Development of latex agglutination test with nucleoprotein as antigen for detection of antibodies to swine influenza virus.

As pigs are susceptible to infection with both avian and human influenza A viruses, they have been proposed to be an intermediate host for the generation of pandemic virus through reassortment. The broad susceptibility of pigs to influenza viruses emphasizes the importance of surveillance of swine influenza virus. Thus, A latex agglutination test (LAT) was developed for rapid detection of antibodies to swine influenza virus. The nucleoprotein (NP) gene of the H9N2 swine influenza virus isolated from local farms was cloned, and expressed in Escherichia coli. Reactivity of the expressed protein was confirmed by Western blot. Subsequently, the NP gene was purified and used as the diagnostic antigen to develop a NP-based LAT for detecting antibodies to swine influenza virus. The LAT is shown to be specific for swine influenza virus and does not cross-react with swine sera that have antibodies to other swine viruses. The NP-LAT and HI test had a high agreement ratio in detecting 10 serum samples from naïve pigs, 28 serum samples from experimentally infected and vaccinated pigs. Compared with the hemagglutination inhibition (HI) test, the corresponding specificity, sensitivity, and correlation were 92.9%, 94.1%, and 94.1%, respectively, in detecting 321 serum samples from vaccinated pigs. The NP-LAT developed in our laboratory is a rapid and simple test suitable for field monitoring of antibodies to swine influenza virus. We conclude that it was specific and sensitive and it has great application potential in China's long-term prevention and control of swine influenza virus.