| Swine influenza is an acute, febrile respiratory disease of swine with high morbidity and low mortality. Three predominant subtypes have been reported throughout the world: the classic H1N1, H3N2, and a reassortant H1N2 swine virus of H1N1 and H3N2. At present, economic losses due to swine influenza virus (SIV) infection are highly significant in the U.S. swine industry. The overall objectives of this thesis were to develop a rapid SIV diagnostic method for field specimens, to genetically characterize recent SIV isolates and to investigate pathogenesis of SIV infection by both in vitro and in vivo studies.; In order to detect and subtype SIV directly from field samples, two multiplex RT-PCR assays specific for H1, H3, N1, and N2 were developed. The RT-PCR assays were shown to be highly sensitive and specific. From this study, the usefulness of multiplex RT-PCR was well demonstrated for detection and identification of influenza A virus subtypes.; Serologic and virologic prevalence of different SIV subtype infection were investigated using swine sera, nasal swabs and lung samples collected between 1998 and 2001. The results demonstrated that H1N1 was the most prevalence subtype as a 66.7%, and H3N2 was 33.7% out of 25,348 tested serum samples. Interestingly, H1N2 subtype was isolated from 1, 8, and 13 samples collected in 1999, 2000, and 2001, respectively. The 22 H1N2 isolates originated from 9 different states of US. Genetic characterization revealed that all 22 H1N2 isolates were reassortments of classical swine H1N1 and triple reassortant H3N2 viruses.; In in vitro pathogenesis study, a novel mechanism of SIV-mediated cytotoxicity on PK-15 and Hela cells was found. The results demonstrated that bcl-2 family proteins were associated with SIV-induced apoptosis, and releasing of cytochrome c from mitochondria was important for SIV-induced cytotoxicity. When this H1N2 subtype was experimentally inoculated in 3-week old pigs seropositive against H1N1 and H3N2, virus was readily infected. Results in this thesis could provide improved service for SIV diagnosis and give a better understanding of the prevalence, pathogenesis for SIV infection. In addition, the results could contribute to the knowledge for the effective control measures to this disease. |
