Inhibition Test in Detecting Serum Antibody against Swine Influenza Viruses Comparison of a Commercial H1N1 Enzyme-Linked Immunosorbent Assay and Hemagglutination

Recently a commercial enzyme-linked immunosorbent assay (ELISA) kit for detecting antibody against H1N1 swine influenza virus (SIV) has been made available to diagnosticians and veterinary practitioners. Because the hemagglutination inhibition (HI) test has been considered the standard test for SIV serology, diagnostic performance of the new ELISA was evaluated using positive (n 5 60) and negative (n 5 188) serum samples from young pigs with known status of SIV infection and compared with that of the HI test. Both ELISA and HI test identified all negative animals correctly. None of the serum samples (n 5 64) from pigs inoculated with H3N2 SIV was positive by ELISA for SIV antibody. The H1N1 SIV antibody detectable by ELISA appears to develop more slowly in comparison with antibody detectable by HI test. Although antibody was detected by HI test in all inoculated animals (n 5 20) by day 7 postinoculation (PI), antibody was detected by ELISA in 0%, 75%, and 100% of the inoculated animals on days 7, 14, and 28 PI, respectively. Discrepancy in test results between the 2 serologic tests appeared to be because of differences in antibody isotypes detected by each test. Enzyme-linked immunosorbent assay mainly detected IgG antibody, whereas the HI test detects IgM antibody very efficiently as well as IgG antibody. Collectively, the commercial ELISA is highly specific for antibody to H1N1 SIV but may not identify positive animals at the early stage of infection as effectively as the HI test, particularly when SIV is introduced to a nai ve swine population. Swine influenza, commonly known as ‘‘swine flu,’’ has become one of the economically significant respiratory diseases in pigs throughout the world because it was initially recognized in the early 1900s.5,6,11,16 The disease is caused mainly by influenza A viruses, which are enveloped RNA viruses with 8-segmented, singlestranded, negative-sense RNA molecules.8 Although detection of swine influenza virus (SIV) or viral antigen in the lung or nasal secretions from clinically affected animals is considered as the definitive diagnosis of swine influenza, serologic testing is often used to detect animals that have been exposed to SIV because the disease has a very short course and the causative agent becomes undetectable in nasal secretions or lungs relatively quickly.9,17 Serology is also used to assess immune status of pigs at various stages within an operation so that the level of herd immunity or timing of vaccination can be determined. Several serologic assays have been used for detecting antibody against SIV: hemagglutination inhibition (HI) test, serum–virus neutralization test, and indirect From the Veterinary Diagnostic Laboratory, Iowa State University, Ames, IA 50011 (Yoon, Janke), Murphy Family Farms, Algona, IA 50511 (Swalla), and Rollins Animal Disease Diagnostic Laboratory, Raleigh, NC 27605 (Erickson). 1Corresponding Author: Kyoung-Jin Yoon, Associate Professor and Head of Virology, Veterinary Diagnostic Laboratory, College of Veterinary Medicine, Iowa State University, 1600 South 16th Street, Ames, IA 50011. fluorescent antibody test.8,17 Among these assays, HI test has been used most commonly in veterinary diagnostic laboratories to detect anti-influenza virus antibody and is considered to be the standard test for international trade of animals by Office International des Epizooties.12 The HI test is designed to detect antibody specific for hemagglutinin (H), which is 1 of 2 major envelope proteins on the surface of SIV and highly immunogenic. Because 15 different H subtypes are known to exist among influenza A viruses,8 the HI test must be tailored for each subtype by using reference strains corresponding to individual subtypes in assays for measuring subtype-specific antibody. At present, H1 and H3 subtypes are of worldwide concern in the swine industry,1,2,13 although pigs are known to be susceptible to all H subtypes.12 The HI test is a relatively inexpensive serologic assay. In general, the presence of HI antibody in animals is indicative of protection against the subtype used in the test, and HI antibody titers appear to correlate with the level of protection. However, the labor intensiveness of the HI test is a major hindrance to its use on a large scale. Recently, a commercial enzyme-linked immunosorbent assay (ELISA) has been developed specifically for detecting antibody against SIV of H1N1 subtype. The following study was conducted to assess the diagnostic performance of the ELISA in comparison with the HI test using a set of serum samples from animals with known status of swine influenza. by guest on May 22, 2011 vdi.sagepub.com Downloaded from