influenzas in pigs, rather than just the classical swine flu which had been around for decades,” says Kurt Rossow, DVM, of the University of Minnesota Veterinary Diagnostic Laboratory.
There are the classical H1N1 and H3N2 strains, plus some definite genetic and antigenic variations within H1N1 and a few variations within H3N2, suggests Bruce Janke of the Iowa State diagnostic lab. A smaller number of H1N2 cases have also been identified.
In Iowa, based on 266 isolates tested in 2001, 70% of the samples were H1N1, 29% were H3N2 and 0.4% were H1N2. For 150 isolates tested in 2002 through September, 74% were H1N1, 23% were H3N2 and 3% were H1N2.
For Minnesota, the predominance of cases in 2001 was H1N1, 94%. H1N1 accounted for 93% of cases through September 2002, says Rossow.
The situation in North Carolina is a study in contrast, explains Gene Erickson, DVM, of the Rollins Animal Disease Diagnostic Laboratory. In 2001, of 320 isolates representing 250 farms, 59% were subtyped H3N2, 39% were H1N1 and 2.4% were H1N2. For 130 isolates representing 114 farms typed so far in 2002, 46% were H3N2, 44% were H1N1 and 11% were H1N2.
This emerging trend in North Carolina was confirmed by Minnesota diagnostic tests, he says.
There are several standard test procedures for SIV diagnosis using tissue, nasal swabs and/or serum. Polymerase chain reaction (PCR; DNA testing), serology, immunohistochemistry (microscopic detection of virus in living tissues), Directigen Flu A test (quick test using nasal or lung swabs) and virus isolation have been shown to provide varying degrees of sensitivity, says Rossow.
Detecting SIV is usually straightforward; however, the new challenge is further characterizing the virus to detect subtypes of strains which have undergone genetic reassortment or variation, he observes. Additional diagnostic tests such as genetic sequencing and new serology tests are being used to help detect and understand the changes.
When pigs are coughing and have pneumonia, lung tissue samples are normally submitted. For diagnosis, the best lung samples are those taken from pigs during the first 3-4 days after the onset of disease.
Nasal swabs are used to do PCR tests to define SIV status or when people don't want to submit whole pigs or euthanize pigs, he notes.
Testing for SIV at state diagnostic labs has proven especially challenging for Pfizer Animal Health, which markets the FluSure family of vaccines for SIV. Other federally licensed, bivalent (H1N1 and H3N2) SIV vaccines are sold by Intervet, Inc. (EndFLUence) and Schering-Plough Animal Health (MaxiVac Excell with Emunade).
The standard or reference serology test used in diagnostic labs doesn't detect SIV antibodies from pigs immunized with the FluSure vaccine, explains Steve Sornsen, DVM, director of U.S. Swine Veterinary Services for Pfizer.
“We know that if we use our virus to make a different serology test, we stimulate a nice antibody response,” he says. “This could have been a big problem for Pfizer, since practitioners often use serology to determine how well pigs have been immunized against SIV.”
Pfizer rectified this situation by sending their virus to many diagnostic labs to make a serology test to detect antibodies to their vaccine. “Using this test, labs have found that they can not only detect pigs vaccinated with FluSure, but also detect some cases that may have been missed,” Sornsen explains.